image of Minimal Impact Design Standards logo
Residential rain barrel - Stillwater, MN
Example of a residential rain barrel - Stillwater, MN

Rain water harvesting is the practice of collecting rain water from impermeable surfaces, such as rooftops, and storing for future use.

Contents

Overview for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Stormwater harvesting and use is part of a larger concept of ‘reuse’, the practice of collecting stormwater, greywater, or blackwater to meet water demands, including but not limited to: irrigation, drinking, washing, cooling, and flushing. The focus of this section will be on the harvesting and use of stormwater, but the harvesting and use of stormwater can be combined with the harvesting and use of greywater and blackwater, for which other regulations and guidelines apply. In Minnesota, please contact the Minnesota Department of Health for questions related to harvest and use of greywater or blackwater.

Introduction

This schematic shows Example Stormwater Harvesting and Use System Schematic
Example Stormwater Harvesting and Use System Schematic

A stormwater harvesting and use system is a constructed system that captures and retains stormwater for beneficial use at a different time or place than when or where the stormwater was generated. A stormwater harvesting and use system potentially has four components (See Example Stormwater Harvesting and Use System Schematic at right): collection system (which could include the catchment area and stormwater infrastructure such as curb, gutters, and stormsewers), storage unit (such as a cistern or pond) treatment system: pre and post (that removes solids, pollutants and microorganisms, including any necessary control systems), if needed, and the distribution system (such as pumps, pipes, and control systems).

The specific components of a stormwater harvesting and use system vary by the harvested stormwater source (rooftops, low density development, traffic areas, etc.) and the beneficial use of stormwater (irrigation, flushing, washing, bathing, cooling, drinking, etc.). Commonly in stormwater harvest and use, rainwater is differentiated from stormwater and is defined as stormwater runoff collected directly from roof surfaces which can have lower levels of pollutants and it often requires less treatment than other forms of stormwater. However, rainwater is still stormwater and depending on the use, may require treatment prior to use. See the Water harvesting and use system matrix table below for a summary matrix of harvested water sources and beneficial uses. The components, design, construction, and operation & maintenance of a water harvesting and use system are described in more detail in the Design Guidance, Construction Sequence, and Operation & Maintenance sections.

The source area of harvested stormwater largely determines the quality of the stormwater supply in a stormwater harvest and use system. As precipitation accumulates and flows over surfaces it collects pollutants and microbial contaminants . The type of and quantity of pollution in stormwater depends on the composition of the surfaces over which stormwater runoff flows and the activities within the drainage area that generate pollution. Water quality considerations of harvested water are described in more detail here.

The quantity of runoff that can be harvested is dependent on the depth and intensity of precipitation as well as the capacity of the source area to shed or retain water. Quantification of runoff that can be harvested from a site is described in more detail in the Calculators section.

The beneficial use of stormwater determines the volume and treatment criteria needed. Common beneficial uses of stormwater are described in this memo under the section Beneficial Use of Stormwater Key Considerations. Methods for estimating beneficial use water volume demand are outlined in the Design Guidance and Calculators section. Water quality criteria for different beneficial uses of stormwater are discussed in more detail here.

A central consideration in any stormwater harvesting and use system is matching the water quality of harvested stormwater with the water quality requirements of the beneficial use of stormwater. Water quality requirements for beneficial uses of stormwater are often context-specific and required treatment will vary depending on source water quality. Water quality requirements for beneficial uses of stormwater are based on the risks posed to human health (i.e., health criteria) and/or to the environment. For some uses, industry-specific standards may also apply. The difference between the water quality of the harvested stormwater and the water quality requirements of the beneficial use of stormwater must be addressed by incorporating appropriate treatment components into the stormwater harvesting and use system. The water quality requirements of common beneficial uses of stormwater and the level of treatment needed for various types of harvested stormwater to meet these requirements are summarized in the Water harvesting and use system matrix table. These concerns are taken up in greater detail in the Water Quality Considerations section.

Finally, the specific components of a water harvesting and use system determine the costs, environmental concerns and long term maintenance of a system. These topics are discussed in more detail in the Costs, Environmental Concerns, and Operation and Maintenance sections.

Water harvesting and use system matrix
Link to this table

Beneficial Uses
Stormwater from rooftops only (rainwater)
Stormwater
Health criteria Level Level of effort1 Health criteria Level Level of effort1
Outdoor Sanitary sewer flushing Limited human exposure at point of use and limited exposure to pathogens upstream of point of use No treatment needed Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal
Irrigation – low exposure risk Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal to Medium
Irrigation – high exposure risk Limited human contact and controlled access at point of use Minimal to Medium Limited human contact and controlled access at point of use Medium to High
Vehicle/building washing Limited human contact and controlled access at point of use Minimal to Medium Limited human contact and controlled access at point of use Minimal to Medium
Fire fighting Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal to Medium
Water features (uncontrolled access) Limited human contact and controlled access at point of use Medium Limited human contact and controlled access at point of use Medium
Street cleaning/ dust control Limited human contact and controlled access at point of use Minimal to Medium Limited human contact and controlled access at point of use Minimal to Medium
Indoor Fire supression Limited human contact and controlled access at point of use Medium Limited human contact and controlled access at point of use Medium
Cooling Limited human contact and controlled access at point of use Minimal to Medium Limited human contact and controlled access at point of use Minimal to High
Process /Boiler Water Limited human contact and controlled access at point of use Minimal to Medium Limited human contact and controlled access at point of use Minimal to High
Flushing Uncontrolled access at point of use Minimal to Medium Uncontrolled access at point of use Medium to High
Washing Uncontrolled access at point of use Medium Uncontrolled access at point of use Medium to High
Drinking water Drinking water standards Drinking water standards Drinking water standards Drinking water standards

1Minimal - pretreatment; Medium - pretreatment + disinfection OR pretreatment + treatment; High - pretreatment + treatment + disinfection

Beneficial use of stormwater key considerations

Beneficial uses of stormwater include any use of water to meet individual or societal water needs, including but not limited to: irrigation, drinking, washing, bathing, cooling, and flushing. Beneficial uses of stormwater pose different levels of human health risk based on whether public access is “restricted” or “unrestricted”. A use is restricted if public access can be controlled, such as irrigation of golf courses, cemeteries, and highway medians. A use is unrestricted if public access cannot be controlled, such as irrigation of parks, toilet flushing, firefighting, or water feature uses. Unrestricted beneficial uses of stormwater have more stringent water quality regulations that limit public health risk and exposure to pollutants and microorganisms than restricted beneficial uses of stormwater (Alan Plummer Associates, 2010; NRMMC et al., 2009; USEPA, 2004). Other ways to classify beneficial uses of stormwater include water quality criteria (potable/non-potable use); setting (indoor/outdoor, urban/rural, residential/municipal/commercial/industrial, etc.), and scale of implementation (private, neighborhood, regional, etc.).

Key considerations for choosing a beneficial use of stormwater include the demand characteristics (seasonal, constant, intermittent, etc.) which influence the design of the makeup supply; exposure level (no contact, limited contact, unrestricted contact) which influence treatment system design; and the scale of implementation (some applications are better suited to multi-residential or commercial settings). In addition, storage availability and distance between the water source and the beneficial use of stormwater can affect cost and therefore adoption rates, but not inherently affect the technical feasibility. Key considerations for specific beneficial uses of stormwater which are represented in the reference literature are discussed categorically in the text that follows. In-depth discussion of considerations for beneficial uses of stormwater can be found in Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits (NCDENR, 2014) and 2012 Guideline for Water Reuse (USEPA, 2012; see Chapter 2).

Outdoor uses

Outdoor uses include irrigation, water features, sanitary sewer flushing, street cleaning/dust control, vehicle/building washing, firefighting, recharge, and ornamental and recreational wetlands. Plumbing codes and requirements for outdoor systems may be less restrictive than those for indoor use; however, in any system, appropriate measures must be taken to prevent contamination of drinking water supply and minimize health risk exposure.

Irrigation

Irrigation is the most common use of harvested water and therefore examples and case studies are more plentiful for this use. In some communities, especially more recently developed suburban areas, the demand for irrigation on the community’s water system can increase significantly during the summer months, at times doubling, tripling, or more the base water demand. Harvested water collected in a community could be used to meet their irrigation water demands, or could be transported via water trucks to meet off-site irrigation needs, such as ultra-urban, downtown settings.

Estimating demand for irrigation water can require complex calculations that take into account not only the size of the irrigation plot, but also the type of plantings and seasonal climatic factors (evapotranspiration, plant water use coefficients, precipitation, humidity, etc.). Given the practicality of harvesting water for irrigation, a wide variety of tools have been developed for estimating irrigation demand. Water demand will be greater for irrigation systems which are susceptible to evaporation losses (sprinkler, spray).

Water quality criteria for irrigation vary depending on the risk of exposure at the point of use (restricted vs. unrestricted public access), the type of crop (food crops vs. non-food crops), and, if applicable, the point of sale of food crops (fresh produce vs. processed food). Water used in animal operations for watering or cleaning may require additional treatment. Additional considerations include maintenance of equipment (potential clogging of spray nozzles) and risk of exposure for wildlife. Stormwater harvested in cold climates can have elevated chloride levels from winter applications of road salts, potentially affecting vegetation growth.

Water features

Water demand for water features (such as decorative fountains, pools or water walls) may be constant (indoor) or seasonal (outdoor). Many water features have high water demand due to evaporative losses. Harvested water may require disinfection for use in water features depending on risk of exposure/ingestion at point of use.

Sanitary sewer flushing

Health criteria for sanitary sewer flushing are less stringent than most beneficial uses of stormwater due to low risk of exposure. Demand for sewer flushing is likely intermittent, but requires large volumes of water per application. Sewer flushing may be a suitable use for water harvested in stormwater impoundments, but pretreatment may be required to prevent sediment from being deposited in sewers. If flushing storm sewers, additional considerations regarding the water quality of a downstream lake or stream (with respect to its ability to meet state water quality standards) may increase treatment requirements.

Street cleaning and dust control

Street cleaning and dust control uses may be intermittent in many cases but possibly regular (daily or weekly washing). Special fittings may be required to fill water tanks as most trucks are fitted for compatibility with fire hydrants. Pretreatment is needed to prevent clogging of spray nozzles and disinfection may be required due to risk exposure.

Vehicle/building washing

Vehicle and equipment washing are common uses of water and there are several examples of using harvested water for vehicle washing in the U.S. (NAS, 2015; US EPA, 2012). Demand for outdoor washing may be seasonal in cold climates. Water harvested for washing may require disinfection due to risk of exposure. Salinity and hardness of harvested stormwater may be a concern for equipment washing.

Firefighting

Harvested rainwater and stormwater that are pretreated is generally suitable for fire suppression, but disinfection may be required if merited by exposure risks (NAS, 2015). Harvested water can be used to fill onboard water tanks. On a larger scale, because firefighting is an emergency use of water, demand for this use will not be predictable; however, wet ponds may provide suitable emergency supply for fire suppression. Use for fire suppression will require a large storage volume on site that is compatible with International Fire Codes (IFC).

Indoor uses

Adherence to plumbing codes impose additional water quality criteria and require a higher level of treatment for indoor uses than similar outdoor uses. Indoor uses are more likely to require a constant supply of water and therefore requires a back up water supply. In cold climates, a secondary supply will most likely be needed during winter months. Indoor uses provide an opportunity to maximize the cycling of water on site since greywater from beneficial uses of stormwater supplied with harvested water can be captured for additional beneficial use.

Flushing

Toilet and urinal flushing has a relatively constant demand throughout the year and account for approximately 24% of household water use (NAS, 2015), however toilet type (e.g., low flush) may affect demand. Use of non-potable water for toilet flushing or other indoor uses requires that the municipal water supply be protected. Water quality criteria are more restrictive due to plumbing code and unrestricted access at point of use. Currently this use is practiced most commonly in multi-residential or commercial setting, due to treatment and plumbing requirements (NAS, 2015). Beneficial use for toilet flushing may contribute to sustainable building certifications such as US Green Building Council (USGBC) and the State of Minnesota’s B3 Guidelines.

Fire suppression

Considerations for fire suppression sprinkler systems are similar to those for firefighting. Systems must be compatible with IFC and indoor plumbing codes.

Washing

Indoor washing applications include laundry (residential, industrial, institutional, etc.), washing of equipment, or other cleaning practices. Laundry accounts for about 22% of household water use in the U.S. (USEPA, 2008). Using harvested water for laundry may reduce household consumption of potable water significantly.

Cooling

Harvested water can be used for cooling water or cooling water makeup supply. Harvested water with high levels of salinity or hardness can cause scaling and should be avoided, whereas rainwater is naturally soft and low salt rainwater is preferred in these applications. Industry specific standards may apply. This use application may be suitable for implementation at a variety of scales.

Process and boiler water

The considerations for process and boiler water are similar to those for cooling water. Industry specific criteria will likely apply and health criteria for process water are dependent on the particular application.

Drinking water

Household water demand for drinking and cooking is fairly constant, but these uses make up a relatively small portion of household use (< 5%; USEPA, 2008). The level of treatment required to meet drinking water criteria and public acceptance are key considerations of drinking water uses. Using treated rainwater for drinking water supplies is practiced in various parts of the U.S. including Virginia, Texas, Georgia, but to a much lesser degree in Minnesota. Public acceptance of harvest and use for potable supply may be greater in cases where potable water is commonly used, but not necessarily required (e.g., laundry and flushing), than in cases where potable water is required (e.g., drinking).

Water harvesting and use benefits

The potential benefits of water harvesting and use for stormwater management are largely tied to the impacts of urbanization. Urbanization can dramatically alter the hydrology and water quality of a watershed or smaller catchment. Increased impervious surface area and other changes in land cover associated with urbanization tend to decrease the attenuation of water on landscapes. This results in increased runoff volumes and peak stream flows following storm events; and decreased groundwater recharge and stream baseflows in the watershed. Furthermore, the quality of stormwater runoff can be degraded when runoff flows over developed or managed surfaces collecting pollutants and pathogens that may cause health risks to plants, animals and humans. By retaining and/or treating stormwater on-site through harvesting and use, the impacts of urbanization on hydrology and water quality can be reduced.

As the human population and urbanization grow, there is also a need to reduce potable water demand (Hatt et al., 2006). Although this goal is most commonly associated with harvest and use programs in arid environments where the availability of freshwater is limited, the cost savings associated with reducing potable water consumption can be a compelling goal even in water rich environments. The harvest and use of stormwater may also reduce stress on existing water and stormwater infrastructure providing cost savings on repair and maintenance or even mitigating the need for expansion of facilities. Additional benefits of water harvesting and use include education opportunities and onsite environmental benefits.

Below is a summary of potential benefits of water harvesting and use in urban areas.

  • Reduce impacts of urbanization on watershed hydrology
    • Reduce runoff volume from the site
    • Reduce peak stream flows following storm events
    • Reduce flooding in downstream waters
    • Increase groundwater recharge
  • Reduce impacts of urbanization on water quality
    • Reduce pollutant loads to downstream receiving waters
  • Increase water conservation
    • Conserve potable water for essential uses
    • Provide alternative to potable water during time of peak demand
    • Reduce or limit withdrawals from ground or surface water supply
    • Maintain reliable water supply in the event of municipal service disruption
  • Reduce stress on existing/need for additional infrastructure
    • Reduce the size of stormwater BMPs needed to achieve regulatory requirements
    • Increase the efficiency or extend the life of stormwater BMPs/infrastructure
    • Reduce stress on municipal water supply systems during peak usage
    • Reduce stress on/cost of water supply and treatment infrastructure
    • Reduce community expenditure on expansion of infrastructure
  • Energy, education, environment, and economics
    • Provide educational opportunities/increase public awareness
    • Attain sustainable design certification/recognition
    • Reduce consumption of potable water for individual cost savings
    • Reduce the energy footprint of water, wastewater, and stormwater infrastructure
    • Take advantage of rainwater quality (low mineral content, no chlorine and ability to reduce or eliminate the use of water softeners)
    • Reduce on-site erosion control and flooding

Water harvesting: use of codes and standards

Current codes and standards for water harvesting and use systems are described below:

Plumbing codes

The new 2015 Minnesota Plumbing Code, Minnesota Rules, Chapter 4714, took effect Jan. 23, 2016. The code includes the design and installation of harvesting rainwater from building roof tops in Chapter 17, Nonpotable Rainwater Catchment Systems. Nonpotable rainwater catchment systems are acceptable for use to supply water to water closets, urinals, trap primers for floor drains, industrial processes, water features, vehicle washing facilities, and cooling tower makeup water provided the design, treatment, minimum water quality standards, and operational requirements are in accordance with Chapter 17 of the code. Designs must be approved by a qualified Minnesota registered professional engineer.

Rainwater catchment systems use for plumbing applications listed above in combination with lawn irrigation must meet the requirements of Chapter 17. System components used solely for lawn irrigation, such as irrigation pumps and piping mounted outside of buildings are not subject to the requirements of Chapter 17. The conveyance of the rainwater catchment system is still governed by the plumbing code.

Minimum water quality standards are now described in Chapter 17 (1702.9.4 Minimum Water Quality). The minimum water quality for rainwater catchment systems shall meet the applicable water quality recommendations in the table below

Minimum water quality
Link to this table

Measure Limit
Turbidity (NTU) <1
E. coli (MPN/100 mL) 2.2
Odor Non-offensive
Temperature (degrees Celsius) MR-measure and record only
Color MR-measure and record only
pH MR-measure and record only

Treatment: 5 micron or smaller absolute filter; Minimum .5-log inactivation of viruses.

To achieve these water standards, it is highly recommended to sample your source water and design your water treatment system around the baseline data. It is recommended to work with suppliers and manufacturers that are trained or have direct and relevant experience treating rainwater. Most UV manufacturers will include an Ultra Violet Transmittance (UVT) requirement for their UV to reach end use requirements. To achieve less than 1 NTU, a 1 micron absolute pre-filter may be required. If the roof is “dirty” gravel, pervious pavers, green roof or carbon filtration may be required to eliminate odors. Other strategies may include inclusion of aeration in the tank to maintain aerobic conditions. The most cost effective and long run approach is to reduce the amount of organic material entering the tank.

Water harvesting and use considerations

Treatment

High levels of pollutant and pathogen treatment, can add cost and can limit the range of practical beneficial uses of stormwater considered in the design of water harvesting and use systems. Partly, for this reason, irrigation has been the most common type of beneficial use application for water harvesting and use systems constructed in Minnesota due to the low levels of treatment required and lack of consistent rules.

Storage versus supply

The relative availability of harvested water supply, storage size, and water demand is often not balanced in society. For example, in highly urbanized sites, the harvested water supply can sometimes greatly exceed the water demands and storage availability, while in less urbanized sites, water demands and storage availability can sometimes greatly exceed harvested water supply. These limitations can be overcome by centralizing water harvesting and use systems over larger areas to bring together areas with excess harvested water supply with areas of high water demand.

Cold climates

In cold climates stormwater and rainwater supply are seasonal. Beneficial uses of stormwater which require a constant supply, will need to rely on a secondary supply during several months of the year as required by the plumbing code for indoor uses. Additionally, outdoor systems not designed with freeze protection will require annual maintenance to prevent damage from freeze/thaw cycles. Systems can be designed for year round supply (see Ontario and Alaska Guidance documents). Two generally accepted approaches are to provide 2 inches of rigid insulation over the entire tank area in a shallow frost protected foundation approach and having all water supply pipes exit the bottom of the tank below frost line or with freeze protected and/or heat traced pipe. In these applications, the prefiltration devices must be chosen that can withstand freeze thaw conditions. If the system is insulated and the conveyance piping is run with consistent slope, there is rarely problems in this application. The overflow of the system can be problematic if the tank and the filter overflow to daylight or a pond. Refer to the Ontario Guidelines for subsurface overflow strategies.

Site constraints

Constructing water harvesting and use systems in fully developed areas can be difficult due to space and cost limitations of retrofitting developed sites with the infrastructure needed to collect, store, and distribute harvested water to the beneficial use.

Public acceptance

Modern society is used to nearly all water supplies being treated to the drinking water level. While this type of treatment for all domestic uses of water may be unnecessary and costly, the public sometimes perceives a high level of risk for using water not treated to drinking water levels. Overcoming this perception of harvested water being ‘dirty’ or ‘dangerous’ will be a large hurdle for this management technique to expand beyond irrigation uses of stormwater.

References


Design criteria for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Stormwater harvest and use can be implemented at a variety of scales, from individual parcels to regional scales, and in a variety of contexts, from ultra-urban settings to new development. The scale and complexity of any harvest and use project depends on several factors, including source water quality, intended application, and water quality recommendations and/or regulations. The design process for each project must be flexible and rigorous enough to address these factors.

Although there is a wealth of information on selection and design of system components (collection, storage, treatment, and distribution systems), guidance offering a concept-to-finish perspective is limited (Met Council, 2011; NC DEQ, 2014; DPLG, 2010). The design process described in this report uses four broad phases: feasibility, pre-design, design, and implementation. In practice the distinction between phases is not strict. Design is an iterative process which should include several rounds of review beginning in the feasibility phase.

The following discussion includes a description of each design phase, a list of activities typically included in each phase, design guidance, and key resources for individual activities.

Overview of broad phases of design

harvest use flowchart
Flowchart illustrating the steps comprising the feasibility, pre-design, and design phases for a harvest and use/reuse system. Click on image to enlarge.

The four broad phases of design include feasibility, pre-design, design, and implementation. This section provides a summary list of components of each design phase. These are discussed in greater detail in the following section. Implementation is discussed only briefly in this section and is covered in greater detail in the construction section.

Feasibility phase

During the feasibility phase, opportunities for stormwater harvest and use are identified and evaluated at a basic level, taking into consideration site characteristics, expected source water quality, and constraints imposed by various use applications. The goal of this phase is to determine if a project is feasible and, if it is, what constraints might exist. Typical feasibility phase activities include the following:

Pre-design phase

During the pre-design phase, stormwater harvest and use opportunities that were identified in the feasibility phase are evaluated at greater depth to determine the most feasible option or best application. Typical pre-design phase activities include the following:

Design phase

The design phase includes the selection, sizing, siting, and design of stormwater harvest and use system components. Generalized design steps are listed below. In practice, design is an iterative process and steps will likely be revisited as the system design is refined in conjunction with site, cost, regulatory, or other considerations.

Implementation phase

The implementation phase includes all post-design work including construction and installation; operations, maintenance, and monitoring; and additional activities which are included in project objectives. Typical implementation phase activities include the following

The implementation phase is discussed in detail in the sections on construction and on operation and maintenance.

Additional resources

More information on the design process and design sequencing can be found in the following resources:

  1. Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
  2. City of Bellingham, Public Works Department. March, 2012. Rainwater Harvesting, Guidance towards a Sustainable Water Future. City of Bellingham, WA.
  3. Department of Environment and Conservation, New South Wales. April 2006. Managing Urban Stormwater: Harvesting and Reuse. Sydney, Australia, ISBN 1 74137 875 3.
  4. Department of Planning and Local Government (DPLG). December 2010. Water-Sensitive Urban Design Technical Manual for the Greater Adelaide Region; Chapter 8 - Urban Water Harvesting and Reuse. Government of South Australia.
  5. North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
  6. American Rainwater Catchment System Association and American Society of Plumbing Engineers. 2013. ARCSA/ASPE/ANSI Standard 63-2013. Rainwater Catchment Systems
  7. American Rainwater Catchment System Association and American Society of Plumbing Engineers. 2015. ARCSA/ASPE78: Stormwater Harvesting System Design for Direct End‐Use Applications
  8. American Rainwater Catchment System Association. 2015. ARCSA Rainwater Harvesting Manual. 1st Edition
  9. Despins, Christopher. 2010. Ontario Guidelines for Residential Rainwater Harvesting Systems. Eds. Leidl, C., and K. Farahbakhsh

Feasibility phase

During the feasibility phase, project objectives are defined and opportunities for stormwater harvest and use are evaluated at a preliminary level. Preliminary evaluations should take into consideration site characteristics, expected source water quality, and constraints imposed by various harvest and use applications. The goal of the feasibility phase is to identify stormwater harvest and use opportunities that are suitable for in-depth feasibility analysis or determine that harvest and use is not feasible.

Identify harvest and use system goals and objectives

The first step in the project is to define objectives. Objectives are typically driven by regional or organizational goals, including the following:

  • decrease the quantity of potable water used;
  • adding resiliency into the water supply system;
  • utilizing stormwater as a resource, instead of a waste product;
  • pollution prevention by capturing rainwater prior to contact with surface;
  • decrease stormwater runoff volumes and/or stormwater runoff rates; and
  • decrease pollutant loads associated with stormwater runoff.

Specific, quantifiable goals which address project objectives should be used to evaluate the feasibility of the proposed system. Examples include the following:

  • capture 90 percent of annual stormwater runoff from a site;
  • capture sufficient water to meet ½ of the irrigation demand for a golf course;
  • reduce phosphorus loading from a site by 80 percent; or
  • capture sufficient water to meet ½ of the toilet flushing demand for a business.

In Minnesota, capture and use of stormwater irrigation use is increasingly being used to meet runoff and phosphorus reductions goals. Irrigation practices utilize evapotranspiration losses of water and plant uptake and soil adsorption of the dissolved fraction of phosphorus. The determination of project objectives and goals may be an iterative process in which project objectives are modified based on feasibility, cost, or other concerns. Identifying project objectives which align with broad regional or organizational goals may be important in garnering stake-holder support or procuring funding for the project.

Additional Resources

  1. Department of Environment and Conservation, New South Wales. April 2006. Managing Urban Stormwater: Harvesting and Reuse. Sydney, Australia, ISBN 1 74137 875 3.
  2. Department of Planning and Local Government. December 2010. Water-Sensitive Urban Design Technical Manual for the Greater Adelaide Region; Chapter 8 - Urban Water Harvesting and Reuse. Government of South Australia.

Consider site opportunities and constraints

At the feasibility phase, a preliminary assessment of site conditions provides an understanding of stormwater harvest and use constraints and opportunities. More in-depth analysis of site conditions and constraints will be required during pre-design and design phases. The following table describes some site considerations and identifies potential constraints.

Water reuse key site considerations
Link to this table

Consideration Notes Potential constraints
Catchment boundaries Identify the area(s) from which water can be captured. This is necessary to compute capture volumes, identify site constraints, and determine water quality. Does the amount of water that can be captured affect the type of possible end use and the size of storage unit?
Existing drainage patterns Identify how water will drain from the catchment area(s). Do any additional drainage features need to be constructed to centralize runoff collection?
Buffers and Setbacks Identify wetlands, streams, shorelines, and buildings/structures in the potential receiving area and determine setbacks from these, including buffer zones. Identify setback distances for infiltration practices described in the Minnesota Stormwater Manual and determine if these are applicable to the project. Are there any portions of the irrigation area that fall within a setback?
Proximity to water supply wells or sensitive water bodies Identify setback distances for infiltration practices described in the Minnesota Stormwater Manual and determine if these are applicable to the project. Are there any setback distances from supply wells or sensitive water bodies to consider?
Existing and adjacent land use/ stormwater hot spots Water quality varies with land use and will impact the level of treatment needed. Runoff from confirmed stormwater hotspots should not be used for harvest and use unless treated to appropriate standards. May need appropriate bypass systems. Are there any portions of the catchment area that need to be bypassed from the collection system or that require additional treatment prior to use?
Future land use changes Changes in the land use of the source area or irrigation areas may change opportunities and hurdles to implementation, such as level of treatment needed or availability of green space for irrigation. Are there any future land use changes that may prohibit or alter the proposed stormwater harvest and use system? (e.g. stormwater hotspots)
Existing infrastructure Maintain easements/appropriate distance from sewer lines, fiber optic cables, etc. Is there any infrastructure that needs to be avoided?
Site slopes Slope may affect runoff calculations and affect the system design (e.g. location of storage tank). Can sufficient runoff be collected from the source area? Can the cistern be placed at the lowest point on site? If not, what modifications need to be made to the collection system to route stormwater to the cistern?
Soil type Infiltration capacity affects runoff calculations and irrigation demand assumptions. Can sufficient runoff be collected from the source area? How do the irrigation area soils affect irrigation demand?
Proximity to building foundations & utilities Leakage from storage tanks could damage building foundations or utilities. Geotechnical engineering may be needed to ensure sufficient support for tanks and to prevent damage to adjacent foundations. Are there any setback distances from building foundations or utilities to consider?
Seasonal high water table Underground components may be constrained by the seasonal high water table. Above-ground systems are generally unaffected by the seasonal high water table. Will the seasonal high water table level affect any underground components?
Maintenance access Need space for maintenance access. Is there sufficient space for maintenance access?
Code or System Expansion Configure the system to allow for expansion if space is available. Follow national standards and simple steps of including a pipe stub in or designed plumbing system to allow for future incorporation of indoor uses in a phased fashion Should we stub in a pipe in case a code or owner changes to allow other uses? Can we configure the plumbing to adapt for indoor use in a second phase or when funding is available?

Additional Resources

  1. Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
  2. North Carolina Department of Environmental Quality (NCDEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
  3. City of Bellingham, Public Works Department. March, 2012. Rainwater Harvesting, Guidance towards a Sustainable Water Future. City of Bellingham, WA.
  4. Department of Planning and Local Government (DPLG). December 2010. Water-Sensitive Urban Design Technical Manual for the Greater Adelaide Region; Chapter 8 - Urban Water Harvesting and Reuse. Government of South Australia.
  5. Cabell Brand Center, 2009

Identify source areas and expected source water quality

Expected stormwater quality from various source areas is described in detail in the section on water quality considerations. Snowmelt from ground surfaces tends to have higher pollutant concentrations compared to runoff at other times of the year (EOR and CWP, 2005). Oberts (2000) discussed the dynamics of snowmelt runoff quality, indicating the initial melt (snow and ice from pavement) can have very high concentrations of soluble pollutants, which can be difficult to remove in traditional stormwater BMPs. Depending on the use of harvested stormwater, it may be desirable to provide a bypass or use a pre-tank filter for spring snowmelt, particularly the initial phase of the melt. Typical pollutants found in stormwater collected from different source areas are summarized in the table below.

Typical pollutants found in stormwater collected from different source areas

Link to this table

Source Area Solids Total suspended solids Nutrients Bacteria Metals Chloride Grease, Oil Pesticides Other chemicals
Hard Roofs1
Green and Brown Roofs2
Paved Surfaces (parking lots, sidewalks, driveways and roadways)
Green Spaces (lawns and park areas)

● = relatively high concentrations
○ = relatively low concentrations
1Metals are highly dependent on roof type-only certified membrane roofs are currently approved for potable through NSF.
2Vegetated roofs and pervious pavers have had poor performance due to discoloration of water and difficulty filtering the water to levels that the UV treatment is effective.

Determine acceptable use(s) of stormwater based on the needs of each use on site

Potential uses of harvested stormwater include the following.

  • Outdoor use
    • Sanitary sewer flushing
    • Irrigation (low or high exposure risk)
    • Vehicle/building washing
    • Fire fighting
    • Water features
    • Street cleaning/dust control
  • Indoor use
    • Fire suppression
    • Cooling
    • Process/boiler water
    • Flushing
    • Washing (e.g. bathing, laundry, dishwashing)
    • Drinking water

General considerations for the use of harvested stormwater are described here. See also the Source and Use Worksheet, C.1w, from the 2011 Met Council Reuse Guide. In addition, if the harvest and use system is required to follow the Minnesota Plumbing Code, check Section 1702.9.3 for prohibited collection surfaces and discharges.

Harvested stormwater beneficial uses
Link to this table

Beneficial Uses Stormwater from rooftops only (rainwater) Stormwater
Health Criteria Level Expected level of effort Health Criteria Level Expected level of effort
Outdoor Sanitary sewer flushing Limited human exposure at point of use and limited exposure to pathogens upstream of point of use No treatment needed Limited human exposure at point of use and limited exposure to pathogens upstream of point of use Minimal (pretreatment)
Irrigation – low exposure risk Limited human exposure at point of use and limited exposure to pathogens upstream of point of use. Minimal (pretreatment)1 Limited human exposure at point of use and limited exposure to pathogens upstream of point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment)
Irrigation – high exposure risk Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use.1 High (pretreatment + treatment + disinfection)
Vehicle/building washing Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment)
Fire fighting Limited human contact and controlled access at point of use.1 Minimal (pretreatment) Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment)
Water features (uncontrolled access) Limited human contact and controlled access at point of use. Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use. Medium (pretreatment + disinfection OR pretreatment + treatment)
Street cleaning/ dust control Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment)
Indoor Fire suppression Limited human contact and controlled access at point of use. Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use. Medium (pretreatment + disinfection OR pretreatment + treatment)
Cooling Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use.1 High (pretreatment + treatment + disinfection)
Process /Boiler Water Limited human contact and controlled access at point of use.1 Medium (pretreatment + disinfection OR pretreatment + treatment) Limited human contact and controlled access at point of use.1 High (pretreatment + treatment + disinfection)
Flushing Uncontrolled access at point of use1 Medium (pretreatment + disinfection OR pretreatment + treatment) Uncontrolled access at point of use1 High (pretreatment + treatment + disinfection)
Washing Uncontrolled access at point of use Medium (pretreatment + disinfection OR pretreatment + treatment) Uncontrolled access at point of use1 High (pretreatment + treatment + disinfection)
Drinking water Drinking water standards Drinking water standards Drinking water standards Drinking water standards

1A higher level of treatment that may be required depending on context
Note: In practice, all indoor non potable uses are typically treated to water quality requirements in code and currently require disinfection.

Consider level of operation and maintenance needed

Key operation and maintenance (O&M) questions to consider during the feasibility phase include:

  • Who will be responsible for O&M?
  • Is there a need for a licensed professional to do certain components of O&M?
  • How often does O&M need to occur?
  • How intensely must the system be operated and maintained?

See the section on operation and maintenance for detailed information pertaining to these questions.

Decide whether to proceed to pre-design

If the goals and objectives are compatible with the identified site constraints, source areas, and appropriate uses, proceed to Pre-design.

Pre-design phase

Complete site survey & information gathering

The following information may be useful or necessary when designing a harvest and use system. Some of the information is general and should be used for initial screening. The information should be recorded in a form (see link; Data Collection Worksheet, C.2w, items 1-3, from the 2011 Met Council Reuse Guide).

  • System maps and surveys
    • Municipal storm sewer maps illustrate stormwater drainage and treatment features, including piping, sewer inlets, treatment devices, and outfalls. These maps can be used to determine drainage patterns and potential catchments for the harvest system. Storm sewer maps are not readily available. Consult with the local municipality to determine if these maps are available. Example maps can be found in the Storm Water Master Plan for the University of Minnesota Twin Cities Campus (2012).
    • Topographic information is used to determine elevations, drainage patterns, and slope. USGS topographic maps illustrate these features and also illustrate other features, such as surface waters. Light Detection and Ranging (LIDAR) allows for high-resolution topographic mapping.
    • FEMA floodplain maps are used to indicate if the site is located in a floodplain. Sites located in floodplains may necessitate specific design considerations and may utilize floodwater as part of the harvest system.
    • County soils maps can be used to determine the permeability of soils and estimate other soil properties, such as depth to water.
    • Karst maps indicate the location of active karst areas. Irrigation over active karst is not recommended without site specific investigation.
  • Municipal, county, and MnDOT highway maps are used to help determine the catchment area.
  • Studies and reports
    • Runoff monitoring data and reports for the area or site provide information on stormwater runoff quantity and water quality, which can be used in determining appropriate level of treatment.
    • Hydrology analyses define rate and volume of stormwater runoff, which can be used in runoff calculations.
    • Stormwater design data for sites with pre-existing stormwater collection systems can be used in laying out the components of the harvest and use system.
    • Pre-existing permits for use in identifying limitations to the project. Permits include but are not limited to land use, zoning, building, plumbing, and stormwater discharge permits (e.g. MS4 permit).
    • Pre-existing site soil borings and other geotechnical analyses that help determine soil infiltration properties, depth to bedrock, depth to water, and other similar features.
  • Site specific data and surveys
    • Site plans or site surveys identify existing structures, roof areas, impervious surfaces, slopes, and drainage patterns.
    • Gopher State One-Call for locating above- and below-ground utilities.
    • Storm sewer surveys include determining the location of stormwater infrastructure, including pipes, inlets, outlets, storage, and treatment structures. Note the size and type of these structures.
    • Dimensions, depth, structures, and operating requirements for stormwater ponds or other BMPs located on the site.
    • Pre-existing storage tank properties including dimensions, control valves, cover, fencing, elevations, access, inlets, outlets, and condition.
    • Presence and location of wells. County Well Index (is now the Minnesota Well Index) is a tool that can be used to identify wells.
    • Building plans that show roof structures, materials, and drainage systems.
    • Precipitation data is used for runoff calculations. Use site information, if available, or data from the nearest station having long-term records.

Harvested stormwater site survey and information worksheet
Link to this table

Item Comments / notes Used
Storm sewer maps
Topographic maps
Floodplain maps
Soil maps
Karst maps
Highway maps
Hazardous waste site maps
Runoff monitoring data
Hydraulic analyses
permits
Soil borings/geotechnical analyses
Site plans
Gopher State One call
Storm sewer surveys
Presence of wells
Building plans
Precipitation data
Existing information on stormwater ponds
Existing information on storage tanks

Estimate the water balance for the site

A water balance must be calculated to determine if the desired capture volumes can be achieved and to properly size the system. A water balance consists of estimating the amount of water that can be captured and the amount of water that is used. Key considerations include balancing the amount of storage unit overflow with the size of the storage unit, and limiting or eliminating the need for a secondary water supply.

For stormwater management, it is recommended to evaluate source areas (harvest and use supply) first and then identify candidate use applications. Calculations of water captured are a function of rainfall and runoff potential from the site.

Detailed information for estimating a water balance is found at the page titled Estimating the water balance for a stormwater and rainwater harvest and use/reuse site. That page provides information on calculating a watershed budget and a phosphorus budget. The page includes equations for calculating water and phosphorus budgets and water demand. The page also provides links, including links to calculators.

Identify required permits and applicable codes

Required permits and applicable codes include the following: (note that not all may apply to every situation)

  • Municipal zone and zoning codes – contact the municipality in which the project occurs to determine if zoning review is required or zoning codes are applicable for harvest systems
  • Municipal permits
    • Plumbing permits may be required for new or altered underground and interior piping. Contact the local building department for more information.
    • Building permits – contact the local Zoning Administrator to determine if a building permit is required.
    • Erosion and sediment control permits may be required. A State Construction Stormwater permit is required if the project results in land disturbance of equal to or greater than one acre or a common plan of development or sale that disturbs greater than one acre. Some municipalities and watershed organizations have more stringent requirements. Contact local units of government to determine if local erosion and sediment control permits are required.
    • Site plan approval is required to ensure compliance with local ordinances. Site plan review and approval is often conducted by a city planner. Contact the appropriate local entity for site plan review and approval.
    • Storm/sanitary/water connections permits are required for new connections to a storm or sanitary sewer system.
  • Minnesota Plumbing Code
  • Watershed District or Watershed Management Organization Rules
  • County Health Department permits
  • Minnesota Pollution Control Agency (MPCA)
    • MPCA’s jurisdiction is typically over the water quality reaching the waters of the state and to ultimately protect the water quality in the lakes, streams, and groundwater. In terms of stormwater management, this typically applies to construction sites disturbing more than one acre of soil, industrial sites that currently have an industrial stormwater permit, or MS4s (Municipal Separate Storm Sewer System operators) trying to meet the requirements of their permits (which can include TMDL—Total Maximum Daily Load—wasteload allocations). Unless the stormwater harvesting and reuse systems for irrigation system is intended to demonstrate compliance with any of the above permits and stormwater management requirements, the MPCA would not be involved in the review of these systems.
  • Minnesota Department of Natural Resources (MDNR) (Per 6/14/17 email conversation with Dan Miller, MDNR).
    • A water appropriations permit is required from the MDNR for all users withdrawing more than 10,000 gallons per day or 1 million gallons per year from waters of the state. The term “waters of the state” is defined by statute as “surface or underground waters, except surface waters that are not confined but are spread and diffused over the land.” This includes water in ponds and basins.
However, a law enacted by the 2017 legislative session exempts stormwater control and reuse facilities from the water appropriation permit requirement. It specifically exempts “appropriation or use of storm water collected and used to reduce storm-water runoff volume, treat storm water, or sustain groundwater supplies when water is extracted from constructed management facilities for storm water.” “Constructed management facilities for storm water” is defined as “ponds, basins, holding tanks, cisterns, infiltration trenches and swales, or other best management practices that have been designed, constructed, and operated to store or treat storm water in accordance with local, state, or federal requirements.”
Previously, a permit was required to use stormwater for irrigation or other uses. Existing water appropriation permits using stormwater for irrigation will be canceled, effective July 1, 2017. Stormwater reuse facilities that use natural, non-constructed water features for irrigation or other uses may still require water appropriation permits from the DNR. If you have any questions contact Dan Miller with the DNR (651-259-5731).

In general, if water is pumped out of a stormwater basin, an appropriation permit is required. If the water is temporarily drained out of the basin via an operable outlet structure, an appropriation permit is not required. However, Minnesota law now provides an incentive for stormwater capture and reuse. Legislation adopted in 2014 directs the Department of Natural Resources to “waive the water use permit fee for installations and projects that use storm water runoff or where public entities are diverting water to treat a water quality issue and returning the water to its source without using the water for any other purpose, unless the commissioner determines that the proposed use adversely affects surface water or groundwater.” (MS §103G.271, subd. 6(g); emphasis added). Therefore, stormwater reuse is exempt from the annual water use reporting fee, although an one-time application fee is required. Under the appropriation permit, monthly water use is measured and reported annually.

A general permit for temporary projects (General Permit 1997-0005) authorizes temporary water use of up to 50 million gallons per year for activities such as construction dewatering, landscaping, dust control, firefighting, and hydrostatic testing of pipelines, tanks and wastewater ponds, provided that activities are completed within one year.

The DNR is currently requesting comments on a proposed general permit for stormwater use In MS4-regulated communities. The general permit would authorize stormwater use for irrigation of residential landscaped areas, cemeteries, golf courses, athletic fields, and similar sites. The MS4-regulated community can utilize the general permit as a method to more quickly get approval of stormwater use projects. In addition, the MS4 community could coordinate the application and reporting of stormwater use projects from private entities, if desired. For further information, contact Dan Miller at Dan.W.Miller@state.mn.us

Identify water treatment requirements

See the section on water quality considerations.

Conduct an environmental and health risk assessment

See the section on environmental concerns.

Decide whether to proceed to design

Use the Pre-design phase checklist to determine whether to proceed to the design phase. In general, you can proceed to the Design Phase if

  • no major site constraints were identified during site survey and information gathering, or if these constraints can be overcome;
  • the harvested water supply is sufficient to meet identified use demands based on available storage, or if not sufficient, a supplemental supply can be used to meet the demands;
  • water treatment requirements and codes can be met;
  • required permits can be obtained; and
  • environmental and health risks are acceptable.

Design phase

The design phase involves identifying the specific components of the harvest and use system.

Storage

Storage is the central and often most expensive component in a stormwater harvesting and use system. Several factors must be considered, including size, type, siting and materials certification (e.g. NSF 61) of the storage system.

Determining the appropriate storage size

Decision-making on storage size can have a strong impact on the economic feasibility of stormwater harvesting (VA DCR, 2009). During the pre-design phase, a water balance should have been completed. This water balance should be refined during the Design Phase to incorporate other factors such as cost, site constraints, aesthetic concerns, or water quality criteria. For these reasons, the selection and sizing of storage units may be an iterative process. Work with an engineer, system supplier or accredited professional with experience using stormwater harvest and use calculators to appropriately size storage units for your stormwater harvest and use system.

For information and guidance on determining approrpiate storage size, see the page titled Determining the appropriate storage size for a stormwater and rainwater harvest and use/reuse system. Methods for sizing tanks are discussed in the section on water balance.

Determining the siting of the storage system

Storage options for stormwater harvesting systems include above- or below-ground tanks or cisterns which are closed to the environment, and above-ground open storage, such as a stormwater retention or detention pond. Depending on the type of storage system, considerations for siting storage units include the following.

For all storage systems

  • Ensure the location allows for proper drainage of rainwater through the conveyance network and the stormwater discharge location.
  • Ensure that inflow and overflow are the same diameter.
  • Ensure the location can accommodate the desired storage capacity.
  • Ensure sufficient access around the perimeter of the storage system to install, inspect and maintain the system and all its components, including access for equipment.
  • Does the storage system require specific location or proximity to source areas and overflow pathways or electrical and water utilities to service pumps, controls, and makeup water supply?
  • Should an outdoor system be provided with a large pump to do all the work or is it more cost effective to pump and treat at a slower rate to a day tank? For flows above 50 gallons per minute, use of a day tank reduces the size of the treatment system, initial cost and long term maintenance costs.
  • Should the storage system be placed so that it can be expanded later? For example, if another tank is added later, should the tanks be configured to deliver and receive overflow?
  • Ensure the location is permitted by applicable provincial codes and regulations and municipal zoning bylaws. Consult local building authorities for details.
  • Consider site topography and storage tank location, which will affect pumping requirements. Locating storage tanks in low areas will make it easier to get water into the cisterns; however, it will increase the amount of pumping needed to distribute the harvested stormwater back into the building or to irrigated areas situated on higher ground. Conversely, placing storage tanks at higher elevations may require larger diameter pipes with smaller slopes but will generally reduce the amount of pumping needed for distribution (Draft District of Columbia Stormwater Management Guidebook).
  • Consider siphonic roof drainage to reduce conveyance pipe costs and when elevation issues are present.
  • Consider if weather proofing of the tank is necessary; will the tank be operational during the winter months to service the stormwater use?

For underground storage tanks

  • Avoid potential conflicts with underground utilities.
  • Provide a union disconnect inside of the access cover that allows disconnection of the pump without entering the tank. After the union, the pump line can exit deep in the tank below frost line for year round applications.
  • Underground, outdoor tanks may need to be located below the frost line, drained or decommissioned during the cold season. The entire tank may not need to be under the frost line. For example, smaller systems may be designed with 2 inches of rigid insulation above the tank and utilization of the Ontario freeze model to identify thickness, depth and flaring of the insulation. Pump lines need to be below the frost line or freeze protected. Systems with bypass or prefiltration with a blind insert can divert water away from the system and do not need to be disconnected in the winter.
  • Are there any safety issues like soil settling, proximity to basements, or others issues that would preclude a tank location?
  • Areas with vehicle traffic should be avoided or the tank needs to be structurally strong enough to support the traffic load. If pre-filtration is used, specify the proper filter top based on the traffic loads present.

For aboveground storage tanks

  • Aboveground outdoor tanks may need to be properly insulted, or drained and decommissioned during the cold season if not placed in a conditioned space.
  • Are there any safety issues like soil settling, blocking access, or others issues that would preclude a tank location?
  • Are there objections from homeowner associations or neighbors concerning locations of the storage system and can these objections be overcome?
  • Are there aesthetic considerations, such as locating the storage system so that it is out of view or does not block features such as windows or shrubs?
  • Will you be using gravity flow to deliver water, which may affect the height at which the storage system is located?

For indoor tanks

  • Review guidelines for underground and aboveground storage tanks to determine if they apply (see above).
  • Ensure provisions (such as floor drains and/or sump pump) are in place to handle potential leaks and overflows from the storage tank.
  • Consult a structural engineer regarding the design and location of all integrated tanks, as well as indoor tanks located anywhere other than the basement or garage.
  • Locate the tank in a temperature-controlled environment such as a heated garage or basement to prevent tank freezing. If the tank is not located in a temperature-controlled environment and is at risk for freezing, winterizing or decommissioning must be performed in accordance with the guidelines below.

Storage siting additional resources

Selecting and designing the storage unit

Storage options for stormwater harvesting systems include above- or below-ground tanks or cisterns which are closed to the environment, and above-ground open storage, such as a stormwater retention or detention pond. Storage tanks or cisterns are commonly used in rainwater harvesting systems and are used for both indoor and outdoor stormwater use applications. Tanks or cisterns will likely provide the best option for indoor stormwater use applications due to ease of siting storage close to point of use. Stormwater ponds are commonly used for outdoor irrigation and in retrofit applications with existing stormwater ponds.

Advantages and disadvantages of above- or below-ground tanks and stormwater ponds are summarized below. The City of Bellingham, WA Rainwater Harvesting Guidance provides a comparison of characteristics of aboveground and belowground tanks (See Table 2 in Appendix A).

Advantages and disadvantages of above- or below-ground tanks and stormwater ponds
Link to this table

Type of storage system Advantages Disadvantages
Open systems (e.g. ponds)
  • Low capital costs
  • Low maintenance costs
  • Provide stormwater treatment
  • Can utilize existing stormwater ponds
  • Public safety concerns if not fenced
  • Habitat for mosquito breeding
  • Loss to evaporation
Below-ground, closed systems
  • Concealed from view
  • Do not consume above-ground space
  • Can be freeze protected
  • Greater capital costs
  • Higher maintenance costs
  • Require stronger structure in traffic areas
  • Require pumping
  • Access can be difficult
Above-ground, closed systems
  • Moderate capital costs
  • Moderate maintenance costs
  • Can be gravity fed
  • Aesthetic concerns
  • More susceptible to weather conditions than below-ground systems (UV, freezing)

Storage tanks and cisterns

General design considerations include the following.

  • Ensure that tank size is appropriate to meet use demand (see above)
  • Ensure tank location has been determined (see above)
  • Select the type of tank
  • Safety considerations

The suitability of different storage tank options is evaluated based on three main criteria: required capacity, potential location, and choice of materials (VA DEQ, 2009). Certain storage tank types may not provide adequate storage volume or structural integrity for large capacity systems. Site constraints may limit the size of above- or below-ground storage units, and tank materials can influence water quality. The following table summarizes comparisons of tank materials. If planned for potable water, tank material should be certified to NSF Standard 61.

Comparison of properties of different types of storage tanks
Link to this table

Tank material Advantages Disadvantages
Plastic
Fiberglass
  • Commercially available
  • Alterable and movable (modular)
  • No leak fittings
  • Durable
  • Minimal maintenance needed
  • Light weight
  • If aboveground, must be sited on smooth, solid, level footing
  • Expensive in smaller sizes
Polyethylene, polypropylene
  • Commercially available
  • Alterable and movable
  • Affordable
  • Available in a variety of sizes
  • Easy to install
  • Little maintenance
  • Can be degraded by UV
  • If aboveground (outdoors), painted or tinted
  • Can be insultated and heat traced.
Barrels and trash cans
  • Commercially available
  • Inexpensive
  • Must use new cans
  • Small storage capacity
  • Rarely installed correctly
  • May lead to foundation water issues
Metal
Galvanized steel tanks
  • Commercially available
  • Alterable and movable
  • Available in variety of sizes
  • Possible corrosion and rust
  • Must be lined for potable use
  • Only above-ground use
  • Must be insulated and heat traced
Steel drums (55-gallon)
  • Commercially available
  • Alterable and movable
  • Verify prior to use for toxics
  • Corrosion and rust can leach metals
  • Small storage capacity
Concrete and masonry
Ferroconcrete
  • Durable
  • Versatile
  • Suitable for above- or below-ground installations
  • Can be incorporated into the foundation of a building.
  • Potential to crack and leak
  • Permanent
Monolithic/poured-in-place
  • Durable
  • Suitable for above- or below-ground installations
  • Neutralizes acid rain
  • Can be incorporated into the foundation of a building.
  • Potential to crack and leak
  • Permanent
  • Need drainage in clay soil
Stone, concrete block
  • Durable
  • Keeps water cool in hot weather
  • Difficult to maintain and sanitize
  • Permanent
  • Expensive to build
  • Could harbor biofilms
Wood
Pine, redwood, cedar, cypress
  • Attractive
  • Durable
  • Can be disassembled
  • Available in variety of sizes
  • Expensive
  • Require skilled technician to build
  • Not for use in hot, dry conditions
  • Can be leaky
  • Only above-ground use
  • Not as easily sanitized

Design considerations for above and below-ground tanks are summarized below.

  • Aboveground tanks:
    • Outdoor tanks should be opaque to prevent algae growth
    • Use material that is resistant to degradation (UV, corrosion)
    • To avoid freeze damage:
      • Outdoor pipes must be drained seasonally and/or insulated;
      • All gutter systems should freely drain (i.e. the use of wet conveyance is highly discouraged
      • First flush systems should not be used
    • Provide adequate ventilation
    • Provide adequate maintenance access
    • Assess the need for a foundation based on the weight of the tank when full of water. The Minnesota State Building Code requires foundations not designed by a structural engineer to have a minimum footing depth of 3 feet in the Twin Cities.
    • Locate foundations away from natural drainage pathways
    • Situate smaller tanks without concrete foundations on a compacted subgrade of granular material such as aggregate
    • Minnesota State Building Code requires storage covers be able to withstand a snow load of 50 pounds per square foot.
    • Create accessible locations for valves and other maintenance devices
    • See and follow all tank installation guidelines. Most manufacturers of commercial tanks can provide stamped foundation plans for review by the engineer of record.
  • Below-ground storage tanks
    • Polypropylene, fiberglass, and concrete are the materials commonly used for below-ground storage tanks. Concrete may need to be used if the tank has to be buried deeper or if the tank is to store a large volume.
    • Tanks should not be buried below the water table unless an adequate foundation drain has been designed to convey the water away from the tank excavation.
    • Tanks that will be used year-round should be located below the frost line and/or be insulated
    • Maintain appropriate distance from underground utilities
    • Tank footing must comply with Minnesota State Building Code
    • The load bearing capacity of the tank must take into account above ground loads
    • Air ventilation, overflow piping, and clean-out ports must be provided for safety and maintenance
    • Assess the need for a foundation based on the weight of the tank when full of water. The Minnesota State Building Code requires foundations not designed by a structural engineer to have a minimum footing depth of 3 feet in the Twin Cities.
    • Avoid underground storage in areas that have highly expansive types of clay due to the potential for damage caused by swelling of the clay. If tanks must be installed in areas having expansive clay, utilize specific installation specifications such as a foundation drain in the base of any subsurface tank.
    • Review utility plans to avoid conflicts and/or the need to relocate utilities
    • Avoid locating the tank where traffic or other heavy loads can cross above the tank. Heavy loads may require the load bearing capacity of the tank to be increased.

For additional information, see Section 2.3 of the Ontario Guidelines and Procedure Tool I.2 from the Metropolitan Council Stormwater Reuse Guide. Section 2.4 Of the Ontario Guidelines for Residential Rainwater Harvesting provides a detailed summary of design and installation guidelines for above- and below-ground storage tanks. The guidelines can be summarized as follows:

  1. Determine the rainwater storage tank capacity (see above)
  2. Determine the type of material utilized for the rainwater tank (see above)
  3. Determine the location of the rainwater storage tank (see above)
  4. Provide tank frost protection
    1. Winterize by providing a heating system to maintain air temperature, a heating system directly inside the tank, or by insulating the tank.
    2. Drain the tank prior to the onset of freezing temperatures and ensure no water will enter the tank during cold weather months.
  5. Provide a minimum access opening of 18 inches unless local codes and standards require otherwise. Ensure the openings have drip-proof, non-corrosive covers that are lockable.
  6. Below-ground tanks should be vented from the top of the tank to a minimum of 6 inches above grade. The vent pipe should be a minimum 3 inch diameter (based on inlet size) and terminate in a gooseneck fitting with a screen to prevent entry of birds, rodents, and insects. Note: we recommend using a stainless steel screen and 2 inch diameter vent pipe.
  7. Indoor tanks must be properly vented to the outside of the building.

The Metropolitan Council Reuse Guide includes these additional design considerations.

  • For small site storage, consider pre-assembled stormwater / rainwater collection storage systems that include pumps.
  • Recommended detention times are 50 days at an average daily temperature of 59oF, 30 days at 68oF, and 20 days at 77oF.
  • Consider using the storage tank as a sedimentation basin to provide additional treatment . Locate the drawdown valve a minimum of 1 foot above the bottom of the tank to allow for sediment settling. Provide access for cleaning and debris removal.
  • Seal unnecessary tank openings with a gasket and bolts.
  • Protect the inlet of the tank from mosquitoes and other insects by using a mesh with spacing no more than 1/16 inches wide.
  • Consider energy dissipation, such as internal baffles or calming inlet, to prevent sediment re-suspension.
  • Provide oxygenation/aeration to the storage system to prevent anaerobic conditions.
  • Design an overflow system for when the tank reaches capacity; the overflow pipe diameter should be the same as the inlet pipe diameter and should be directed away from the foundation and towards a surface that will not erode.
  • Create operation and maintenance accessibility, including a manhole for larger tanks or access for a hose/siphoning equipment for smaller tanks.
  • Provide back-flow prevention valves if system has cross-connection with potable water supply.
  • Consult with Minnesota Plumbing Code for other requirements.
  • Submit plans to the Department of Labor and Industry, or designated municipality, for compliance with Minnesota Plumbing Code.
  • Develop an operations and maintenance plan that includes
    • an inspection schedule;
    • a cleaning schedule;
    • a winterization plan and schedule;
    • a pump maintenance schedule; and
    • a site plan.

See the section on operation and maintenance for more detailed guidance on developing an operations and maintenance plan.

Stormwater ponds

Stormwater ponds can be used as the storage component of a stormwater harvest and use system. These ponds are multi-purpose, providing stormwater retention, sedimentation, and storage for later use. In this way, stormwater harvest and use systems can be part of a treatment train approach for stormwater management. Existing ponds can be retrofitted to serve as a water source for a harvest and use system.

Design considerations:

  • Adequate sediment storage must be provided to preserve reservoir capacity for intended use(s)
  • Pond must be properly designed. This includes consideration of local codes and watershed district rules, water quality targets for both the intended use, and water quality goals for water captured by the pond but not used for the intended use (e.g. water discharged to a surface water body via the storm sewer system).
  • Is lining the pond with topsoil or clay necessary to hold water for use or prevent infiltration in Groundwater Protection Areas? If soil infiltration rates are high in the underlying soils, have infiltration BMPs been considered for stormwater management?
  • What is the depth of the pond at natural water level (NWL) and after drawdown for irrigation?
  • Does a drawdown limit need to be set with a flow to maintain sufficient water levels for pond aesthetics? Can a buffer of tall, native vegetation be used to improve aesthetics during drawdown?
  • Will there be limitations to vegetation established due to water level bounce and drawdown?
  • Consider the potential for erosion from inlets under low pond water levels.
  • To discourage the growth of algae and other microorganisms, ponds should be sized such that detention times are not excessive during warm weather. As temperatures increase, the recommended maximum detention time decreases (Met Council, 2011). The following table is from the Met Council Reuse Guide Storage Systems Toolbox I.2, originally adapted from New South Wales Department of Environment and Conservation, Managing Urban Stormwater, Harvesting and Reuse, April 2006:

Maximum Detention Time - Average Daily Temperature
Link to this table

Maximum Detention Time (days) to limit algae blooms: Average Daily Temperature (F)
50 59
30 68
20 77

Design resources

Footings and buoyancy calculations

Footing (above and below ground tanks) and buoyancy (partly submerged below ground tanks) calculations are required for safe design of stormwater harvesting systems that use tanks. All below ground tank companies will provide information on buoyancy. The designer must consult appropriate standards in designing footings and determining tank buoyancy. The civil engineers will confirm that the weight of the specified backfill counteracts the buoyant force of the tank. Simple tank buoyancy calculations can be found in the literature (US EPA; FEMA (example C7); Oregon DEQ; ExcelCalcs; ARCSA/ASPE 63).

Overflows and bypass

All harvest and use systems have a limited capacity to convey and store harvested stormwater. For this reason, stormwater harvesting systems must include a bypass valve or overflow structure to safely convey excess stormwater to downstream stormwater flow paths and networks when runoff exceeds design capacity.

Design considerations

  • Overflow should be directed to pervious areas, a storm sewer system (which may include traditional stormwater treatment practices such as bioretention), or an infiltration system (including soakaway pits). Advantages and disadvantages of these alternatives are summarized in the following table (Source: Ontario Guidelines for Residential Rainwater Harvesting Systems Handbook).
  • Adequate erosion control should be provided for surface overflow pathways.
  • Storage tank outflow capacity should meet or exceed inflow capacity.
  • The location of storage tank outlet/overflows should be easily accessible.
  • Overflow should not pose a risk of cross-contamination with drinking water supplies and may require backflow prevention.
  • If the bypass of the system runs to daylight, provide a backflow prevention valve to keep rodents or other animals out of the tank.

Rainwater harvesting overflow discharge location - methods
Link to this table

Overflow Discharge Locations / Methods Advantages Disadvantages
Discharge to grade via gravity flow (most recommended
  • Simplest method to design, install and operate
  • Low probability of rainwater backing up the overflow drainage piping
  • If discharge location not preparedd properly, may cause soil erosion at site
  • May pose a nuisance/safety issue if discharging large volumes from big catchment surfaces.
  • Overflow drainage piping may freeze if large sections are above the frost penetration depth; ice may build up at the point of discharge if not designed properly.
Discharge to storm sewer via gravity flow
  • Ideal for below-ground tanks as storm-sewers are also located below grade.
  • Storm sewers are specifically designed to collect roof runoff and direct it to an apprpriate location off-site
  • Design must prevent backflow from storm sewer into rainwater tank.
  • Stormwater discharges can have negative environmental impacts on recieving water bodies
Discharge to soakaway pit via gravity flow
  • Permits the handling of stormwater on-site, which contributes to maintaining pre-development drainage regimes.
  • Environmantal benefits of groundwater discharge
  • In newer housing developments, an infiltration trench, serving multiple lots, may be built by the developer
  • Soakaway pits require extensive site work to design and install (high in cost)
  • Large rainfall events can exceed the infiltration capacity of the soil, requiring a separate overflow from the soakaway pit.
  • Suitable only for permeable soils

Design resources

Collection

In a stormwater harvest and use system, flowing stormwater must be intercepted and conveyed to storage through a collection system: gutters, downspouts, pipes, drains, channels, or swales. The character of the collection system is determined in large part by the source area - rooftop runoff is typically conveyed via roof drains, gutters and downspouts which may be internal or external to the building structure; ground surface runoff is typically conveyed via pipes and channels. Collection system design is also influenced by the size, type, and location of the storage unit – pipes to underground storage tanks must be water tight to prevent leakage that results in saturation of soils and damage to building foundations. Site topography also affects the storage system. The elevation drops associated with the various components of a stormwater harvesting system and the resulting invert elevations should be considered early in the design, in order to ensure that the stormwater harvesting system is feasible for the particular site. The collection area may also affect treatment requirements if there are locations in the treatment area that will contribute larger quantities of pollutants. The collection system may include multiple treatment practices.

For the harvesting system to perform as intended, the sizing of conveyances must take into account both the volume and flow rate of intercepted stormwater. Intense rain storms may cause damage or pose a safety risk if conveyances and contingency overflows are undersized or otherwise deficient. Designers must follow appropriate design protocols and standards of practice in the sizing and design of all conveyances and fittings. Guidance on sizing and design of conveyances and fittings can be found here.

Ground surface collection

The design of ground surface conveyance systems should follow established stormwater management design standards and protocols. Water harvested from ground surface source areas is typically stored in stormwater BMPs (stormwater ponds) or underground tanks rather than aboveground tanks. The conveyance of stormwater to the storage system is via standard stormwater practices (stormsewers, ditches, and swales) and treatment is provided by the conveyance system (e.g. swales) or storage system (e.g. constructed stormwater ponds).

Design considerations:

  • Catchment peak runoff rate (catchment area, slope, runoff coefficients)
  • Local hydrology – number and size of high-intensity storms
  • Conveyance to tank – wet or dry conveyance, conveyance slope
  • Debris filtration and removal – Need for first-flush diverters, in-line debris removal, or filtration strategies. Debris filters should be installed upstream of first flush diversion
  • Position of tank relative to other components - piped inflow to tanks should be calmed to minimized water agitation
  • Relative size of inflow and outflow pipes – overflow pipe capacity must be greater than or equal to inflow capacity

Design resources:

Rooftop runoff collection

  • System components: A typical rooftop conveyance system uses gutters and/or scupper and downspouts to convey stormwater from rooftops to the harvest system storage unit. Gutters, which are mounted to the eaves of a building, are typically used for a pitched roof since water can be easily conveyed directly from the roof to the gutter trough. Gutters must be hardy enough to withstand the weight of water delivered from rooftop and must have adequate flow capacity to capture runoff generated during typical rainstorms or system design storms. Scuppers, which are used to drain flat roofs, may be used in conjunction with gutters or may drain directly to external or internal downspouts. The appropriate size of gutters and downspouts depends on the roof surface area, slope, and configuration; design rainfall intensity; the number of downspouts; and the number of type of in-line treatment components that will be incorporated. Gutters and downspouts can be sized using manufacturer’s sizing chart for a given set of sizing criteria. Sizing criteria should be chosen based on local codes and regulation, local rainfall characteristics, and storage capacity. For roofs that experience problems with ice dams, additional considerations may be necessary, such as using heated gutter guards.
  • Collection system materials: Common materials for gutters and downspouts include PVC, vinyl, aluminum, and galvanized steel (Lawrence et al., 2009). Designers should consult local building and plumbing codes and health codes in choosing conveyance materials. Some materials, such as those containing copper and lead, should not be used.
  • In-line treatment components: Depending on the roof type and the intended use application, treatment components may be included in-line with rooftop runoff conveyance. Debris or leaf screens are commonly integrated in gutter systems. First flush diverters and filtration units can be incorporated along downspouts upstream of the storage unit and should be able to be operated year round. Vortex filters are typically used when multiple downspouts are joined above or below grade. Rooftop stormwater treatment should be designed in conjunction with the collection and in tank smoothing inlet and overflow siphon system.

Roofs - design considerations

  • Consider siphonic roof drainage to reduce conveyance pipe costs and when elevation issues are present. Applicable rules include Minnesota State Statute 4715.2790- Siphonic Roof Drainage System. The Minnesota Department of Labor has published a design checklist for siphonic roof drainage systems based on this statute.
  • Non-porous roofing materials, such as metal roofs, will provide a greater runoff yield for harvest than porous material such as wood shingles.
  • Some degree of pitch is advantageous for rooftop collection. Runoff flows more easily across a pitched roof. Flat roofs may require roof drains. Flat roofs may also be more susceptible to build up of organic debris and dirt than pitched roofs.
  • Roof pitch influences design flow rates of gutters and downspouts, with roofs having a greater pitch delivering water faster to the gutter system.
  • Certain roof materials impact water quality adversely and are not suitable for harvesting. These types include asphalt or asbestos shingles, wood shingles, roofs containing copper, lead or other toxic metals, and other roofing materials which contain chemicals that pose health risks (see the section on WQ Considerations). Other examples to consider include roofs that have cooling units on them-units that leak or discharge chemicals as they are operating or roofs that function as a programmed space such as a rooftop patio with a grill.
  • If the harvesting and use system is required to follow Minnesota Plumbing Code, only roof surfaces without a prohibited discharge are acceptable for collection (Section 1702.9.3). Prohibited discharges include: overflows and bleed-off pipes from roof-mounted equipment and appliances, condensate, and other waste disposal (Section 1702.9.3.1).
  • Green roofs will have a lower yield of runoff and, if soil based, may produce discolored runoff suitable for irrigation only (Lawrence et al., 2009). Green roof runoff may also have elevated concentrations of phosphorus and nitrogen or discolor water so that a finer micron filter will be required for disinfection.
  • Consider the potential for leaf litter and debris to collect on the roof and select appropriate in-line treatment devices that are compatible with the conveyance system (see Pre-storage treatment section).
  • Roof catchment characteristics are described in the Virginia Rainwater Harvesting Manual, 2nd Edition: Design Guide, pp 22 – 25, ‘Roof’

Gutters and downspouts - design considerations

  • Gutters and downspout sizing
    • Rainfall intensity must be considered in the sizing of gutters, scuppers, and downspouts. Design guides generally recommend using a rainfall intensity associated with a 10-year or greater return period for a short duration storm (1 hour or less). Local rainfall characteristics should be taken into account in this consideration. Gutters can be sized using gutter sizing tables. Sizing information is also available through manufacturers.
    • Example gutter and downspout sizing
      • Step 1. Identify the rainfall design criteria – rainfall return period and storm duration - to be used for gutter sizing. In general, for a given return period, higher rainfall intensity is experienced during storms of short duration. For siphonic roof drainage systems, per Minnesota statute 4715.279, the minimum pipe size must accommodate a rainfall rate of 4 inches per hour. For gutters and downspouts, the appropriate design criteria may vary depending on the application and setting. Rainfall depths for various return periods and durations for a given location in Minnesota can be retrieved from NOAA’s National Weather Service NOAA Atlas 14 Pont Precipitation Frequency Estimates. The average rainfall intensity (in/min or in/hr) can be calculated by dividing the design storm rainfall depth (in) by the storm duration (min, hr).
      • Step 2. Calculate the roof catchment area (Area = Length x Width).
      • Step 3. Use standard hydrologic and hydraulic methods and published sizing tables to choose appropriate size/geometry of gutters based on rainfall intensity and gutter slope. Gutter sizing tables can be found through manufacturers or gutter sizing design guides (see design resources for examples).
      • Step 4. To ensure that rainfall will not backup in the system, calculate the pipe friction loss using standard hydraulic methods (equivalent length method, Hazen-Williams Equation; see Section 8.10.4 of MnDOT Drainage Manual)
  • Design considerations
    • To prevent flooding, downspout flow capacity must meet or exceed gutter capacity. Large roofs may require multiple downspouts to convey water safely to the storage unit.
    • Friction loss in pipes should be accounted for to prevent water from backing up in pipes during high intensity rainfall events.
    • Some materials, such as those containing copper or lead, are not suitable and should be avoided in choosing gutters, downspouts, and pipes.
    • Material properties such as UV-resistance, temperature/corrosion tolerance, or flexibility/rigidity may be important considerations in material choice.
    • Pre-storage treatment systems should be designed in conjunction with the conveyance systems since they are typically in-line with the conveyance system.
    • Horizontal pipes should have a minimum slope of 1 percent and pipe capacity must meet or exceed upstream components. To achieve greater capacity downstream, the slope can be increased at the downstream end.
    • Half-round and trapezoidal gutters are favored over square gutters because they drain a greater roof area with the same amount of material used to make the gutter.
    • Siphonic roof drains generally require fewer and smaller downpipes and less underground piping compared to conventional roof drain systems. See page 27 of the Virginia Rainwater Harvesting Manual. These roof drains are approved in Minnesota Plumbing Code and function on the principal of full pipe flow. They are ideally suited for flat roof buildings that incorporate rainwater harvesting as the first stormwater BMP in a building. Local examples include IKEA and Target.
    • Cold weather maintenance may be required for gutters and downspouts. Maintenance includes cleaning out all debris, removing ice/snow dams, inspecting seams and anchors to ensure there are no leaks, checking for and if necessary repairing any structural damage, and ensuring that downspouts and diverters are functioning properly. Gutters that experience ice dam problems may be outfitted with heat tape or other components designed to capture and melt a portion of snowfall.
  • Design resources for siphonic roof drain systems
  • Design resources for Sizing gutters and downspouts

Implement cooperative goals (education and outreach, etc.)

While designing the controls system, specify if the system is to serve a public education objective. Controls packages can be delivered to tie to the internet and show the system operation in real time

Treatment

Stormwater harvesting and use systems will typically contain one or more water quality treatment components to protect equipment, meet stormwater treatment objectives, minimize risk of exposure to stormwater pollutants, or meet end use water quality criteria. Terminology on treatment can be confusing and there is no consistent terminology in the literature. The following discussion is based on the location where treatment occurs relative to storage.

  • Pre-storage treatment occurs prior to water being delivered to the storage unit. Treatment is designed to remove trash, gross solids, and particulate matter. Treatment typically occurs with practices described in the pretreatment section of this manual.
  • In-storage treatment typically consists of sedimentation practices and is most applicable to constructed ponds and wetlands, although some settling occurs in tanks.
  • In-storage treatment in tanks may also occur by introducing oxygen to the bottom of the tank with the use of a smoothing inlet and the overflow and evacuation of pollens and floating debris with an overflow siphon.
  • Post-storage refers to biological or chemical treatment as well as advanced filtration or disinfection practices located downstream of the primary storage component.

The type and number of treatment practices included in the system will depend both on the end use and the quality of runoff entering the system. The quality of harvested stormwater prior to any treatment is influenced by many factors including the catchment surface, surrounding land use, and drainage area activities (e.g. amount of road salt applied, presence of vehicle fueling areas, etc.). Information on typical pollutants found in stormwater can be found here. The following table summarizes typical pollutant concentrations in stormwater from different land uses. Harvesting stormwater from confirmed stormwater hotspots is not recommended.

In addition to source area considerations, harvesting and storing stormwater in a storage pond or basin may attract unwanted organisms which can degrade the quality of harvested water or the surrounding environment. Harvesting systems should be designed to minimize the potential for water quality to degrade during collection and storage. Some additional BMPs and maintenance guidelines for minimizing the risk of water quality degradation in harvesting systems are summarized in Table 3.4 (page 19) of the Ontario Residential Rainwater Harvesting Guidelines.

The Toolbox R.4: Treatment in the 2011 Metropolitan Council Stormwater Reuse Guide includes a comprehensive summary of treatment practices including target pollutants, treatment alternatives, pros and cons of treatment options, and considerations for the design, operation, and maintenance of treatment systems. The Minnesota Stormwater Manual contains a discussion of the applicability of several traditional stormwater BMPs and includes design information for these BMPs. Additional resources may need to be consulted for the proper design and sizing of treatment components not covered in the Minnesota Stormwater Manual. Source water quality, environmental concerns in harvesting and use systems, and operation and maintenance considerations are discussed in greater detail in the Water Quality Concerns, Environmental Concerns, and [Operation and Maintenance] sections, respectively.

Generalized steps in the design of water quality treatment systems for stormwater harvest and use (Ontario guide) include the following.

  1. Determine the quality of harvested stormwater.
  2. Determine the level of treatment required to meet end use water quality criteria.
  3. Select treatment components based on level of treatment needed and the harvest and use system design. Determine if there are constraints imposed by the designed system and ensure these constraints can be managed. Ensure there is maintenance access for water quality treatment components.
  4. Determine peak inflow and peak demand to size treatment components.

Pre-storage treatment

Pre-storage treatment practices are used upstream of the storage unit. These practices reduce particulate and particulate-bound pollutant loads, and remove gross solids from stormwater. By reducing particulate and gross solids loads, these practices also preserve the function and extend the maintenance life of downstream components in the system (tank, water treatment, distribution). For some outdoor use applications, pre-storage treatment will be sufficient to meet water quality regulations (Metropolitan Council, 2011). Treatment may be achieved using a single practice or more than one practice in series. Common treatment practices used in stormwater harvest and use systems are described in the table below.

Common pre-storage practices used in stormwater harvesting and use systems
Link to this table

Practice Description
Swales
Photo of vegetated swale city of Wayzata
A swale is a wide, shallow, vegetated depression in the ground designed to channel drainage of water. Swales can reduce the velocity of flowing stormwater which allows larger particle to drop out of suspension in the water column. Swales are considered permanent primary treatment practices. Information on design, construction, maintenance, and performance assessment can be found here.
BMPs with an underdrain
schematic showing biofiltration system.
BMPs having an underdrain are designed to filter water prior to being discharged to the underdrain. These BMPs include bioretention, permeable pavement, sand filters, and tree trenches/boxes. Most of the water entering these BMPs is passed to the underdrain. In a harvest and use system, water passing through the underdrain must be discharged back to the harvest and use system. As water passes through the filter media, particulates are trapped and nutrients are removed via plant uptake. Note that media mixes with high organic matter may contribute nutrients and impart a dark color to the discharge water. Information on design, construction, maintenance, and performance assessment can be found in the appropriate section of this manual.
Manufactured screens and filters
Filtration practices make use of porous media, mesh, or screens to trap pollutant as water flows through the filter. Filtration practices utilize sand filters or bio-filters, and include a number of proprietary devices. These devices are typically classified as pretreatment screen practices. Information on design, construction, maintenance, and performance assessment can be found here.
Debris Screens
Screens prevent large debris (gross solids) and animals from entering the collection system and contaminating harvested water. These devices are typically classified as pretreatment screen practices. They have limited utility in removing suspended solids and associated pollutants and additional treatment is needed prior to discharge to the storage unit. These devices may be prone to freezing over with ice in the winter.
First Flush Device First flush devices reduce pollutant loads to the storage unit by diverting initial stormwater flows to other drainage networks. A majority of pollutants in urban runoff are carried in the initial runoff from a site (MPCA Stormwater Manual). Since these devices do not treat stormwater but instead divert untreated water away from the harvest and use system, they are technically not treatment practices. Special care must be taken to avoid problems with freezing, such as diverting water below the frost line and ensuring the system is drained and shut down in the winter.
Gutter Guards/Leaf Screens Gutter guards and leaf screens prevent organic and other large debris (gross solids) or animals from entering roof gutters. This both prevents clogging of gutter and minimizes stormwater contact with potential sources of pollution. These devices are typically classified as pretreatment screen practices. They have limited utility in removing suspended solids and associated pollutants and additional treatment is needed prior to discharge to the storage unit.
Separators
Separators are flow-through structures with settling chamber or sediment traps that remove particulates and gross solids from stormwater. Some devices also remove floatable or include grease traps. These devices are typically classified as pretreatment settling practices. Information on design, construction, maintenance, and performance assessment can be found here.
Settling
Settling.jpg
Settling is the simple practice of slowing the flow rate of water so that particulates will fall out of the water column.

Settling practices include pretreatment settling practices such as settling basins, catch basins with sumps, and settling chambers. Information on design, construction, maintenance, and performance assessment of pretreatment practices can be found here.

Design considerations

  • Pre-storage treatment units should be properly designed, including sizing, to accept the peak inflow that is anticipated for design storms. Guidance on design and sizing can be found in the appropriate sections of this manual (see table above). If treatment is undersized, additional maintenance may be required to clear accumulated sediment from the storage unit periodically. It is recommended that particles larger than 0.4 millimeters be filtered out before entering tank storages (VA DEQ, 2009).
  • Adequate maintenance access must be provided for all treatment components.
  • For rooftop runoff, the Virginia rainwater manual recommends diverting the first 1 millimeter (25 gallons per 1000 square feet) of runoff. Fixed volume diverters capture rainfall until a holding chamber is full, at which time rainwater is then diverted into the tank. These are not recommended in a freezing climate. The chamber periodically must be cleaned to rid the unit of debris. First flush filters divert rainwater until a stainless steel mesh is wet. Some first flush diverters may be fitted with an optional spray nozzle to clean the filter, which can be problematic in a freezing climate.
  • For some diverters, the quantity of water diverted is proportional to the intensity of rainfall and may more accurately match the true first flush volume (VA DEQ, 2009). Note that multiple treatment practices used in series (e.g. a separator and constructed pond used in series) may also produce high quality water, although increased maintenance of the treatment practices may be needed. Water diverted with a first flush diverter is untreated and should be discharged to a pervious surface or to other BMPs in the storm sewer system. Diverted water can be combined with the tank overflow at a Y to have one pipe exiting the tank location.
  • Flat roofs may present a greater challenge for debris exclusion if water drains through a parapet wall because large debris can block openings in the wall.
  • Filtration units located outdoors should be disconnected or decommissioned during winter months to prevent freeze/thaw damage unless they are designed otherwise.

Design resources

Post-storage treatment

Post-storage treatment is used to remove fine particulates (clay), dissolved pollutants (nutrient, metals, organics), and microorganisms (algae, bacteria, viruses) from harvested stormwater. This additional treatment is needed to comply with plumbing codes for all indoor uses and to comply with health criteria for most indoor uses and some outdoor uses. Common post-storage treatment practices used in stormwater harvest and use systems are listed in the table below. A schematic of treatment sequencing is provided in Figure R.4a: Stormwater Treatment Processes for Multiple End Users in the 2011 Met Council Reuse guide.

Design considerations:

  • Point of use health and safety criteria is the primary consideration in the selection of treatment components.
  • Treatment units should be sized to serve peak use demand.
  • Sizing of treatment components should take into account removal efficiencies of pre-storage and storage components. Provide adequate filtration or sedimentation if not provided upstream of or in the storage component.
  • Treatment components must be accessible for inspection and maintenance.
  • The location of electricity sources should be considered in siting secondary treatment components (site electrical components near source).
  • Interior piping must be color coded in compliance with Minnesota plumbing codes. (See Section 601.2 - Minnesota Plumbing Code)
  • The system must provide valves and backflow prevention in compliance with Minnesota plumbing codes.
  • Harvested water may require pH treatment to prevent corrosion if it is moved through metal pipe. The pH should be tested at point of use (VA DEQ, 2009). Alternatively, test the pH and design the treatment system around the baseline water quality.

Summary of treatment practices used in stormwater harvest and use systems
Link to this table

Treatment Method Primary Purpose Location in System
Pre-storage and in-storage treatment
Filtration/Biofiltration Reduce particulates and nutrients Before storage
Grassed Swale Reduce particulates Before sedimentation
Leaf Screens and Strainers1 Remove organic solids Gutters and downspouts (before sedimentation)
Vortex Filters Remove organic and gross solids, particulates and debris down to 280 microns. Before storage and connected to smoothing inlet in tank.
Screens Reduce gross solids Before sedimentation
Settling Basin/Sedimentation1 Remove particulates Before or during storage
Smoothing Inlet Prevents the resuspension of settled solids and introduces oxygen to the tank to enhance aerobic conditions In tank
Floating Filter Filter particulates before water enters a submersible pump (protects pump) In tank
Skimming Overflow Siphon and Trap Remove floating debris, (e.g. pollen) and creates a water trap to prevent animals and downstream odors from entering the tank. In tank
Aeration Unit Used to introduce oxygen to tank which aids in the formation of aerobic beneficial bacteria that have been shown to reduce nutrient and metals concentrations. In tank or can be set up in a venture injection application.
Post-storage treatment
Activated Charcoal1 Remove chlorine, reduce odor, remove organics After sediment filtration, before use and before or after UV filter
Boiling/distilling1 Kill microorganisms Before use
Chemical treatments (chlorine) 1, a Kill microorganisms During storage or at distribution. Most commonly injected post filtration at point of distribution or in day tank. Should not be required in tank if adequate prefiltration is implemented
Electrodialysis2 Remove dissolved ions (salts) Between storage and end use

Not required in rainwater systems

In-line filters Removes sediment, reduces turbidity, increases ultraviolet treatment Between storage and end use and prior to UV treatment. Recommend 1 micron to meet current indoor water quality criteria of less than 1 ntu or sample the water and send to the UV manufacturer.
Microfiltration2 Remove suspended solids and microorganisms Between storage and end use
Ultrafiltration Remove suspended solids,organics Between storage and end use
Nano-filtration2 Remove multivalent ions, organics microorganisms, viruses and proteins Between storage and end use
Reverse Osmosis2 Remove monovalent ions, organics, microorganisms and viruses. May remove beneficial nutrients and produce highly corrosive water that needs to be reconditioned. Between storage and end use
Sand filtration Between storage and end use
Ozonizationb Inactivate microorganisms Before use typically applied through a venture and injected into the tank or a smaller contact tank.
Ultraviolet Disinfectionb Inactivate microorganisms Before use

1Georgia DCA, 2009
2Met Council, 2011
aBefore activated charcoal (if included)
bAfter activated charcoal (if included). Refer to specific UV and carbon filter manufacturer for guidance,br> 3 VA Rainwater Harvesting Manual, Cabell Brand center, 2009

Design resources

  • Selecting treatment
    • Toolbox I.3: Treatment Met Council Reuse Guide
    • Toolbox R.4: Treatment (Met Council Reuse Guide)
    • Section 4: Water Treatment, particularly Water Treatment Systems table on page 37 (Hawaiian Guidelines)
    • Table 3.3: Summary of post-storage treatment options (Ontario Guide)
    • Section 5.3: Water Quality Treatment for Non-potable Outdoor Use Systems (Georgia Rainwater Harvesting Guidelines)
    • Section 5.4: Water Quality Treatment for Non-potable Indoor Use Systems (Georgia Rainwater Harvesting Guidelines)
  • Guidelines for sizing and design of treatment components

Distribution

If the harvest and use system is being used for drip irrigation by gravity flow, the distribution system may simply be a length of drip tubing. However, if the system operates under pressure, there are additional components.

Some design considerations for pressurized distribution systems are similar to considerations for harvested stormwater conveyance systems (material properties, hydraulic calculations). Others, such as backflow prevention and signage, take into account the role of the distribution system as the link between harvested water and end users. The discussion in this section is limited to general considerations for pressurized distribution systems. Codes, regulations, and design standards that apply to distribution systems may vary considerably depending on the use application. Distribution systems for indoor uses, such as toilet flushing and laundry, must follow appropriate plumbing and building codes. Distribution systems for commercial or industrial uses may be subject to additional, industry specific standards. Designers should consult with a licensed plumbing professional for design of indoor and specialized distributions systems.

Design considerations:

  • Distribution systems should be designed to minimize risk of exposure.
  • Design must ensure that there is no cross-contamination of harvested water with potable/drinking water mains.
  • Separation distances of buried piping.
  • Design should minimize the risk of contamination between the last treatment process and the point of use.
  • Rainwater systems for indoor use should filter and disinfect the water prior to any cross connection with municipal water. The system pressure tank should also be placed downstream of the treatment process. If designed correctly the water quality from the system should be at or above that of the municipal system even if only utilized for currently accepted non potable applications.
  • Pipe color coding and signage must follow local codes and regulations (warning signage may be required for non-potable use applications). Pressure piping for stormwater harvest and use systems is typically colored purple for outdoor applications. Insert signage requirements for indoor use, including confined space entry and non-potable pipe labeling.
  • Distribution pipe materials should meet approved standards for the proposed use.
  • Drains and sump cleanouts should be positioned to allow the system to drain completely for maintenance or seasonal shut-down.
  • For outdoor applications, drainage should be directed towards pervious areas or normal stormwater pathways.

Pumps

Pump selection and sizing
transfer station pump detail
Eagle Valley Stormwater Transfer Pump Station detail. Image courtesy HR Green and Water in Motion. See case study associated with this image.

Harvest and use systems typically require a pump to deliver water from the storage unit to a point of use at a higher elevation. The type and size of pump used depends both on the energy required to transfer water to the point of use and the pressure required at the point of use. In most cases, a pressure pump, which can deliver low flows at high pressure, will be needed to meet end use water pressure requirements. Larger systems with back-up or secondary storage may also require transfer pumps to move water from one area of storage to another at low pressure.

Cost, system configuration, space constraints, ease of operation, and anticipated maintenance are additional factors that may influence the choice of pump type. Submersible pumps, which are placed at the bottom of water tanks, may have greater efficiency than surface pumps, which are located outside the water storage unit; however, pump maintenance may be more costly since pump units must be physically removed to perform maintenance. Provide a union disconnect (detachable joint fitting) or a rail and chain system to allow pump removal without entering tank to reduce maintenance costs. Additional advantages and disadvantages of submersible and surface pumps are summarized in Table 5.2 of the Ontario Rainwater Harvesting Guidelines.

In addition to point of use water pressure requirements, important design elements for pump systems include the following:

  • adequate backflow protection to prevent to protect cross-contamination of drinking water supply;
  • dry run protection to prevent pump failure and/or a shut off float in the pond when a certain water level is reached;
  • adequate pretreatment to prevent overburdening pump filter systems;
  • pump cooling jackets to cool pumps and maximize water use prior to dry run protection set point;
  • stainless steel pumps should be utilized in rainwater harvesting applications. Cast iron pumps should be avoided as the slightly acidic water tends to shorten the lifespan; and
  • stainless steel braiding (wrapping) to protect cables and wire from rodent damage.

Improper installation and operation of pumps can result in serious damage to pumps or catastrophic failure. Qualified professionals should be consulted in the design of pumping systems and manufacturer’s guidelines should be followed in the installation, operation and maintenance of all pumps.

Pump demand

Pump systems should supply sufficient pressure to meet peak demand rate of the end use. Recommended minimum demands from the Minnesota Plumbing Code are shown in the table below. The peak demand can be determined by multiplying the minimum demand per fixture by the number of fixtures and adding the totals for all fixtures. See Appendix A of the Minnesota Plumbing Code. The pump head, typically expressed in units of length, equals the required system pressure plus the total dynamic head. The total dynamic head equals the static lift + static height + friction loss, where static lift is the height from the water level to the pump (applicable only for jet pumps) and static height is the height from the pump to the furthest fixture. Since the head loss due to friction depends on flow rates through the system, peak demand for water should be used in pump demand estimates.

The pump head, Hpump, is given by (Ontario Guide, Chapter 5)

<math> H_{pump} = Required System Pressure Head + TDH </math>

where TDH is the total dynamic head, given by

<math> TDH = H_L + H_S + H_f </math>

where

HL is the static lift (the height that water must be lifted from the reservoir to the pump), in feet,
HS is the static head (the height that water must be lifted from the pump to the furthest fixture), in feet, and
Hf is the head loss due to friction, in feet. Head losses due to friction are dependent on the type of fluid being pump; the length of pipe and pipe material; and the fluid flow rate.

The required system pressure is dependent on the use of harvested water. System pressure requirements can be obtained through fixture manufacturers. Examples are included in the following table.

Sample of general system pressure requirements for common harvest and use applications.
Link to this table

End use Pressure (head-feet) Pressure (pounds per square inch)1 Reference
Drip system irrigation, lower range 35 15 ARCSA, Table 13.2
Drip system irrigation, higher range 58 25 ARCSA, Table 13.2
Garden hoze nozzle irrigation 81 35 ARCSA, Table 13.2
Pressure washer 1300-3300 Greenworks
Toilets 34-46 20-80 (max) Kohler
Dishwashers 46-51 20-120 GE Appliances
1 Pressure(psi) = 0.433(Head-ft)(SG), where SG = the specific gravity of the fluid being pumped. For water at ambient temperature SG ≈ 1.0).
Estimating peak demand for irrigation

Irrigation rates should take into consideration plant water requirements and soil characteristics. Peak water demand for irrigation occurs under drought conditions when plant water demand might be supplied primarily through irrigation. Under certain conditions, typically on coarse textured (A) soils, supplemental irrigation may occur throughout the season for the purpose of infiltrating stormwater rather than just for plant demand. Peak demand can be estimated by multiplying the maximum recommended irrigation rate by plant and soil type by the area to be irrigated. Guidance on recommended irrigation rates can be obtained through university agriculture and extension services (University of MN Extension Services; also see [1]) and using non-potable water demand calculators (San Francisco Non-Potable Water Calculator).

Estimating peak demand for non-potable residential/commercial use

For indoor non-potable use, peak demand can be estimated using recommended minimum water demand for common use applications from the Minnesota plumbing code (Table 2). The peak demand can be determined by multiplying the minimum demand per fixture by the number of fixtures and adding the totals for all fixtures. Appendix A of the Minnesota Plumbing Code includes a sample calculation of peak water demand. Sample calculations are also included in Example 1 below.

Water supply fixture units (WSFU) for typical non-potable applications.1
Link to this table

Appliances, appurtenances, or fixtures Minimum fixture branch pipe size (inches) Water supply fixture units (WSFU)
Private Public Assembly
Clothes washer 1/2 4.0 4.0
Dishwasher, domestic 1/2 1.5 1.5
Hose Bibb 1/2 2.5 2.5
Lawn sprinkler, each head 1/2 1.0 1.0
Urinal, 1.0 gpf2 flushometer valve 3/4 3.0 4.0 5.0
Urinal, > 1.0 gpf flushometer valve 3/4 3.0 4.0 5.0
Urinal, flush tank 1/2 4.0 5.0 6.0
Toilet, 1.6 gpf gravity tank 1/2 2.5 2.5 3.5
Toilet, 1.6 gpf Flushometer tank 1/2 2.5 2.5 3.5
Toilet, 1.6 gpf flushometer valve 1 5.0 5.0 8.0
Toilet, > 1.6 gpf gravity tank 1/2 3.0 5.5 7.0
Toilet, > 1.6 gpf flushometer valve 1 7.0 8.0 10.0

1Values excerpted from Table 1 2.1, Minnesota Plumbing Code (2015) Appendix A.
2 gpf = gallons per flush

Sample calculations
Caution: Minnesota Plumbing code states: “Rainwater catchment systems must be designed by a Minnesota Registered Professional Engineer.”

The design of pump and delivery systems (pipes, fittings, and fixtures) must be completed by an appropriately certified professional. Sample calculations included in examples 1 and 2 below have been provided for the purpose of estimating pump peak demand during feasibility or preliminary design stages. These examples include simplifications which may not provide adequate detail for final design.

Example 1 - –Estimate of peak pump demand flushing, commercial building

Step 1. Determine the peak demand flow rate: Determine the total water supply fixture units (WSFU) to be supplied using harvested water. Find the building supply demand using demand load curves (Chart A.2.1, Chart A.2.1(1)) in Appendix A of the Minnesota Plumbing Code.
Kind of fixtures Number of fixtures Fixture unit demand Total units Building supply demand (gpm)
Toilets (public, 1.6 gpf flushometer valve) 16 5.0 80
Urinals (public, 1.0 gpf flushometer valve) 4 4.0 16
TOTAL 22 96 65
Step 2. Determine System Pressure Requirement: Identify the required/recommended system pressure for each fixture type using manufacturer’s data or professional resources. For toilet and urinal flushing with flushometer valve, the minimum residual pressure is 15 psi (34.6 head-ft) (Item A.3.1, Minnesota Plumbing Code, Appendix A).
Step 3. Determine the Static Lift (HL) – For a pump located level with tank HL = 0 ft
Step 4. Determine the Static Head (HS) – In this example, utilities will be located one floor above the harvest and use tank, Hs ≈15 ft (approximately 6.45 psi).
Step 5. Estimate friction loss in pipes (Hf) – The total head loss due to friction in pipes depends on the total length of pipe, including equivalent pipe length for joints and fittings, and the cross-sectional area of the pipe along each section. For the purpose of this example, the pipe system is assumed to be comprised of 6 lines of 3/4 inch (interior diameter) smooth pipe (purple colored), each with an average distance to fixture of 75 feet, a total of four, 90-degree elbow joints, and a flush valve for each fixture. Estimated equivalent pipe length for joints and fittings can be found in Table A 3.4, Minnesota Plumbing Code, Appendix A.
Equivalent pipe length for the system:
L = (6 * (75 ft + 4(2.5 ft per elbow joint))) + (22 x 1.1 ft per valve) ≈ 535 ft
D = ¾ -inch = 0.0625 ft
Estimate fluid velocity, V (ft/s) = Q (cfs)/A(ft2), where Q = fluid flow rate (cfs) and A = pipe cross-sectional area (ft2)
If the water demand for each line is ¼ the total demand from step 1, then:
Q = 65 gpm/6 = 10.8 gpm
Using Chart A 4.1 in Appendix A of the Minnesota Plumbing Code, the estimated head loss for ¾ inch smooth pipe at the demand flow rate (10.8 gpm) is approximately 22 psi/100 ft.
Hf = 22 psi/100 ft x 535 ft = 118 psi
Step 6. Calculate the Peak Pump Head:
HPump = HL + HS + Hf + System Pressure Requirement
HPump (psi) = 0 + 6.45 + 118 + 15 ≈ 140

For other non-potable uses such as vehicle washing, or cooling tower make-up, peak flow demand can be estimated based on the fixture demand rate and anticipated maximum number of simultaneous uses. Non-potable water demand calculators may be used to estimate peak demand for preliminary design purposed. Examples of daily use rates for some commercial and industrial non-potable end uses are available through the US Department of Energy.

Example 2 – Heavy Equipment Washing

Step 1. Determine required flow rate
Water use (self-service, high use) = 200-300 gal/vehicle
Vehicle washed per day (self-service) = 2 vehicles/day
Total water use ≈ 500 gal/day
Demand Rate: If the total water run time is 1 hr per vehicle, the demand rate is 250 gal/hr, or 4.2 gpm.
Step 2. Determine the System Pressure Head ≈ 1500 - 2000 psi
Step 3. Determine the Static Lift (HL) – n/a, pump located at same level as tank.
Step 4. Determine the Static Head (HS) – ≈12 ft -15ft
Step 5. Determine friction loss in pipes/fixtures (Hf) - The total head loss due to friction in pipes depends on the total length of pipe, including equivalent pipe length for joints and fittings, and the cross-sectional area of the pipe along each section. For the purpose of this example, the pipe system is assumed to be comprised 100 feet of ½ inch hydraulic hose plus required valves fittings. Using the equivalent pipe length method, at the proposed peak water demand rate (4.2 gpm), the friction losses in pipes and fitting are relatively negligible compared to the required system pressure.
Step 6. Calculate the Pump Head: HTotal ≈ System Pressure Requirement, 2000 psi
Design considerations
  • Jet pumps and booster pumps should be set up adjacent to and lower than the storage tank to ensure flooded suction and to maintain prime.

Determine if the pump system requires a pressure switch and pressure tank. The Virginia rainwater harvesting manual recommends the pressure tank size should be three times the gallons per minute of the pump for a standard single speed pump. Variable frequency drive pumps ramp up to meet varying demands and pump manufacturers should be provided TDH, GPM and PSI when sizing a variable frequency drive pump.

  • Pumps should include automated dry-run protection. Some pumps include dry run protection; however, many pumps require manual resetting. A normally open float switch provided with the pump or added to the pump assembly automatically resets when the tank refills. Float switches are inexpensive, but may not be as reliable as a transducer. Consider transducer for larger applications.
  • Pump intake elevation should be high enough to allow for sediment accumulation between tank or pond maintenance times.
  • Pump system must provide back-flow prevention
  • A floating cistern filter is provided with most rainwater pumps and pulls water from 6 to 8 inches below the surface of the water where the highest quality of water exists. The pump is stopped with the no-float at the lowest level to ensure proper pump cooling and to not suck the bottom sediment accumulated in the biofilm at the bottom of the tank.
  • Cistern float filters draw water from the upper part of the storage tank to avoid drawing water from the lower part of the tank, where sediment accumulates and pollutant concentrations may be elevated. Filters should be designed to never clog, often by using a larger pore size than was used in the pre-tank filtration. Filter fabric should also be high-quality stainless steel, like the first flush filter.
  • Pumps should be inspected regularly. Pump maintenance should be performed according to manufacturer’s directions.
  • Outdoor pumps may require winterization or seasonal shut-down. Pumps with sufficient frost protection may remain in water during winter months.
  • Consider 3 phase electrical system if available. If not, the Variable Frequency Drive (VFD) discussed above may be needed.
  • Pump impeller diameter can be critical, as too small of a impeller will not allow for high flow pumping, and too large of a impeller will not allow for low flow pumping.
Design resources

Irrigation systems

Irrigation is the most common use application of harvested stormwater (EOR, 2013). Rainwater in particular is desirable for irrigation since it is generally low in chlorides and is slightly acidic. At small scales, hose watering can be used to passively transfer water from a storage tank to garden or lawn plots. This simply requires a valve at a low tank elevation and a hose nozzle to control flow. At larger scales, hose watering becomes impractical, and irrigation systems design can become complex. Designers should consult trained professionals in designing irrigation systems. Irrigation system plans should be reviewed by a licensed plumbing professional, certified irrigation designer or professional engineer as part of the permitting process.

Design considerations
  • Irrigation systems which use automated backup supplies must have backflow protection to prevent cross-contamination of drinking water supply.
  • Design pressure of the system should be verified and pressure regulators included in the design, if needed.
  • Shut-off valves should be accessible for maintenance and seasonal shut-down.
  • Water efficient irrigation heads should be included in the design.
  • An in-line filter and/or sediment filter should be used to ensure that drainage nozzles do not become clogged.
  • Pipes and spigots must be labeled in accordance with the Minnesota Plumbing Code. The current code 4715.1910 requires non-potable piping be painted yellow and/or be tagged with metal tags (section 4715.1910).
  • Requirements for landscape designer/irrigation specialist
  • Use of an EPA approved smart controllers on irrigation systems that are designed to meet stormwater requirements.
Codes and regulations
  • Minnesota Plumbing Code (submit plans to Minnesota Dept of Labor and Industry of designated municipality)
Design resources

Makeup water supply system and backflow prevention

Most harvest and use systems will require a secondary water supply, or ‘makeup supply’ (also called backup supply) to meet demand when harvested water is not available or insufficient to meet demand (e.g. during summer). For small, decentralized irrigation systems or other outdoor uses, it may be acceptable to access the secondary supply by manually disconnecting from the harvest system storage and connecting the secondary supply. For other applications an automated makeup supply will be required. For example, a toilet flushing system cannot be turned off when harvested stormwater is unavailable. In most cases, the secondary supply will be either municipal drinking water or a private well.

The primary concern with makeup systems is that typically they require potable water to be brought into close proximity with harvested stormwater, which introduces a risk of a cross-connection between the two supplies (Despins, 2012). Cross connection of the two supplies can lead to harvested water being drawn into the potable water supply if there is a drop in pressure in the potable water distribution system. Whenever there is a risk of cross-connection between the two supplies, adequate backflow prevention must be installed to prevent contamination of the potable water supply (EPA, 2012). Designers are responsible for following applicable plumbing and electrical codes.

For automated systems, the design must also take into account stormwater management objectives and pump intake elevation. The makeup system must be triggered before [the pump head drops below operating levels], but makeup water should not displace storage volume required [for water quality or volume control objectives]. Design considerations for automated makeup supply systems are outlined in the Design Considerations for Automated Makeup Water Supply Systems table below.

Design Considerations for Automated Makeup Water Supply Systems
Link to this table

Field Observations For some practices, field observations may be required to determine the design demand. For example, a washing station may require field observation of the time spent washing each vehicle.
Air-gap An air gap and other cross-connection requirements included in relevant building codes must be followed when combining potable and non-potable waters. An air gap physically separates two sections of pipe and is open to the atmosphere. The air gap must be located higher than the overflow drainage piping from the tank and the overflow drainage piping must remain free of blockage so that excess rainwater flows to the overflow system and does not back up and overflow at the air gap. Air gaps are not generally utilized or recommended in large outdoor storage tank applications due to freeze protection and loss of volume storage due to required air space.
Minimal make-up water storage Stormwater management objectives must not be compromised if a secondary or makeup water supply is used. The design must provide adequate storage for the next design storm. Make-up systems must place a minimal amount of volume in the storage at any one time. Utilizations of an air gap in a day tank is one option to address this issue.
Level Indicators Automatic make-up is typically triggered by a float switch, pressure transducer, or level indicator. The water elevation that triggers the make-up must be high enough to avoid running the pumps dry and must be lower than the passive draw-down orifice (if applicable). Float switches are preferred for critical operations including dry run protection and as fail safe cut off when transferring water to a day tank. Level indicators can be used for informational levels and non critical devices and are prone to failure in moist environments.
Wet wells An option for larger, underground storage tanks is to have a separate wet well at a lower elevation. This eliminates the storage of make-up water in the harvest storage tank.

Key design resources

Required Storage Capacity

  1. Storage Tanks, Tank Sizing - Lawson,Sarah; and others. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition. Compiled by The Cabell Brand Center, Salem, VA.
  2. Section 25.4.8 – Section 25.4.11 - North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
  3. Section 6.3.1, Storage Volume - Department of Environment and Conservation, New South Wales. April 2006. Managing Urban Stormwater: Harvesting and Reuse. Sydney, Australia, ISBN 1 74137 875 3.

Storage Unit

  1. Toolbox 1.2 – Storage Systems - Toolbox I.1b – Byass/Overflow - Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
  2. Step 9-d. Storage container – tank composition and sizing - Table 2 – Comparison of characteristics of aboveground tanks and belowground tanks City of Bellingham, Public Works Department. 2012. Rainwater Harvesting, Guidance Toward a Sustainable Water Future, V1|3.6.2012. Bellingham, WA
  3. Chapter 2, Rainwater storage tank sizing - Appendix D, Overflow Provisions and Stormwater Management Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1
  4. Design Guide, Storage Tanks - Lawson,Sarah; and others. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition. Compiled
  5. Section 2, Water Storage - Macomber, Patricia S. 2010. Guidelines on Rainwater Catchment Systems for Hawaii, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. ISBN 1-929325-23-1
  6. Section 25.4.17 – Section 25.4.19 - North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
  7. Chapter 2, Rainwater Harvesting System Components – Storage Tanks - Texas Water Development Board. 2005. The Texas Manual on Rainwater Harvesting, 3rd Edition. Austin, TX

Collection System

  1. Appendix A, Rainwater Catchment and Conveyance Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1
  2. ‘Gutters and downspouts or roof drains’ - Lawson,Sarah; and others. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition. Compiled by The Cabell Brand Center, Salem, VA.

Treatment System

  1. Toolbox R.4 – Treatment - Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
  2. Chapter 3, Rainwater quality treatment - Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1.
  3. Section 4.4 and Section 5.3 – Section 5.4 - Georgia Department of Community Affairs (DCA). 2009. Georgia Rainwater Harvesting Guidelines. Atlanta, GA.
  4. Design Guide, First flush diversion and pre-tank filtration - Design Guide, Additional treatment - Lawson,Sarah; and others. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition. Compiled
  5. Chapter 4, Water Treatment - Macomber, Patricia S. 2010. Guidelines on Rainwater Catchment Systems for Hawaii, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. ISBN 1-929325-23-1
  6. Section 25.4.16 and Section 25.4.22 - North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
  7. Chapter 3, Water Quality and Treatment - Texas Water Development Board. 2005. The Texas Manual on Rainwater Harvesting, 3rd Edition. Austin, TX

Distribution System

  1. Toolbox I.4 - Distribution - Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
  2. Moving Water Out of the Tank - City of Bellingham, Public Works Department. 2012. Rainwater Harvesting, Guidance Toward a Sustainable Water Future, V1|3.6.2012. Bellingham, WA.
  3. Chapter 5, Pumps and Pressurized Distribution Systems - Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1
  4. Section 4.7, Pumps and Controls - Georgia Department of Community Affairs (DCA). 2009. Georgia Rainwater Harvesting Guidelines. Atlanta, GA.

Makeup Water Supply System & Backflow Prevention

  1. Chapter 4, Make-up water system and backflow prevention - Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1
  2. Section 25.4.6 – Section 25.4-7 - North Carolina Department of Environmental Quality. April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.

Design considerations for ponds used for harvest and use

image
photo of reuse pond
Photo of pond used for capturing stormwater and irrigating Eagle Valley and Prestwick Golf Club. Photo courtesy of Emmons and Olivier Resources.

Stormwater ponds can be used as the storage component of a stormwater harvest and use system. These ponds are multi-purpose, providing stormwater retention, sedimentation, and storage for later use. In this way, stormwater harvest and use systems can be part of a treatment train approach for stormwater management. Existing ponds can be retrofitted to serve as a water source for a harvest and use system.

The first question to ask before selecting a constructed stormwater pond as the BMP is whether a pond is the most appropriate BMP. If the goal is to meet a volume retention requirement and the retention requirement can be met through infiltration of stormwater, then stormwater infiltration practices should be considered. On soils conducive to infiltration and where site constraints do not exist, infiltration will typically be the most appropriate BMP. However, if site goals include other factors, such as replacing a water supply or irrigation of vegetation, harvest and use is an appropriate BMP.

schematic of pond design
Example pond design for a harvest and use/reuse system.

Ponds should be designed following guidance in pond design guidance section of this manual. However, there are or may be specific design considerations for stormwater ponds used in harvest and use/reuse systems. These design considerations are summarized below.

  • Reuse of stormwater from a pond treating runoff from potential stormwater hotspots may pose a public safety & welfare concern, as well as may be cost prohibitive to pre-treat if special filter devices are required.
  • Stormwater and rainwater harvest and use/reuse systems may require a water-supply well to supplement irrigation needs when runoff is not available. A minimum horizontal distance of 35 feet may apply.
  • Important to maintain a permanent pool depth below which no pumping occurs to prevent resuspension of sediment.
  • Multiple aquatic benches may be necessary for ponds that experience repeated bounce or drawdown due to irrigation reuse. An alternative to multiple aquatic benches would be mild side slopes of 5:1 from the bench downward to the permanent pool elevation, then grade downward as necessary. These considerations are dependent on aesthetics, adjacent land use (residential vs. commercial, etc.), and objectives for operations and maintenance.
  • Ponds that experience repeated bounce or drawdown due to irrigation reuse may create an environment for invasive vegetation species. Some of these species may include an abundance of volunteer sandbar willow and cottonwood, which may need to be removed.
  • Pump house, control panels, intake and discharge pumps, electrical controls, etc. need to be secure to prevent public access.
  • The operator will need to access the reuse system for operations & maintenance, therefore a well thought out landscape plan needs to be prepared.

Additional considerations

Below is a list of additional considerations that are not specifically addressed above.

  • Some stormwater pond owners prefer inlet pipe inverts be submerged to reduce erosion. Consideration should be given to pipe material and diameter. Reinforced concrete pipe (RCP), with tied joints may last longer than high density polyethylene pipe (HDPE) or corrugated metal pipe (CMP), which both can become buoyant when submerged, and even damaged from repetitive ice heave. In addition, the diameter of the pipe entering the pond may be oversized to account for submerged inverts, and reduced capacity. This is applicable to all constructed ponds.
  • Adequate sediment storage must be provided to preserve reservoir capacity for intended use(s)
  • Pond must be properly designed. This includes consideration of local codes and watershed district rules, water quality targets for both the intended use, and water quality goals for water captured by the pond but not used for the intended use (e.g. water discharged to a surface water body via the storm sewer system).
  • Is lining the pond with topsoil or clay necessary to hold water for use or prevent infiltration in Groundwater Protection Areas? If soil infiltration rates are high in the underlying soils, have infiltration BMPs been considered for stormwater management?
  • What is the depth of the pond at normal water level (NWL) and after drawdown for irrigation?
  • Does a drawdown limit need to be set with a flow to maintain sufficient water levels for pond aesthetics? Can a buffer of tall, native vegetation be used to improve aesthetics during drawdown?
  • Will there be limitations to vegetation established due to water level bounce and drawdown?
  • Consider the potential for erosion from inlets and sideslopes under low pond water levels.
  • To discourage the growth of algae and other microorganisms, ponds should be sized such that detention times are not excessive during warm weather. As temperatures increase, the recommended maximum detention time decreases (Met Council, 2011). This is not likely to be a concern for reuse ponds since detention times are typically short. The following table is from the Met Council Reuse Guide Storage Systems Toolbox I.2, originally adapted from New South Wales Department of Environment and Conservation, Managing Urban Stormwater, Harvesting and Reuse, April 2006:

Maximum Detention Time - Average Daily Temperature
Link to this table

Maximum Detention Time (days) to limit algae blooms: Average Daily Temperature (F)
50 59
30 68
20 77


Implementation phase

The performance of stormwater harvest and use systems depends not only on sound design, but also on appropriate installation, operation, and maintenance. Considerations for implementation phase activities are discussed briefly below.

Construction activities

See Construction Sequence. Construction oversight should include a professional familiar with installation of rainwater or stormwater harvesting systems and installation of all manufactured components should follow manufacturer’s specifications. Construction should be sequenced to ensure that stormwater is managed appropriately while the harvest and use system is off-line.

Implement operation and maintenance and monitoring plans

Depending on the context of the harvest and use application, designers may be required to submit O&M plans for approval by municipal, regional, or state agencies. This should be done prior to installation and in cooperation with parties who will hold ownership of the system during construction, site development, or post-construction. See Operation and maintenance for stormwater and rainwater harvest and use/reuse.

Implement cooperative goals (education and outreach, etc.)

Harvest and use projects may provide an opportunity to offer public education and outreach on stormwater management or integrated water management issues - particularly in commercial, municipal, or institutional settings which are generally accessible to the public. In such cases, it may be advantageous to seek local partners who can provide support for outreach goals. Partnerships should be developed early in the design process to maximize potential education and outreach opportunities. The benefits of public participation in stormwater harvest and use projects are discussed in Chapter 8, Public Outreach, Participation and Consultation of the EPA Water Reuse Guide.

References


Construction specifications for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Unlike many other green infrastructure stormwater best management practices (BMPs), typical harvesting and use systems are not as sensitive to the particular sequence of construction, as long as temporary or construction phase stormwater treatment is addressed (i.e., the storage component can serve both temporary or construction phase stormwater treatment needs as well as long-term harvesting needs). Also, installation of harvest and use systems should be coordinated with any concurrent or related construction. An example construction sequence is shown below.

Typical construction sequencing

The timing between construction steps can vary depending on the project, with months to years from the start of construction until an entire system is in place. This is often the case for stormwater pond and irrigation use systems. For example, initial pond construction and pump installation in new developments could be completed months to years prior to pump installation and final site work (landscaping and irrigation system installation).

  1. Verify that all necessary certifications, licenses, permits and approvals have been obtained, including those required by the Minnesota Plumbing Code.
  2. Schedule and attend a preconstruction meeting with all relevant parties responsible for design, permitting and construction of the proposed facilities. This is the time to make sure that all aspects of the design, schedule, submittals and permits are understood and any ambiguities resolved. Suggested attendees include, but are not limited to, engineers, code officials, system provider, plumbers, electricians and excavators.
  3. Determine how site access will be achieved, determine location of discharge and overflow routes, obtain permission for access or work as needed from adjoining property owners.
  4. Install erosion and sediment control BMPs as needed or required by construction documents and permits. Protect discharge and overflow routes from compaction and erosion. Harvesting storage structures can often be used as sediment control structures, as long as design capacity is restored at the end of the construction/stabilization phase.
  5. Install pre-storage treatment structures and systems and any appurtenant frost and buoyancy protections.
  6. Install harvesting and use storage components, including storage ponds, above and below ground tanks, in tank treatment devices and footings and buoyancy protection devices. It is recommended to install pre-storage and storage components at the same time to coordinate any elevation changes. Ideally, the installing tank company can be contracted to run conveyance piping and prefiltration as opposed to different parties.
  7. Install harvesting and use distribution components, including pipes, pumps, valves, treatment systems, and other utilities and elements indicated in design documents. The specific construction sequence of these components should be determined based on the contractor’s means and methods.
  8. Install post-storage treatment systems per manufacturer warranty specifications.
  9. Perform interim tests and inspections as required by governing agencies including pressure testing of pipe/joints and tanks if required, cross connection testing, rpz testing and start up water quality testing.
  10. Construct and stabilize discharge and overflow routes and verify that insect and rodent screens are installed on all exposed pipe and other openings and that backflow valves are installed if overflow pipes go to grade and that a trap is included on the overflow from the tank.
  11. Inspect and clean all conveyance and storage elements immediately prior to system testing. Verify that the distribution intake within storage device(s) is clear of sediment and will not entrain any sediment once flow is initiated.
  12. Perform and document all tests needed to verify that system components function as designed. Do not allow discharges to overflow routes until they are vegetated or otherwise stabilized (e.g. rip rap channel).
  13. Complete any remaining permit and approval conditions and verify warranties for all system components, and perform a final punch list review and project closeout.
  14. Assist with preparation of operation and maintenance documents and ensure that copies of these documents are in the possession of the owner/operator and the local stormwater regulatory jurisdiction.

You can access an example inspection report by clicking on the following link:

File:MPCA Stormwater Harvest and Use System Example Inspection Report.docx


Operation and maintenance for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

While many stormwater systems are designed to be relatively passive with minimal oversight needed, harvesting and use systems require managed operation where the goal is to move water from the storage unit to a point of use so that there is sufficient storage available to receive runoff from subsequent rainfall events. The timing and management of the water storage and use operation needs to be integrated into the system design. With an actively managed operating system, regular maintenance is also important to preserve the end use water quality, maintain system safety and efficiency, and minimize costs associated with repairs and downtime.

Operation and maintenance agreements

Stormwater harvesting and use systems rely heavily on being properly operated and maintained and therefore having a formal agreement in place upfront is critical. An operation and maintenance (O&M) agreement should:

  • clearly state the responsibilities of the owner in operating and maintaining the system,
  • describe the technical ability of the owner to perform the operation and maintenance,
  • verify that a third party providing O & M services is approved by the city (or other local jurisdiction),
  • identify which O&M activities need to be performed by a licensed plumber, if any,
  • address how the transfer of responsibilities will be managed should ownership change in the future, and
  • provide for an on-going Operation and Maintenance Plan as described below.

Operation and maintenance plans

In general, stormwater harvesting and use O&M plans should include the following items.

A. Site plans showing

  • Location of system components
  • Maintenance access points for all functional elements of the system
  • Location and type of all operational controls (pumps, valves, sensors, etc.)
  • Location of underground utilities


B. As-built drawings showing

  • Layout of pipes
  • Location of all controls
  • Location and type of all components that require periodic replacement (e.g. filters)

C. Operation and troubleshooting guidelines for system controls including:

  • Distribution systems (pumps, valves, sensors, etc.).
  • Treatment systems (monitoring requirements, dosages, etc.)
  • Overflow and bypass systems (valves, water level sensors, etc.).
  • Applicable regulations

D. Statements describing when a licensed/certified professional is needed for repair or maintenance of tanks, pumps, pipes, controls, or other components.

E. Description and schedule of inspection activities for all system components including

  • Spring start-up and winter decommissioning
  • Regular inspections – annual, seasonal, monthly, etc.
  • Inspection guidelines for special circumstances (very large runoff events, electrical outages, etc.)
  • Safety protocols to be followed during inspection

F. Description and schedule of maintenance activities for all system components including

  • Spring start-up and winter decommissioning
  • Regular maintenance – annual, seasonal, monthly, etc.
  • Replacement schedule for control elements of system (rain sensors, level loggers, pumps, etc.)
  • Safety protocols to be followed during maintenance

G. Manufacturer’s literature for all controls, replaceable components, and prefabricated components (pumps, sensors, treatment components, prefabricated storage units, etc.).

H. Component-specific O&M plan details

  • Collection surfaces
    • Sweeping schedule for paved surfaces
  • Collection and pre-treatment system
    • Instructions for correct operation of any manual valves
    • Location and type of filters
    • Location of debris collection sumps
  • Storage system
    • Cleaning schedule
    • Statement describing the method and frequency of water level monitoring
  • Treatment system
    • Point of use water quality criteria/standards
    • Schedule of required water quality monitoring
    • Inspection schedule for all treatment components, per manufacturer’s guidelines
    • Schedules for cleaning of filters, replacement of chemicals, etc.
    • Safety protocols including special or hazardous materials handling
    • Requirements regarding how long a storage unit should flush out in the spring before irrigation or other end uses begin so high chloride levels don’t kill vegetation or corrode metal components
  • Distribution system
    • Instructions for correct operation of any manual valves
    • Required color coding or labeling of pipes carrying harvested water
    • Backflow preventer maintenance and inspection requirements
  • Irrigation system (if applicable)
    • Controller programing guidelines
    • Irrigation map showing application rates appropriate for soils/plantings
    • Any Material Safety Data Sheets (MSDS) that are relevant for operation
    • Inspection requirements for all components of the system
    • Assumption used in sizing the system
    • Applicable laws, codes and permits

I. Monitoring Plan

  • Use rates should be monitored at least monthly for at least three years. This should be compared to the water budget analysis of the design to determine whether the modeled level of performance is being achieved.
  • Required water quality monitoring

J. Inspection forms/maintenance logs for all regularly scheduled inspection and maintenance activities.

K. Statements outlining the roles and responsibilities of all parties participating in the operation, monitoring, and maintenance of the system, including which party must follow up on items that are outside the normal operating procedures.

Operation and maintenance considerations

Operation and maintenance plans will vary depending on the configuration and components of the harvest and use system. Operation and maintenance considerations are described by system component below. Guidelines for inspection and maintenance timeframes and activities are provided in the following table: General Inspection and maintenance guidelines for stormwater harvest and use systems.

Collection surfaces

Collection surfaces should be inspected to identify sources of contamination and determine maintenance needs. Source control of pollutants and large debris at collection surfaces improves the quality of harvested water and can also extend the maintenance life of downstream components by reducing sediment and associated pollutant loads delivered to conveyances .

Rooftops
  • Rooftops should be kept free and clear of debris
    • Organic debris from trees (leaves, pollen, flowers, seeds, twigs, etc.) can degrade the quality of harvested water. Overhanging tree branches should be trimmed as needed to reduce these inputs.
    • Animal feces can degrade water quality. Any nests should be removed and measures to discourage animal activity should be implemented as possible.
  • Rooftops should be kept in good repair
    • Roof material may degrade over time due to exposure to UV light, repeated freeze-thaw cycles or accumulated storm damage. This may negatively impact the quality of harvested water (TWBD, 2010).
Green space
  • Maintain healthy vegetation and minimize application of chemicals to protect water quality
  • Monitor for land disturbance and provide erosion and sediment control
Paved surfaces
  • Residual de-icing chemicals, sand, and salt should be cleaned from paved surfaces prior to spring startup of the system
  • Pavement should be swept regularly (generally once per month)
  • Any spills on the pavement should be cleaned immediately and/or contained to prevent contamination of runoff collected in the storage unit

Collection system and pre-storage treatment components

Collection systems should be kept free of debris to prevent clogging and should be inspected to identify leaks. Clogging decreases the capture efficiency of the harvest and use system and can result in flooding or damage to collection surfaces or upstream facilities. Debris that has built up in conveyances may act as a source of pollution in harvested water, increasing the burden on downstream water treatment components to meet end use water quality criteria. Leaks in the system also decrease the capture efficiency of the system and may cause damage to foundations or structures located near the collection system.

The primary function of pre-storage treatment components is to reduce sediment and adhered pollutant loads in harvested water. Since primary treatment components are designed to collect sediment, these components must be inspected and maintained to preserve sediment storage capacity and maintain functionality of the collection system .

Collection systems are often designed in a passive form using gravity, so operational needs are limited. If collection systems include pumps, then the needs would be similar for distribution systems.

  • Rooftop collection systems
    • Gutter and downspouts can be damaged by high winds, ice dams, or intense storms. Additional inspection is recommended following very large storms or extreme conditions.
    • Leaves, twigs, and other organic debris should be removed from screens at a minimum in the spring and fall, but additional cleaning may be required where there is significant tree canopy.
  • Ground surface collection systems
    • Ground surface collection systems should be inspected annually for debris accumulation and erosion and repaired as needed.
  • Pre-storage treatment components
    • Sediment and debris accumulations in pre-treatment storage components should be monitored periodically during the first year of operation to determine the rate of accumulation and develop an appropriate sediment removal schedule.
    • Joints and fittings which connect collection and pre-storage treatment components should be inspected annually to look for loose fittings and leaks.

Storage Components

Collection, distribution, makeup supply, and overflow systems all interact with the storage unit in some capacity. For this reason, regular inspection and monitoring of the storage system is an important diagnostic tool for monitoring system function as a whole.

  • Tanks should be inspected periodically during operation to monitor for growth of microorganisms, presence of mosquitos, or formation of sludge, since these conditions can degrade water quality or may pose a health risk.
  • Sediment accumulation in tanks should be monitored monthly during the first year of operation to determine the rate of accumulation and expected cleaning frequency. If possible, design your system to prevent the tank from having to be cleaned
  • If tank entry is required for inspection or repair, this work must be done by a licensed and trained contractor or otherwise qualified professional following all safety standards and regulations. Likewise, fiberglass tanks should only be repaired by authorized personnel (ARCSA, 2015). Tank systems can be designed using union disconnects and manways to access tank components. This will reduce maintenance costs and confined space entry permits.
  • Water that is stored for extended periods of time in storage tank may acquire an odor, particularly if there is any organic matter, such as pollen, in the water. Tanks may need to be flushed periodically and disinfected to correct the problem (NCDENR, 2014). An alternative to disinfection which can kill the beneficial aerobic bacteria if present is to provide additional aeration to maintain a healthy tank system.
  • Allow the system to overflow to other best management practices during extreme events.
Underground systems

Special consideration may be needed for underground systems

  • Removal of sediment generally requires a vacuum truck.
  • Repair of underground tanks should be performed only by a qualified professional.
  • Tanks may crack if post-installation above ground loads are higher than load rating (equipment, vehicles) or if activities such as landscaping do not take into account the tank specification or limitations.

For additional information on inspection schedules and activities for underground systems see Section 3.5 of New York City’s Guidelines for the Design and Construction of Stormwater Management Systems.

Stormwater ponds
For information on the monitoring and maintenance of stormwater ponds see the MN Stormwater Manual section on operation and maintenance of stormwater ponds.

Post-storage treatment system components

Post-storage treatment systems generally include consumable components – filters, bulbs, chemicals – that must be periodically cleaned, replaced, or replenished to maintain the performance of the water treatment system. Maintenance considerations will vary depending on the number and kind of treatment components that are included in the system. Examples include cartridge filters, reverse osmosis filters, UV disinfection light bulbs, ozone disinfection, and chlorine disinfection. See Table 6.5 of the Texas Rainwater Harvesting Manual for additional information.

  • Some treatment systems may require special training or certification for operation.
  • Depending on the end use, it may be necessary to demonstrate compliance with water quality regulations or health codes through regular water quality monitoring at point of use.
  • Some systems may use materials that require special handling and storage. In these cases Material Safety and Data Sheets (MSDS) must be available onsite.
  • Any handlers of potentially hazardous material must have appropriate training (ARCSA, 2015).

Distribution components

Monitoring, maintenance, and repair of pumps, pipes, and controls may require a certified or licensed professional. The O&M plan should outline the conditions under which a licensed professional is needed.

Pumps

Most often a problem with the pump and pressurized distribution system is due to associated components (water level sensor, valves, pressure tank, makeup supply, or automatic bypass) and not the pump itself (Despins, 2012). Problems with associated components might be diagnosed by evaluating the water level in the tank and operation of these components (Is the tank empty when it shouldn’t be? Is the makeup supply on or off when it shouldn’t be?). Maintenance concerns will vary depending on the type of pump (e.g. submersible or non-submersible). Potential maintenance concerns for pumps include the following.

  • Malfunction of dry run protection (level sensor or built-in dry run protection)
  • Pump intake becoming clogged
  • Microbial growth on filters at pump intakes
  • Pressure tank not supplying sufficient pressure to keep the pump operating
  • Pressure sensor not functioning properly
  • Electrical connections
Pipes
  • Maintain required coloring and labeling of pipes per plumbing codes (or other regulations).
  • Pipes that are not fully winterized should be drained when the system is decommissioned for cold weather
  • Inspection and repair of pipes may require a licensed technician or contractor.
Makeup water supply system and backflow prevention
  • Local regulations should be consulted regarding potential cross connections when combining rainfall runoff with potable water systems. In most cases, cross connections are not allowed and an air gap must be provided between sources to limit the potential for contamination of the potable water supply.
  • Automated make-up supply - faulty float switches or solenoid valves can cause the makeup water supply to be activated or shut-off when the action is not required.

Overflow and bypass systems

Overflow must be monitored periodically after rainfall events to ensure the system is capturing events it was designed to capture. Overflows that are not functioning properly may cause erosion, flooding, or damage to control systems (makeup supply, pump controls). Specific issues include the following.

  • Clogging or damage at overflow/bypass intakes will cause water to short-circuit the system which may result in damage to nearby structures.
  • Overactive overflow/bypass systems may be an indication of maintenance needs upstream in the system, for example, a pump failure that is preventing the storage from being drawn down or storage capacity compromised by accumulated sediment

Additional information can be found in Section 6.5, Overflow provision and stormwater management, Management Guidelines, Ontario Guidelines for Residential Rainwater Harvesting Systems.

Inspection and maintenance guidelines

  • The general inspection and maintenance guidelines for stormwater harvest and use systems table below provides a summary of general inspection and maintenance guidelines.


General inspection and maintenance guidelines for stormwater harvest and use systems
Link to this table

Component Timeframe What to look for during Inspection Maintenance 1
Source Area/Collection Surface Annually Changes in land use or land disturbance Implement source control BMPs as needed to help meet pre-storage water quality targets
Pollution hot spots
Damage to roofing materials Repair as needed
Overhanging branches Trim overhanging branches
Nests or other evidence of animal activity Remove nests and implement additional measures to discourage animal activity
Monthly or as needed General condition of pavement Adjust street sweeping schedule as needed to maintain clean pavement
Collection System Spring startup and fall General condition of gutters, downspouts, and conveyances Clean accumulated debris in fall prior to winter operations or seasonal shut-down, and as needed.
Debris clogging inlets, gutters, downspouts, and other conveyances
Evidence of leaks at junctions or along conveyances
After large storms General condition of first flush and high-flow diverters (bypass system) Repair as needed
Soil erosion or flooding along diversion flow pathways Provide appropriate erosion control measures (rip rap, check dams, etc.)
Storage System Spring start up General condition of all storage system components Clean, repair and replace as needed
Position and function of valves Test per manufacturer’s guidelines
Function of operational structures and controls
Periodically following startup Tank ventilation Clean, repair and replace as needed
Excess soil moisture near tanks or other evidence of leaks Repair leaks per manufacturer’s guidance
Growth of algae or microbes Drain and clean tank per manufacturer’s guidelines
Intrusion of mosquitos or small animals Implement appropriate pest control as needed
Monthly Sediment level in tank or pond Remove sediment when the tank sediment storage volume has reached 50% of capacity
Treatment Systems Spring startup General condition and function of all treatment system components Conduct testing per manufacturer’s guidance

Clean and repair as needed

Twice per year Clogging from accumulated dirt and debris in pre-storage treatment components Clean as needed
Evidence of leaks from loose fittings, joints Repair as needed
3 times per year AND after each rain event that exceeds the design capacity of the collection system Clogging of intake and filters in first flush diverters, especially during pollen season (filter clogging) lean and replace as needed
Monthly or as required Performance of water treatment system Test water quality at point of use and at other points in the system (outlet of collection system, in-tank) as required
Adjust treatment parameters to meet any water quality deficiencies
Per manufacturer specifications and as needed Condition of replaceable components in the treatment systems (filters, cartridges, bulbs, etc.) Replace/repair per manufacture’s guidelines and as needed. Typical UV treatment is annual and filters are started on a quarterly basis or when differential pressure drops.
Distribution System Spring startup and fall General condition of all distribution system components Repair/replace as needed
Position and function of valves Test per manufacturer's guidelines
Function of operational controls
Presence of leaks (test)
Function and performance of pump Complete all startup inspection and operations per manufacturer’s guidelines
Monthly Presence of biofilms or sediment accumulation on filters Replace/disinfect as needed
Per manufacturer and as needed Function of pumps and other control equipment Test all control components per manufacturer's guidelines or as needed to diagnose problems in the system
Overflow/Bypass Systems Annual (above-ground)/ As needed (below-ground) Clogging or damage at overflow/bypass intakes Clean and repair as needed
Erosion of downstream receiving area Stabilize erosion, repair overflow system as needed, check for failures in other upstream components
Proper pump control and operation Repair and replace as needed

1See also decommissioning and winter maintenance tasks

Winter decommissioning and maintenance

Throughout Minnesota, temperatures can drop below freezing (0°C) during the winter months. If stormwater harvesting systems are not fully winterized to withstand seasonal temperature fluctuations, systems should be decommissioned before the cold weather season. Winter decommissioning also provides an opportunity to preform annual inspection and maintenance. The table below provides a summary of winter decommissioning and maintenance tasks.

Typical winter decommission and maintenance tasks

Link to this table

Component Typical decommissioning and winter maintenance tasks needed prior to spring startup 1
Source Area/Collection Surface
  • Clean residual de-icing material (sand, salt) from pavement prior to spring startup
Collection System
  • Drain and disconnect conveyances to prevent freeze/thaw damage
Storage System
  • Drain all water from above-ground outdoor storage tanks
  • Disconnect downspouts and/or pipe upstream of the tank to prevent rainwater/snowmelt from entering the tank during winter months
  • Disconnect electrical supply that controls equipment
  • Shut off makeup water supply to prevent water from entering the tank
Treatment System
  • Drain and disconnect any pre-storage treatment devices that should be decommissioned during the winter
  • Decommission treatment system components per manufacturer’s guidelines
Distribution System
  • Switch end use supply from harvested water to public water supply (if necessary)
  • Drain all water from pumps and conveyance to prevent freeze/thaw damage
  • Disconnect electrical supply to the pump and control equipment
  • Complete all recommended winter maintenance for pumps and controls
Overflow/Bypass Systems
  • Drain and disconnect to prevent freeze/thaw damage

Tracking operation and maintenance

Record keeping is part of regular operation and maintenance. Record keeping is important for

  • documenting the system history (parts replacement, maintenance activities, and other observations),
  • demonstrating compliance with regulatory codes,
  • evaluating system performance and the performance of various components, and
  • evaluating the cost of operation and maintenance, including labor and replacement parts.

Because harvesting systems can be configured differently, there is no ‘one size fits all’ form for inspections. Inspection forms and maintenance logs that are suited to the particular context should be included in the O & M plan. Example forms for spring inspection and maintenance, regular monitoring and maintenance, and inspection following storm events are included in this section.

Inspection forms should include, at a minimum, the following information:

  • Type of inspection (annual, seasonal, monthly, special conditions)
  • Date of inspection
  • Name and contact information of inspector
  • Items to be inspected
    • criteria for passing inspection
    • maintenance to be performed at the time of inspection
  • Record of inspection - pass/fail for each item on the list
  • Record of maintenance performed during inspection
  • Record of any problems noted during inspection and follow-up actions that are required


Maintenance logs should include, at a minimum, the following information:

  • List of items which require regular maintenance
    • frequency of required maintenance
    • description of maintenance to be performed
  • Record of maintenance performed
    • name and contact information of maintenance provider
    • date maintenance performed
  • Record of any problems noted during maintenance and follow-up actions that are required

It may be helpful to include additional information from the O & M plan, such as replacement part type, manufacturer, or service provider contact information, on inspection and maintenance forms. Any actions required after inspection or maintenance must be brought to the attention of the system manager or party who holds responsibility per the operation and maintenance plan.

Sample inspection & maintenance forms

Click on these to download these sample forms

File:Spring Inspection & Maintenance Form Stormwater Harvesting and Use Systems.docx

File:Treatment System Maintenance Log Stormwater Harvesting and Use Systems.docx

File:Post-Storm Inspection Form Stormwater Harvesting and Use.docx

References

  • Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1.
  • North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.

Resources for operation and maintenance

  • Metropolitan Council. Fall 2011. Stormwater Reuse Guide, prepared by Camp Dresser & McKee, Inc. and others. St. Paul, MN.
    • Toolbox I.1b - Ground Runoff Collection, Operation & Maintenance Plans
    • Toolbox I.2 – Storage Systems, Operation & Maintenance Plans
    • Toolbox I.3 – Treatment, Operation & Maintenance Plans
  • Toolbox I.4 – Distribution, Operation & Maintenance Plans
  • Despins, Christopher. September 2012. Guidelines for Residential Rainwater Harvesting Systems Handbook. Canada Mortgage and Housing Corporation (CMHC). ISBN 978-1-100-21183-1.
    • Section 1.5 - Rainwater catchment and conveyances, Management Guidelines
    • Section 2.5 - Rainwater storage and tank sizing, Management Guidelines
    • Section 3.5 - Rainwater quality and treatment, Management Guidelines
    • Section 4.5 - Makeup water system and backflow prevention, Management Guidelines
    • Section 5.5 - Pump and pressurized distribution systems, Management Guidelines
    • Section 6.5 - Overflow provision and stormwater management, Management Guidelines
  • American Rainwater Catchment Systems Association (ARCSA). 2015. Rainwater Harvesting Manual, 1st Edition. Editor: Ann Audrey.
    • Section 7-4.3 – Pipes and Fittings - Operation, maintenance and repair Considerations
    • Section 10-4 – Above Ground and Underground Storage Tanks – Overview of installation and operation
    • Section 11-4 – Small-scale Storage Tanks – Overview of installation and operation
    • Section 12-4 – Large-scale Storage Tanks – Overview of installation and operation
    • Section 13-4 – Pumps and Controls – Overview of installation and operation
    • Section 14-4 – Sanitation and Water Treatment – Overview of installation and operation
    • Section 15-4.3 – Irrigating with Rainwater – Operation, maintenance and repair Considerations
  • North Carolina Department of Environmental Quality (NC DEQ). April 2014. North Carolina Stormwater BMP Manual, Chapter 25, Rainwater Harvesting. Draft document.
    • Section 25.6 - Maintenance
  • New York City of Environmental Protection (NYCDEP). July 2012. Guidelines for the Design and Construction of Stormwater Management Systems. Developed in conjunction with New York City Department of Buildings.
    • Section 3.5 Operations and Maintenance for Subsurface Systems


Water quality considerations for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Pollutants in harvested water

The composition of stormwater is highly variable in space and time due to differences in land use and rainfall events. This variability is an extremely important consideration when evaluating the feasibility of a stormwater harvesting and use system and determining what level of treatment is necessary to achieve the water quality criteria of the end use.

Common pollutants in stormwater runoff include nutrients, sediments, heavy metals, salinity, pathogens, and hydrocarbons. Water quality of stormwater varies depending on the type of land uses in the drainage area, such as commercial, industrial, residential and parks/open spaces. Typical urban stormwater quality characteristics for the Twin Cities and two other cities are summarized in the following tables.

Typical stormwater pollutants, summary of sources and potential concerns for harvest and use
Link to this table

Pollutant Sources Potential Concerns
Nutrients
  • Nitrogen
  • Phosphorus
  • atmospheric deposition, sediment (adsorbed nutrients)
  • organic debris
  • fertilizer runoff
  • animal feces
  • combined sewer overflows
  • Support growth of algae or unwanted microbial growth on the water surface in the storage unit.
Organic Matter
  • Organic debris (leaves, flowers, pollen, twigs, insect carcasses, etc.)
  • Decomposition in tank can result in low dissolved oxygen levels, nuisance odors, and release of pollutants from sediments
Suspended Sediment
  • paved surfaces
  • areas of bare soil/poor vegetative cover
  • construction activity
  • stockpiles
  • May clog pump intake or distribution
  • Increases maintenance of storage
Chlorides
  • De-icing chemicals
  • water softening chemicals
  • Corrosive to metal pipes/plumbing
  • Plant toxicity (irrigation)
Pathogens
  • animal feces (including bird feces on rooftops)
  • insects/vector organisms
  • drainage area activities such as waste management
  • sewage overflows or leaking sewers
  • Human health risk
Metals
  • Vehicle exhaust
  • Roofing materials
  • Drainage area activities that are potential sources of metals (e.g. vehicle fueling or repair)
  • Plant toxicity
Organic Chemicals
  • Pesticides/herbicides
  • Industrial chemicals and solvents
  • Petroleum-derived chemicals
  • Drainage area activities that are potential sources of organics (e.g. herbicide/ pesticide use or waste management)
  • Plant toxicity
  • Human health risk
  • Animal health risk

Typical concentrations of pollutants in stormwater runoff and snowmelt runoff for select cities
Link to this table

Constituent (concentrations reported in mg/L) Annual Twin Cities Snowmelt4
Twin Cities1 Marquette MI2 Madison WI Storm Sewers Open Channels Creeks NURP5
Cadmium 0.0006 0.0004
Copper 0.022 0.016
Lead 0.060 0.049 0.032 0.16 0.2 0.08 0.18
Zinc 0.111 0.203
Biological Oxygen Demand 15
Chemical Oxygen Demand 169 66 169 82 84 91
Total Kjeldahl Nitrogen 2.62 1.50 3.52 2.36 3.99 2.35
Nitrate + Nitrite 0.53 0.37 1.04 0.89 0.65 0.96
Ammonia 0.2
Total Phosphorus 0.58 0.29 0.66 0.7 0.56 0.54 0.46
Dissolved Phosphorus 0.20 0.04 0.27 0.25 0.18 0.16
Chloride3 230 49 116
Total Suspended Solids 184 159 262 148 88 64
Volatile Suspended Solids 66 46 15

1 Event mean concentrations; Reference: Brezonik and Stadelmann 2002
2 Geometric mean concentrations; Reference: Steuer et al. 1997
3 Geometric mean concentrations; Reference: Waschbusch et al. 1999
4 Reference: Oberts, G. (Met Council). 2000. Influence of Snowmelt Dynamics on Stormwater Runoff Quality.
5 Reference: Median concentrations from more than 2,300 rainfall events monitored across the nation; EPA, 1983

Water quality considerations of runoff from specific source areas

The source area from which stormwater is collected largely determines the water quality characteristics of harvested stormwater. Most stormwater is collected from a mix of source areas, however stormwater harvested for use can often be collected from one dominant source area since the catchment area of the systems tend to be smaller than other larger scale stormwater BMPs. This section discusses the unique water quality considerations for stormwater harvest and use systems for the following source areas

  • Hard roofs
  • Green and brown roofs
  • Paved surfaces
  • Green spaces

Summary of pollutants typically found in stormwater by source area
Link to this table

Source Area Solids Total Suspended Solids Particulate Nutrients Dissolved Nutrients Bacteria Metals Chlorides Grease, Oil Pesticides Other Chemicals
Hard Roofs
Green and Brown Roofs
Paved Surfaces
Green Spaces
Sedimentation Basins and Detention Ponds

● = relatively high concentrations
○ = relatively low concentrations

Hard roofs

Hard rooftops may be composed of a variety of materials (ex. clay/concrete tile, asphalt/composite/wood shingles, metal, slate, or rubberized roofs). See table below.

Common roofing materials and water quality considerations
Link to this table

Roofing Material Water Quality Considerations
Metal Roofs
  • Runoff may contain high levels of zinc, copper, and lead (see here)
Sheet Roofing (PVC)
  • Recommended for non-potable use only
Tile roofs (clay, ceramic, cement, fiberglass)
  • Need periodic cleaning - debris may accumulate between tiles
  • Recommended for non-potable use only
Shingles
  • asphalt
  • composite
  • three tab asphalt
  • Recommended for non-potable use only
  • Some shingles manufactured prior to 1980 may contain asbestos in trace amounts
  • May not be suitable for irrigation if shingles have been treated for mold eradication or with herbicides
Shingles – cedar shakes/wood shingles
  • Recommended for irrigation only
  • Shingles retain moisture, support mold, algae, and insects, may be treated with fire retardant or other chemicals

The table below provides a summary of typical roof runoff quality in Minneapolis and Wisconsin.

Typical Roof Runoff Quality in Minneapolis and Wisconsin
Link to this table

Constituent Minneapolis1 Wisconsin2
E. coli (#/100 mL) 764
Total Solids (mg/L) 126
Total Solids (mg/L) 10 19
Total Hardness (mg/L) 44
Total Nitrogen (mg/L) 0.421
Ammonia-N (mg/L) 0.268
Nitrate-N (mg/L) 0.586
Total Phosphorus (mg/L) 0.104 0.24
Total Dissolved Phosphorus (mg/L) 0.076 0.11
Soluble Reactive Phosphorus (mg/L) 0.065
Cadmium (mg/L) 0.0004
Copper (mg/L) 0.0075 0.01
Lead (mg/L) 0.0032 0.01
Zinc (mg/L) 0.101 0.363

1 Arithmetic mean concentrations; Reference: Minneapolis Public Works, City of Minneapolis Neighborhood Rain Barrel Partnership Project, 2008 2 Highest geometric mean concentration reported; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992

High metal concentrations in rooftop runoff are a major water quality consideration for harvest and use systems. See table below. Although runoff collected from rooftops is generally high quality compared to other sources of stormwater (NAS 2016), certain roof materials may adversely affect the quality of harvested rainwater (Common roofing materials and water quality considerations Table). Other water quality concerns for rooftops include pathogens which may be found in bird or animal feces and organic litter from tree canopy which may contribute to biological oxygen demand.

Concentrations of Zinc, Copper, and Lead in Roof Runoff Based on Roof Material Type
Link to this table

Metal Roof Materials Runoff Concentration (mg/L)
Zinc New uncoated galvanized steel 0.5-10
Old uncoated galvanized steel 1-38
Coated galvanized steel 0.2-1
Uncoated galvanized aluminum 0.2-15
Coated galvanized aluminum 0.1-0.2
Other (aluminum, stainless steel, titanium, polyester, gravel) <0.002
Copper Uncoated copper 0.002-0.175
Uncoated galvanized steel <0.003
Clay tiles 0.003-4
New asphalt shingles 0.01-0.2
New cedar shakes 1.5-27
Aged/patinated copper 0.9-9.7
Lead Uncoated galvanized steel 0.001-2
Coated and uncoated galvanized steel <0.0001-0.006
Painted materials <0.002-0.6

Zinc data: Clark et al. (2008a,b); Faller and Reiss (2005); Förster (1999); Gromaire-Mertz et al. (1999); Heijerick et al. (2002); Mendez et al. (2011); Schriewer et al. (2008); Tobiason (2004); Tobiason and Logan (2000); Zobrist et al. (2000)
Copper data: Clark et al. (2008a); Gromaire-Mertz et al. (1999); Karlen et al. (2002); Wallinder et al. (2009); Zobrist et al. (2000)
Lead data: Clark et al. (2007); Davis and Burns (1999);Förster (1999); Gromaire-Mertz et al. (1999); Good (1993); Gumbs and Dierberg (1985); Mendez et al. (2011); Shriewer et al. (2008)

Green and brown roofs

photo of green roof on the Target Center in Minneapolis, MN
Green roof on the Target Center in Minneapolis Minnesota. Image Courtesy of The Kestrel Design Group, Inc.

Filtrate from green and brown roofs may require little or no treatment since green and brown roofs are effective at removing sediment, although soluble nutrient concentrations (nitrogen and phosphorus) may be elevated and water may be colored.

Paved surfaces

Paved surface source areas include parking lots, sidewalks, driveways, and roadways. The table below provides a summary of water quality characteristics for several types of paved surfaces. Runoff from paved surfaces can contain higher levels of chlorides, solids, and hydrocarbons. Harvest and use systems collecting runoff from paved surfaces will likely require some sort of first flush diverter to bypass very high concentrations of pollutants in spring snowmelt, and potential toxic spills in the drainage area, Treatment may also require filtration units capable of removing fine solids and hydrocarbons.

Urban Stormwater Quality Characteristics from Paved Surfaces
Link to this table

Constituent (concentrations reported in mg/L) Wisconsin Data1 Twin Cities Highways2
Arterial Street Feeder Street Collector Street Collector Street Residential Driveway
Cadmium 0.0028 0.0008 0.0017 0.0012 0.0005 0.0025
Chromium 0.026 0.007 0.013 0.016 0.002
Copper 0.085 0.025 0.061 0.047 0.02 0.023
Lead 0.085 0.038 0.062 0.062 0.02 0.242
Zinc 0.629 0.245 0.357 0.361 0.113 0.123
Nitrate-Nitrite 0.77
Total Phosphorus 1.01 1.77 1.22 0.48 1.5 0.43
Total Dissolved Phosphorus 0.62 0.55 0.36 0.07 0.87
Chloride3 11.5
Total Suspended Solids 993 1152 544 603 328
Suspended Solids 875 1085 386 474 193

1 Arithmetic mean concentration; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992
2 Reference: University of Minnesota Water Resources Center, Assessment of Stormwater Best Management Practices Manual, 2008
3 Data represents chloride concentrations during monitoring season, typically April through October. Chloride concentrations in winter snowmelt grab samples have been found to be as great as 3,600 mg/L.

Green spaces

Green space source areas include lawns and park areas. Green spaces typically have lower concentrations of pollutants compared to stormwater source areas. Due to the presence of pets and/or wildlife (particularly Canadian geese), these areas may have very high concentrations of pathogens and require disinfection treatment for certain end uses.

Seasonal considerations

In addition to variability in stormwater quality from different source areas, stormwater quality also varies with season. Link here for a summary of water quality characteristics of snowmelt in the Twin Cities Metropolitan Area. Seasonal considerations include the following.

  • Snowmelt can have very high concentrations of chlorides and sediment from winter road de-icing practices.
  • In spring, organic litter including pollen may increase Biochemical Oxygen Demand (BOD) and residual solids on pavement may increase TSS.
  • In fall, leaf litter may contribute to BOD, nutrients and solids.

Treatment requirements

Water quality criteria have been developed for stormwater harvest and use systems in many states and are summarized in Toolbox R.3b of the 2011 Met Council Stormwater Reuse Guide. The only existing criteria in the State of Minnesota are for stormwater harvest and use systems regulated by Chapter 17 of the Plumbing Code (see below). The National Water Research Institute (NWRI) is currently sponsoring an Independent Advisory Panel to develop national risk-based treatment requirements for stormwater harvest and use systems (see below). Treatment requirements will be based on risk of exposure to harvested stormwater instead of based on achieving end-use water quality criteria. For each level of risk, certain levels of treatment and risk barriers will be required. A risk-based approach for stormwater harvest and use treatment is more appropriate than end use-based water quality criteria due to the wide range of harvested stormwater quality, pathways for exposure, and project specific circumstances. Please check for up-to-date treatment requirements during the design of each stormwater harvest and use system project at NWRI's website. Post-storage treatment processes available to reduce risk of exposure are described below.

Risk-based treatment requirements for stormwater harvest and use systems

Link to this table

Risk Level Description Site Barriers Filtration Disinfection Other treatment barriers
Low exposure No direct physical contact None
Medium exposure Direct physical contact Signage
High exposure Ingestion or inhalation Restricted access

National Water Research Institute (NWRI) Independent Advisory Panel findings

The primary public health concern with stormwater reuse systems is pathogenic microorganisms. Traditionally, water and wastewater systems have been monitored using fecal indicator organisms (FIO) such as E. coli. The presence or concentration of FIO in a water or wastewater samples was assumed to be indicative of other waterborne pathogens. The FIO were useful because they were expected to be present in water contaminated with fecal waste. However, there are a number of limitations with the use of FIO including (a) FIO may not always be present in stormwater, (b) FIO are not necessarily representative of other pathogen groups, (c) grab samples analyzed for FIO cannot be used for continuous monitoring, and (d) FIO are more difficult to measure consistently than other surrogate parameters. Therefore, state agencies are currently working on procedures for designing and monitoring stormwater reuse systems in more effective ways.

Water quality monitoring and control systems are used commonly to assess the operation, performance, and status of a given component or process. The fundamental purpose of performance target monitoring of a stormwater reuse system is to ensure that the treatment barriers that have been put in place to meet the specified water quality targets are operating as intended.

Most non-potable water systems utilize a number of unit processes in series to accomplish treatment, known commonly as the multiple barrier approach. Multiple barriers are used to improve the reliability of a treatment approach through process redundancy, robustness, and resiliency.

When multiple treatment barriers are used to achieve pathogen removal, the contribution from each barrier is cumulative. In addition to these treatment barriers, operational and management barriers are used to ensure that the systems are in place to respond to non-routine operation. The technical barriers can be monitored using operational and critical control points.

This image shows Risk characterization stormwater reuse
Risk characterization stormwater reuse

Minnesota Plumbing Code

The new 2015 Minnesota Plumbing Code, Minnesota Rules, Chapter 4714, took effect Jan. 23, 2016. The code now includes the design and installation of harvesting rainwater from building roof tops in Chapter 17, Nonpotable Rainwater Catchment Systems. Nonpotable rainwater catchment systems are acceptable for use to supply water to water closets, urinals, trap primers for floor drains, industrial processes, water features, vehicle washing facilities, and cooling tower makeup water provided the design, treatment, minimum water quality standards, and operational requirements are in accordance with Chapter 17 of the code. Designs must be approved by a Minnesota registered professional engineer.

Rainwater catchment systems use for plumbing applications listed above in combination with lawn irrigation must meet the requirements of Chapter 17. System components used solely for lawn irrigation, such as irrigation pumps and piping mounted outside of buildings are not subject to the requirements of Chapter 17. The conveyance of the rainwater catchment system is still governed by the plumbing code. Minimum water quality standards are now described in Chapter 17.

“1702.9.4 Minimum Water Quality. The minimum water quality for rainwater catchment systems shall meet the applicable water quality recommendations in Table 1702.9.4”

Minimum water quality for rainwater catchment systems

Link to this table

Measure Limit
Turbidity (NTU) <1
E. coli (MPN/100 mL) 2.2
Odor Non-offensive
Temperature (degrees Celsius) MR*
Color MR
pH MR

* MR = measure and record only; Treatment: 5 micron or smaller absolute filter; Minimum .5-log inactivation of viruses

Treatment systems

Stormwater harvest and use systems require some level of pretreatment, similar to other stormwater BMPs, such as:

  • source control
  • debris/coarse solids removal, and
  • suspended solids removal.

These pretreatment processes are not discussed here. For a full discussion of pretreatment, link here. The following table describes post-storage treatment process considerations, including

  • dissolved solids removal,
  • disinfection, and
  • other additional treatments (such as chlorine residual removal and pH adjustments).

Post-Storage Treatment Process Considerations
Link to this table

Post-Storage Treatment Process Description and Considerations Treatment Alternatives Target Pollutants Capital Cost O&M Level Energy Needs Advantages over Alternatives Disadvantages over Alternatives
Dissolved Solids Removal Filtration generally is used to remove residual solids that will not settle spontaneously from harvested water through sedimentation or which may become re-suspended in storage. Filters come in a variety of different types and sizes. The type of filter depends on the class of pollutants targeted for removal. Coarse & fine filters
  • Suspended solids
Med Med Med
  • Lower overall O&M costs than other filtration
  • Does not remove micro-organisms
Micro-filtration
  • Suspended solids
  • Micro-organisms
Med Med Med
  • Smaller footprint required
  • May reduce disinfection requirements
  • Captures microorganisms
  • Higher capital costs
  • Higher O&M costs including membrane replacement, energy, performance monitoring, and residuals disposal
Nano-filtration
  • Dissolved salts
  • Bacteria/ viruses
  • Proteins
Med Med High
  • Requires less energy than reverse-osmosis and ion-exchange filters
  • Requires large amount of pretreatment to remove metals that cause scaling and particulates that cause biofouling
  • Produces a larger waste stream than reverse-osmosis
Reverse-osmosis
  • Dissolved salts
  • Dissolved solids
  • Ions
  • Bacteria/ viruses
High High High
  • Highest removal efficiency
  • Produces the smallest waste stream
  • Commercially available
  • Requires more energy than nanofiltratoin
X X
Ion-exchange filter
  • Charged ions
  • Dissolved salts
High High High
  • Requires the least amount of pretreatment
  • Produces the largest waste stream
  • Requires more energy than nanofiltration
  • Make-up water is required to continuously wash membranes
Disinfection Disinfection processes kill, remove, or deactivate pathogenic microorganisms in harvested water. Chlorination – injects chlorine into stormwater
  • bacteria
  • viruses
  • other pathogenic organisms
Low Low Low
  • Most common disinfection technology
  • Least cost
  • Requires calibration of dosage control devices
  • Does not kill cysts
Ultra-violet light (UV) radiation – stormwater is passed over an ultraviolet lamp
  • bacteria
  • viruses
  • other pathogenic organisms
Med High High
  • No byproducts
  • Minimal energy requirements compared to chlorination/ozonation
  • Requires cartridge filters ahead of the UV light, with routine cleaning of filters – UV is ineffective on unfiltered stormwater
  • UV lamps must be replaced periodically
Ozonation – diffused ozone released through a fine bubble diffuser at the bottom of the storage tank (possible with stormwater but rarely used) Med Med Med
  • Also removes dissolved organics
  • More effective disinfectant than chlorination
  • Treatment of off-gases required
  • High energy requirements
  • Corrosion protection required
  • Requires monitoring of influent to adjust doses
  • Requires routine check for leaks
Other treatments (e.g., pH adjustment) Treatment for pH adjustment may be needed if the end use of harvested water requires a neutral pH or if harvested water will come in contact with metal pipes or surfaces. Rainwater tends to be slightly acidic and harvested stormwater may retain this characteristic. Acidity can cause metal pipes to corrode leading to contamination of harvested water. Chemical additive
  • acidic or alkali substances
Low Low Low
  • N/A
  • N/A


Environmental concerns for stormwater and rainwater harvest and use/reuse

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

The construction and operation of stormwater harvest and use systems can pose potential risks from the pollutants and toxins found in stormwater and harvest and use system materials. These risks are largely addressed via water quality standards, plumbing and building codes, stormwater rules and regulations, required signage, and the engineering review process. Stormwater harvesting is, however, an emerging practice in water resource management and existing regulations may not fully address the risks associated with harvest and use practices.

Therefore, a risk assessment should be completed during the pre-design phase to ensure that potential risks are properly managed through system design, operation and maintenance. According to U.S. EPA, there are two types of risk assessments:

  1. A human health risk assessment, which is the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future.
  2. An ecological risk assessment, which is the process for evaluating how likely it is that the environment may be impacted as a result of exposure to one or more environmental stressors such as chemicals, land change, disease, invasive species and climate change.

The following factors should be considered when assessing human health and ecological risks of stormwater harvesting and use systems (NAS, 2016):

  • Geology and climate of the harvesting site
  • Potential environmental hazards located within the rainwater/stormwater source area
  • Source water quality
  • Potential exposure to harvested water (indoor/outdoor, restricted access, unrestricted access)
  • End use of harvest water

Potential human health and environmental risks of stormwater harvest and use systems, and ways to manage those risks through design, operation and maintenance are summarized briefly below. For further guidance, refer to Chapter 5 of Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits (NAS, 2016) and the US EPA Risk Assessment webpage.

Specific guidelines for addressing health and environmental risks associated with stormwater harvest and use systems have been developed in Australia. See Figure 1.1 on page 6 of the Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2), Stormwater Harvesting and Reuse for a flow-diagram guide to their risk assessment approach.

Human health risks

Human health risks include any adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future. Potential hazards to human health from stormwater harvest and use systems include:

  • Chemical pollutants and pathogens in stormwater before and during harvest. This includes
    • nutrients, sediment, microbes, salts, oil and grease, and metals typically found in stormwater runoff. See Water Quality – Pollutants in Stormwater Runoff
    • Pollutants associated with spills
    • Metals or other chemicals leaching from rooftops, conveyances, or tanks
  • Bacteria or other pathogenic organisms colonizing in the collection or storage systems
  • Waterborne diseases
    • Bacterial pathogens (e.g., Escherichia spp., Salmonella spp., Campylobacter spp., Legionella sp.). See Table 8 in Water Quality Considerations for E. coli water quality criteria.
    • Parasites (helminths)
  • Opportunistic pathogens (e.g., Mycobacteria spp., Pseudomonas spp.)
  • Mosquito-borne diseases (e.g., West Nile virus, La Crosse Encephalitis)

Potential exposure and health effects

The main health concern with harvest and use of stormwater is exposure to pathogenic microorganisms. Studies have consistently reported high concentrations of fecal indicator microorganisms across different source areas of stormwater; however, the occurrence and fate of human pathogens in stormwater is not well characterized (NAS, 2016). The nature and severity of human health effects depend on the type of exposure (skin contact, ingestion, inhalation, etc.) as well as the duration and magnitude of exposure (Table 1). Treatment requirements will be stricter for beneficial use applications which have a high chance of exposure compared to those which have a low risk of exposure. Additional information on exposure and dose-response assessments can be found on the EPA Human Health Risk Assessment webpage. This webpage also provides detailed guidance on how to complete the following steps of a human health risk assessment:

  • Planning and Scoping process, which includes the following questions:
    • Are there hazard(s) of concern?
    • What is the source of the hazard(s)?
    • How does exposure occur?
    • Who/what is at risk?
    • When/where are they at risk?
    • What are the biological pathway and health effects of the hazard(s)?
    • Under what conditions is a harmful effect likely to occur?
  • Step 1 - Hazard Identification: Examines whether a stressor has the potential to cause harm to humans and/or ecological systems, and if so, under what circumstances.
  • Step 2 – Dose-Response Assessment: Examines the numerical relationship between exposure and effects.
  • Step 3 – Exposure Assessment: Examines what is known about the frequency, timing, and levels of contact with a stressor.
  • Step 4 – Risk Characterization: Examines how well the data support conclusions about the nature and extent of the risk from exposure to environmental stressors.

Examples of potential exposure types and pathways
Link to this table

Exposure Types Exposure Pathways
Skin contact
  • Hand watering with harvested stormwater.
  • Contact with irrigation mist or runoff
  • Contact during use (washing, flushing)
  • Contact during recreation
Direct ingestion
  • Cross-contamination of drinking water supply with harvested stormwater
  • Ingestion of irrigation mist or spray
  • Ingestion of irrigated fruits and vegetables
Indirect ingestion
  • Ingestion of bio-accumulated pollutants in food crops irrigated with harvested water
  • Ingestion via hand-to-mouth after contact with irrigated areas
Inhalation
  • Irrigation mist
  • Water agitation

Ecological risks

Ecological risks include any impact on the environment from the result of exposure to one or more environmental stressors such as chemicals, land change, disease, invasive species and climate change. Potential ecological risks from stormwater harvest and use systems include impacts to the following.

  • Plants – Pollutants found in stormwater runoff (e.g., heavy metals, salts, and hydrocarbons) may decrease the productivity of or even kill certain plant species under high irrigation rates or long-term exposure from irrigation (EPA, 2012; National Academy of Sciences (NAS), 2016). The Minnesota Stormwater Manual lists plant species with known tolerance to salt. These species may also have some tolerance to sediments and petroleum which are commonly associated with salt in road runoff. Salt tolerance has also been shown in some of the aggressive and invasive species found in the Midwest. One concern with using stormwater for irrigation is that the higher pollutant loads will increase susceptibility to exotic and invasive species, such as common buckthorn, box elder, and reed-canary grass. (MPCA). See Table 8 in Water Quality Considerations for chloride and metal water quality criteria.
  • Soil - Salinity in stormwater is a key concern for soil health. Sodium in harvested stormwater that is applied for irrigation can replace calcium and magnesium in soils. Over time, this process can negatively impact soil structure, making the soil less permeable and more erodible, particularly soils with high clay content (NAS, 2016). See Table in Water Quality Considerations for chloride water quality criteria.
  • Local hydrology – Stormwater harvest and use systems can impact local hydrology via (US EPA, 2012)
    • increased baseflow to surface waters if extensive land application of water increases groundwater elevations,
    • increased runoff volumes or peak rates during wet periods if extensive land application of water shifts soil conditions from ‘dry’ to ‘wet’ preceding rainfall events, and
    • decreased local flow due to harvested stormwater for indoor uses being routed to the sanitary sewer system instead of discharging to surface waters.
  • Equipment degradation - Some [stormwater quality constituents] can negatively affect the performance of the harvest and use system increasing maintenance needs and potentially reducing the useful life of components. Stormwater quality should be characterized in the pre-design phase so that design choices optimize performance of the system. Ultimately, a system that does not perform as intended may pose risks to health or to the environment. The stormwater quality constituents that could affect the operation of the harvest and use system equipment and structures include (from Toolbox R.1a in the 2011 Met Council Reuse Guide):
    • Debris and particulates associated with sediment and leaves could potentially block or clog pipes, irrigation nozzles or drip irrigation systems, or damage pumps. See Table in Water Quality Considerations for turbidity and TSS water quality criteria.
    • Organic matter (measured by BOD, COD, or TOC), for example from glass clippings, that causes reduced dissolved oxygen levels through decomposition could result in odors and release of pollutants from sediments.
    • Nitrogen and phosphorus could support algal growth in open storage facilities, which can lead to higher turbidity and/or create algal blooms with biofilm characteristics that could clog irrigation equipment.
    • Iron concentrations could clog irrigation systems and decrease the effectiveness of the disinfection system.
    • Hardness could result in clogged irrigation systems.
    • Anaerobic conditions or high salt concentrations could result in corrosion of system components. See Table in Water Quality Considerations for chloride water quality criteria.

Managing risk

Potential risks must be managed through proper design, operation, and maintenance of stormwater harvesting systems-see table below. If potential risks cannot be addressed through cost-effective design or operation and maintenance, the goals and objectives of the stormwater harvest and use system should be reconsidered in the pre-design phase.

Elements of design, operation, and maintenance that address potential risks associated with stormwater harvesting and use
Link to this table

Risk Type Design Considerations O & M Considerations
Human Health Risks
Source area pollutants
  • Bypass runoff from pollutant hotspots in the source area
  • Pre-storage treatment and treatment systems
  • Source control BMPs
  • Inspection and maintenance of source area
  • Regular clean-out of accumulated sediments in storage ponds, if applicable
Hazardous spills in the source area, including sudden air releases of hazardous substances that could deposit in the collection and storage systems
  • Incorporate at least a 72-hour residence time of harvested stormwater in storage unit to contain hazardous spills in the source area prior to distribution (NRMMC et al. 2008)
  • Incorporate an emergency bypass in the collection system
  • Regular inspection of source area for hazardous spills
  • Coordination with local agencies responsible for responding to hazardous spills in the source area
  • Incorporate an emergency spill response plan in the O&M document
Metals and other chemicals from roofing materials (link to table 4 in WQ considerations)
  • Some roofs may leach chemicals at levels of concern
  • Regular inspection and cleaning of system components to prevent degradation
Bacteria, viruses
  • Source control BMPs
  • Disinfection or filtration treatment systems
  • Regular cleaning and maintenance of storage and treatment units
  • Water quality monitoring
  • Signage and/or access restriction to irrigation areas or other exposure controls
Mosquito and other vector-borne illnesses
  • Install insect screens on exposed pipe and other openings
  • See the following websites for information on mosquito control in storage ponds:
  • Regular monitoring
  • Regular mosquito control treatment of storage unit
Ecological Risks
Plant communities
  • Source control BMPs
  • Pre-storage treatment and treatment systems
  • Use of salt tolerant plant species in irrigated areas
  • Irrigation rates that optimize the saturation of soils without interfering with plant growth.
Soils
  • Avoid irrigating high-clay soils with high-salinity stormwater. Salt reduces the permeability of clay soils by increasing the stickiness of clay soils when wet and forming hard clods and crusts upon drying (See this)
  • Regular monitoring of stormwater runoff and soil quality if salt contamination of soils is a concern
Aquatic ecosystems
  • Avoid inventoried public waters and wetlands for storage of harvested stormwater
  • Install aeration systems in storage ponds to minimize algal blooms, if allowed by local code or ordinance
  • Pre-storage treatment systems
  • Regular monitoring of storage pond for adverse ecological impacts
Local hydrology
  • Design storage residence times and overflow volumes such that downstream hydrology is not adversely impacted
  • Regular inspection and maintenance of storage, overflow and distribution system components
Equipment degradation
  • Source control BMPs
  • Pre-storage treatment and treatment systems
  • Regular inspection and cleaning of system components

Key resources


Cost-benefit considerations for stormwater and rainwater harvest and use/reuse

What are the total costs and cost per element of a harvesting and use system? Costs can be highly dependent on the situation and context. The total cost of a harvest and use system can be divided into the four major components of a harvesting and use system:

  • Collection
  • Storage
  • Treatment
  • Distribution

The individual components required to construct each of the four systems usually depends on the site and/or use of the water. For the collection component, storm sewer pipes and roof drains may already be part of the design, thus reducing cost. The storage component is typically the largest cost item. If storage already exists at a site, such as existing wet ponds, providing storage for a harvest and use system can be done at minimal cost. Treatment costs can vary dramatically, depending on the source water and the end use, from virtually no treatment to meeting drinking water standards. Costs for distribution are usually associated with connection to an irrigation system. The location and elevation of the irrigation site in proximity to the source and storage areas affect the amount of pipes and pumps needed. If a site already has an irrigation system in place drawing from a potable water source, the distribution portion of the system costs may be minimal.

The site setting affects the cost of harvest and use systems. For example, in highly urban areas the choices for stormwater treatment may be limited and components such as storage (which is often an underground cistern) may be quite expensive on a cost/unit treatment basis. However, harvest and use may still more cost-effective than other stormwater management techniques such as green roofs or underground infiltration facilities. It is difficult to compare unit costs of harvest and use systems across different settings.

Total system costs

The total cost of a stormwater harvest and use system varies due to the large range in the size and scale of these systems. In a Minnesota Pollution Control Agency (MPCA, 2016) survey for stormwater harvest and use systems in the Twin Cities Metropolitan Area, 26 respondents provided total system cost information, summarized in the MPCA survey responses of total stormwater harvest and use system costs graph below. Total costs ranged from 💲1,500 to 💲1,500,000, with eight systems over 💲400,000. Of the 26 systems with cost information, 22 were irrigation systems (💲1,500 - 💲1.5M), 1 was a toilet flushing system (💲300,000), 1 was a toilet flushing and vehicle washing system (💲57,500), and 2 were irrigation and vehicle washing systems (💲10,000 - 💲425,000).

This graph shows MPCA survey responses of total stormwater harvest and use system costs
MPCA survey responses of total stormwater harvest and use system costs

Individual component costs

Major individual component costs of stormwater harvest and use systems include land acquisition, excavation and material removal, and the storage/treatment systems. Very little detailed component cost information is currently available because costs for many of the storage and treatment systems are packaged together. Examples of harvest and use system itemized costs are discussed below.

The 2011 Met Council Stormwater Reuse Guide developed a list of stormwater harvest and use system construction activity components and cost units for developing system cost estimates, reproduced in the Stormwater Harvesting and Use Component Checklist and Cost Units table. A cost analysis of different cistern materials was summarized by CONTECH Inc. in their 2011 Cistern Design Considerations for Large Rainwater Harvesting Systems Professional Development Advertising article, reproduced in the Comparison of materials used for rainwater harvesting systems table below. Some itemized component cost information was also compiled in the Texas Manual on Rainwater Harvesting, summarized in the Itemized stormwater harvest and use system component costs table below. These itemized costs include cistern and gutter costs on a per volume/length basis, and treatment system consumables (such as filters and cartridges) that must be replaced regularly as part of normal system operation and maintenance.

Comparison of materials used for rainwater harvesting systems

Link to this table

Material Cost low - high Installation hard - easy Longevity short - long Durability low - high Maintenance Access hard - easy Best Use Capacity (gallons)
Underground FiberglassX 💲💲💲💲💲 ●●●○○ ●●●●○ ●●○○○ ●●●●○ 5,000 to 30,000
Polyethylene 💲 ●●●●● ●●●○○ ●●●○○ ●●○○○ > 5,000
Steel Reinforced Polyethylene (SRFE) 💲💲💲 ●●●●○ ●●●●● ●●●●● ●●●●○ 10,000 to 100,000+
Plastic Crates 💲💲💲 ●●●○○ ●●○○○ ●○○○○ ●○○○○ 5,000 to 50,000
Concrete 💲💲💲💲💲 ●●○○○ ●●●●○ ●●○○○ ●●●●○ 30,000+ (with high loading)
Fabricated Steel 💲💲💲💲 ●●●●○ ●●●○○ ●●●●○ ●●●●○ not recommended
Waterproof Corrugated Metal 💲💲 ●●●●○ ●●●○○ ●●●●● ●●●●○ 5,000 to 30,000
Above-Ground Monolithic 💲💲💲 ●●●●○ ●●●●● ●●●●○ ●●●●● Up to 20,000
Plate Assembled On-Site 💲💲💲 ●●○○○ ●●●●● ●●●●○ ●●●●● 15,000+

Itemized stormwater harvest and use system component costs

Link to this table

System System Component Cost Cost Recurrence
Materials Tanks 💲0.50/gallon for fiberglass to 💲4/gallon for wielded steel tank
Gutters 💲0.30/foot for vinyl/plastic to 💲6 - 12/foot for aluminum/galvalume
Annual maintenance (costs will be dependent on system size) Cartridge Filter 💲20-60 Filter must be changed regularly
Reverse Osmosis Filter 💲400-1,500 Change filter when clogged (depends on turbidity)
UV Light Disinfection 💲350-1,000; 💲80 to replace UV bulb Change UV bulb every 10,000 hours or 14 months
Ozone Disinfection 💲700-2,600; 💲1,200+ for in-line monitor to test effectiveness
Chlorine Disinfection 💲1/month manual dose or a 💲600-3,000 automatic self-dosing system

Stormwater Harvesting and Use Component Checklist and Cost Units

Link to this table

Phase Component Unit Check if required for system:
Collection Cleaning of roof (if retrofit project) Square foot
Roof washing system Each
Gutters Linear foot
Gutter screens Linear foot
Downspouts Linear foot
Scuppers Each
Catch basins Each
Catch basin filters Each
Manholes Each
Oil/water separators Each
Storm sewers Linear foot
Bypass valves Each
First flush diverter Each
Storage – Ponds/ basins Site demolition Varies
Excavation Cubic foot
Disposal of excess soil Cubic foot
Vegetation restoration Square foot
Baffles at outlet Linear foot
Filters at outlet Each
Outlet structure Each
Pumping system including pump, motor, valves, and pressure tank (for non-gravity and pressurized systems) Varies
Aeration Varies
Electrical supply (for pumps or aeration) Varies
Below-ground storage Site demolition Varies
Excavation and backfill Cubic foot
Imported aggregate bedding material Cubic foot
Disposal of excess soil Cubic foot
Vegetation or pavement restoration Square foot
Pre-fabricated tanks Each
Baffles, calming inlet, and/or filters, if not supplied with pre-fabricated tank Each
Cast-in-place concrete tank Varies
Pumping system including pump, motor, valves, and pressure tank (for non-gravity and pressurized systems) Varies
Maintenance access manhole Each
Electrical supply (for pumps) Varies
Treatment systems Piping Linear foot
Valves Each
Flow meter (when needed to regulate chemical feed) Each
Electrical supply Varies
Maintenance access manhole (if located underground) Each
Backflow prevention valves (if connected to potable water for supplemental supply and/or for filter backwash) Each
Suspended & Colloidal Solids Removal Systems
  • Chemical feed
  • Tank with baffles or mixing device (if chemicals not fed into inline mixing device)
  • Settling basin with dewatering valves for solids removal
Varies
Residual Suspended Solids Removal Systems
  • Filter chamber containing activated carbon or other filter media, including piping and valves for bypass and backwash (for filtration systems)
  • Tank with micro-bubble diffusion, dewatering valves, and surface skimmer (for dissolved air flotation systems)
Varies
Residual Colloidal Solids Removal Systems
  • Filter chamber containing multi-media, including piping and valves for bypass and backwash (for ultrafiltration systems)
Varies
Dissolved Solids Removal Systems
  • pH feed (for reverse osmosis systems)
  • Chemical feed (for electrodialysis systems)
  • Filter chamber containing semi-permeable membrane, including piping and valves for bypass and backwash (for all systems)
  • Solids disposal system
Varies
Disinfection
  • Chemical feed (for continuous chlorine disinfection)
  • Tank with baffles or mixing device (for batch disinfection with chlorine)
  • Contact tank with UV lights and piping (for ultraviolet disinfection)
  • Tank with piping, valves, ozone diffuser (for ozone disinfection)
  • Off-gas ozone destructor tank (for ozone disinfection)
Varies
Distribution Pumping system including pump, motor, valves and pressure tank Varies
Piping for distribution Linear foot
Valves for pressure control, and regulating flow Each
Valve boxes Each
Sprinkler nozzles – impulse, spray, rotating, bubbler, or drip Each
Irrigation controllers with wiring to each sprinkler (for automated control systems) Varies
Drain plug (for winterization) Each

Harvest and use system construction bid estimate examples

Due to the large variability in harvest and use system costs, construction bid estimates are provided as examples of itemized costs.

  • Installation of 3 – 29,000 gallon underground storage tanks: The Contractor bid averages for installation of three 29,000 gallon underground storage tanks table below summarizes the average of four contractor bids for the installation of three 29,000 gallon underground storage tanks (120” diameter x 50’ length) for an irrigation harvest and use system designed for an approximate 8 acre drainage area and an approximate 3 acre irrigation area. This estimate is for storage (3 tanks) components only. The average capital cost for one gallon of storage is 💲1.46 per gallon. The underground storage tank components were 71 percent of the total installation bid.
  • Installation of 1 – 1,500 gallon aboveground tank harvest and use system: The table below summarizes the engineers estimate for installation of one 1,500 gallon aboveground corrugated steel storage tank that captures rainwater from a 2,100 square foot roof connected to a hose for on-site exterior water use. This estimate is for collection (gutters), storage (tank), and distribution (hose) components. The total capital cost for one gallon of storage is approximately 💲20 per gallon. The aboveground storage tank components were 46 percent of the total construction estimate.

Contractor bid averages for installation of three 29,000 gallon underground storage tanks

Link to this table

BASE BID ITEM ESTIMATED QUANTITY UNIT UNIT PRICE AVERAGE TOTAL AVERAGE BID
Part 1 - General and Erosion Control
MOBILIZATION 1 LS 💲23,340 💲23,340
SEDIMENT CONTROL LOG -- INSTALL, MAINTENANCE AND REMOVAL 267 LF 💲5 💲1,335
TEMPORARY FENCE -- INSTALL AND REMOVAL 160 LF 💲8 💲1,280
STABILIZED CONSTRUCTION EXIT -- INSTALL, MAINTENANCE AND REMOVAL 1 LS 💲2,410 💲2,410
EROSION CONTROL BLANKET 424 SY $4 $1,696
STORM DRAIN INLET PROTECTION -- INSTALL, MAINTENANCE AND REMOVAL 2 EA 💲387 💲774
DUST CONTROL 10 HRS 💲121 💲1,210
Total Part 1 💲32,045
Part 2 - Removals
REMOVE EXISTING SEDIMENT CONTROL LOG OR SILT FENCE Total Part 2 268 LF 💲4 💲1,072
Total Part 2 💲1,072
Part 3 - Grading
COMMON EXCAVATION -- INCLUDES TANK TRENCH EXCAVATION AND FILL TO PROPOSED GRADE 1955 CY $9 $9
REMOVAL OF EXCAVATED MATERIAL 1389 CY $10 $13,890
AGGREGATE BACKFILL 1009 CY $41 $41,369
Total Part 3 💲72,854
Part 4 – Underground Storage Tank Components
RAINWATER HARVESTING TANK (120" DIA. X 50-FEET) 3 EA 💲77,639 💲232,917
CONSTRUCT DRAINAGE STRUCTURE 6 EA 💲1,153 💲6,918
CONCRETE, REINFORCED COLLAR (RISER MANHOLE CAP) 6 EA 💲1,085 💲6,510
CONCRETE, REINFORCED COLLAR (AT RISER CONNECTION TO TANK) 6 EA 💲1,248 💲7,488
INSTALL CASTING 6 EA 💲1,221 💲7,326
18" HDPE PIPE 115 LF 💲73 💲8,395
SOIL DENSITY COMPACTION TESTING 12 EA 💲430 💲5,160
Total Part 4 💲274,714
Part 5 – Site Restoration
RAPID STABILIZATION 0.42 AC 💲3,173 💲1,333
PERMANENT SEEDING 0.42 AC 💲6,336 💲2,661
TURF ESTABLISHMENT 1 EA 💲2,500 💲2,500
Total Part 5 💲6,494
Total Bid 💲386,590
PRICE PER TANK (3 – 29,000 gallon tanks) 💲129,060
PRICE PER TANK PER YEAR OVER 25 YEAR 💲5,162
PRICE PER GALLON (87,000 gallons) 💲4.44
PRICE PER GALLON PER YEAR OVER 25 YEARS 💲0.06

Engineers estimate for installation of one 1,500 gallon aboveground corrugated steel storage tank

Link to this table

ITEM ESTIMATED QUANTITY UNIT UNIT PRICE TOTAL COST
RAINWATER HARVESTING PACKAGE: 1,500 GALLON ABOVE GROUND CORRUGATED STEEL TANK AND ASSOCIATED FITTINGS & ACCESSORIES, INCLUDING PUMP AND FILTER SYSTEM. (72" DIA. X 9') 1 EA 💲15,000 💲15,000
REINFORCED CONCRETE FOUNDATION ON IMPROVED SUBGRADE 1 EA 💲2,500 💲2,500
REMOVE EXISTING GUTTER 60 LF 💲9 💲540
5" BOX GUTTER 222 LF 💲10 💲2,220
GUTTER DOWNSPOUTS 45 LF 💲15 💲675
6" HDPE PIPE 110 LF 💲30 💲3,300
RODENT GUARD 1 EA 💲350 💲350
SCOUR STOP MAT 32 SF 💲30 💲960
EROSION CONTROL BLANKET 14 SY 💲20 💲284
ENGINEER'S REPORT 1 EA 💲2,500 💲2,500
O&M GUIDELINES 1 EA 💲1,500 💲1,500
Total 💲29,829

Funding sources

There are many sources of funding that can be used to finance stormwater harvest and use systems (Stormwater Harvest and Use Funding Sources table). Due to the high cost of these systems, more than one source of funding is often needed. Of the 26 respondents to the 2016 MPCA stormwater harvest and use system survey that provided cost and funding source information, 16 respondents utilized two or more sources of funding to finance their harvest and use system.

Stormwater harvest and use funding sources
Link to this table

Funding Source Funding Type
Watershed Organization Implementation and Cost-Share Programs
  • Watershed Management Organization
  • Watershed District
  • Joint Power Agreement
State Agency Grants and Loans
County Funds
  • County program funds
Municipal Funds and Utility Fees
  • Enterprise fund
  • Stormwater utility fees
  • Surface water management utility fees
Other Public Financing
  • University environmental fee
  • School district
  • Student housing fees
  • Parks operating budget
Private Financing
  • Developers (to meet stormwater requirements on a restricted site or to maximize developable area)
  • Local organizations (e.g. youth soccer club)

Financial incentives and benefits

There are many financial incentives and benefits that should be factored in to the global net cost of a stormwater harvest and use system. These include:

  • Points toward U.S. Green Building Council LEED or Institute of Sustainable Infrastructure Envision ratings which demonstrate how well public entities are sustainably managing energy, material, and water resources (LEED) and planning for climate change and long-term resiliency in public infrastructure investments (Envision)
  • Reduced potable water utility costs
  • Reduced downstream stormwater infrastructure costs
  • Increased resiliency in stormwater management and potable water systems due to reductions in stormwater volume and potable water demand
  • Ecosystem services

Additional cost resources

Cost-Benefit case studies

System costs


Case studies for stormwater and rainwater harvest and use/reuse

image
Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Cottage Grove City Hall

  • Location: Cottage Grove, MN
  • Owner: City of Cottage Grove
  • Designer: Wold Architects (City Hall), SkyHarvester (reuse system)
  • Year of Completion: 2012
  • Total Drainage Area: 0.9 acres of rooftop
  • Total Construction Cost: $120,000
  • Pretreatment/Methods of Filtration: Rainwater filter (screen to remove large particles leaves, debris, and sediment), Fine filter (filter fine particles, down to 5 microns), UV Treatment (provides microbial disinfection)
  • Documented Maintenance Practices: Replacement of filter media and UV bulbs as needed, general winterization of irrigation system
  • Pollutant Removal: Runoff volume control approximately 1.8 acre-feet (570,000 gallons) per year. MIDS calculator estimates pollutant reductions of approximately 1.5 pounds of phosphorus (0.85 pounds particulate phosphorus and 0.70 pounds dissolved phosphorus), and 282 pounds total suspended solids per year.
  • Is the site publicly accessible: Yes, an interpretive sign is on the rear patio of City Hall for public viewing.
  • Special Design Features: Rainwater Harvester Control Panel to regulate operation of systems, Irrigate planting beds on 7-acre site, education signage about system

In 2012 the City of Cottage Grove completed construction of the new City Hall and Public Safety complex, designed by Wold Architects. The building is situated in a growing part of the community, directly adjacent to Cottage Grove Ravine Regional Park, a natural resource and recreation amenity area which features unique habitat and a beloved fishing lake. In an effort to reduce water use and impacts on stormwater runoff, the building design incorporates a stormwater harvester system. The harvester reuses stormwater collected from the building's roof for irrigation of the green space around the building and the Veterans Memorial fountain located at the building entrance. This system collects runoff from the 0.9 acre roof and stores it in an underground storage tank. From the storage tank, the water is filtered and treated with ultraviolet light then pumped through the irrigation system for use on the 7-acre City Hall grounds, and at the fountain. The purpose of the system is to provide the dual benefits of reducing the use of groundwater for landscape irrigation and minimizing stormwater runoff to Ravine Lake.

Eagle Valley and Prestwick Golf Club, Woodbury

Photo of harvest and use system Woodbury
Plan for the harvest system showing storage and transport components. Image courtesy Emmons and Olivier Resources, HR Green and Water in Motion.
transfer station pump detail
Eagle Valley Stormwater Transfer Pump Station detail. Image courtesy HR Green and Water in Motion.
pump intake detail
Irrigation pump station intake flume detail. Image courtesy HR Green and Water in Motion.
  • Location: Woodbury, MN
  • Owner: City of Woodbury/Prestwick Golf Club Inc
  • Designer: HR Green, Water in Motion
  • Year of Completion: 2014
  • Design Features:
    • Eagle Valley - large storage pond and babbling brook landscape feature for holding and moving stormwater runoff and reuse
    • Prestwick – creation of one large storage pond from two smaller ponds for holding and moving stormwater runoff for irrigation reuse
  • Total Drainage Area: 430 acres (Eagle Valley), 130 acres (Prestwick)
  • Total Construction Cost: $700,000.00 (funding from South Washington Watershed District/Clean Water Fund)
  • Pretreatment: screen filter on intake pump
  • Documented Maintenance Practices: winterize irrigation system each year, clean pump screen if it gets clogged (has not been needed during first three years of operation)
  • Pollutant Removal: 99 lbs phosphorus per year, and 9.6 Acre/ft per event volume reduction
  • Is the site publicly accessible: Yes
Project Background

In 2014, the City of Woodbury, with funding from the South Washington Watershed District, constructed stormwater reuse for irrigation systems at Eagle Valley and Prestwick golf clubs. Each system collects runoff from a large drainage area containing roads, housing developments, and a golf course and stores it in a centralized pond. A pump then draws water from the pond for use in golf course irrigation. The total cost of both projects was $700,000.00. The catalyst for the projects was the planned reconstruction of CSAH 19 (Woodbury Drive), changing it from a rural 2-lane road to an urban 4-lane road. The goals for the project were to have no measurable downstream stormwater impacts from the road reconstruction project, and to help achieve Colby Lake’s target TMDL standard to reduce phosphorus (TP) inputs by 30 lbs per year.

Eagle Valley Golf Course

Prior to the reuse for irrigation project Eagle Valley Golf Course pumped 30 million gallons of well water annually to irrigate approximately 60 acres of golf course turf and landscaping. This project collects stormwater runoff from a 430 acre drainage area covering the golf course, surrounding neighborhoods, and Woodbury Drive into a storage pond on the course. The drainage area is 33.8% impervious. The reuse system can pump approximately 22.5 million gallons of stormwater for irrigation from the pond annually. The remainder of irrigation water that is needed will be supplied by the existing well. Stored water can also be routed through a babbling brook landscape feature on the golf course. The P8 modeled water quality benefits of the project at Eagle Valley were reductions in TP to Colby Lake of 56 lbs per year, and a volume reduction of 6.1 acre/ft per event.

Prestwick Golf Club

Prestwick golf club regularly irrigates up to 75 acres of turf and landscaping. Previously, course managers pumped 35 million gallons of water annually from a course well to accomplish this. The stormwater reuse system which was installed in 2014 can now supply approximately 17.5 million gallons of water from the storage pond annually. The 130 acre drainage area for the pond is 27% impervious. The remainder of irrigation water that is needed will be supplied by the existing well. The P8 modeled water quality benefits of the project at Prestwick were reductions in TP of 43 lbs per year, and a volume reduction of 3.5 acre/ft per event.

Conclusion

The City of Woodbury far exceeded their goals for water quality and volume reduction with the implementation of these two reuse projects at Prestwick and Eagle Valley. In total the projects reduce TP loading to local lakes by almost 100 lbs per year, and also reduces volume by 9.6 acre/ft for each event. The goals were 35 lbs of TP per year, and 1.84 acre/feet per event. In addition, both projects combine to reduce pumping from local aquifers by 40 million gallons annually by utilizing surface storage features (ponds) that add beauty and challenge to both golf courses.

Carver County Club West Development

image reuse plan
Carver County Club West storm sewer reuse plan. Image courtesy Alliant Engineering.
images of system components
Rainwater harvest components. Images courtesy Alliant Engineering.
  • Location: Harvest Estates, Chaska, MN
  • Project Owner: Club West Partners LLC
  • Project Designer: Alliant Engineering
  • Year of Completion: 2015
  • Design Features:
    • Gallons used for irrigation annually: Permitted for 22.2 million gallons
    • Use for water: Irrigation of Lawn in Park, Boulevard Plantings and Flower Beds
    • Rainwater Harvester Control Panel: Regulates operation of irrigation systems
  • Total Drainage Area:
    • Pond 1 = 28 acres
    • Pond 3 = 25 acres
  • Drainage Area Surface Type: Residential (roads, driveways, turf, etc.)
  • Pond Size:
    • Pond 1 = 18,600 sq ft
    • Pond 3 = 22,413 sq ft Irrigated area
    • Pond 1: 30,807 square feet
    • Pond 3: 42,267 square feet
  • Cost Savings Per Year: $3,000 on irrigation water (compared to City potable water rates)
  • Pretreatment: NURP pond and screen filter in intake pump
  • Methods of Filtration: Filter Screens
  • Documented Maintenance Practices: winterize irrigation system each year, clean clogged pump screen (annual startups and blow outs)
  • Pollutant Removal: 4.37 lb TP and 1,503 lb TSS per year
  • Is the site publicly accessible? Yes - Park and boulevards are central features to the neighborhood. With the systems in close proximity to the neighborhood and park entrances, awareness of the system is heightened.

Club West Partners, the developer of Harvest Estates, worked with the Carver County Watershed Management Organization (CCWMO) to implement a stormwater harvest and use system that would meet stormwater, TP, and TSS reduction requirements (90% TSS and TP reduction, per CCWMO rules) for that subwatershed. Designed by Alliant Engineering, the harvest and use project utilizes stormwater runoff draining from a new development, Harvest Estates, for irrigation of common areas within the development. The site is located in southwest Chaska, MN, in close proximity to the Minnesota River, where population growth trends continue to encroach on undeveloped areas of the watershed. This project is one of five sites in Carver County where stormwater is being used to irrigate green space in order to mitigate the impacts of population growth on groundwater withdrawals.

At Harvest Estates, stormwater is collected into three stormwater ponds. Two intake pumps draw the stormwater from two of the three ponds to irrigate a central park and boulevard at the entrance of the development. By incorporating the harvest and use system into the construction of the new development, overall stormwater runoff impacts are partially mitigated and use of potable water for irrigation is reduced. Up to 22.2 million gallons of harvested stormwater can be used to irrigate the park and boulevards. The project goal is to harvest and use up to 12,500 cubic feet of stormwater per week, or equivalent to a depth of ½”of stormwater over the surface area of the new impervious paving in the development (18.8 acres total). The harvest and use system uses a 1.5-2.0 HP centrifugal stormwater pump, which is then transferred and managed through irrigation controller pedestals that allow for the efficient harvest and use of stormwater. The system reduces the need to pump irrigation water from conventional sources, with a bypass switch for municipal water only in cases of decreased rainfall. Harvested stormwater pumped from the two ponds goes through a filter system to remove sediment prior to irrigation uses. The modeled water quality benefits of the project are TP reductions up to 4.37 lb per year and TSS reductions up to 1,503 lb per year. To date, roughly 1.5 million gallons of stormwater have irrigated the site through the harvest and use system, with an overall cost savings of $3,000 each year due to the decreased use of potable water.

Mississippi Watershed Management Organization (MWMO)

This picture shows a cistern located at Mississippi Watershed Management Organization
Cistern located at Mississippi Watershed Management Organization. Photo by MWMO Staff. To enlarge, click on image.
This picture shows a cistern located at Mississippi Watershed Management Organization
Cistern located at Mississippi Watershed Management Organization. Photo by MWMO Staff. To enlarge,click on image.

Name of Project: Mississippi Watershed Management Organization (MWMO)
Type of Reuse System: Cistern
Overview: The cistern, located at the MWMO, utilizes rooftop runoff from the main office area roof to water trees in a trench system.
Location: Mississippi Watershed Management Organization, 2522 Marshall Street NE, Minneapolis 55418
Owners: Mississippi Watershed Management Organization
Contractors: Meisinger Construction-general contractor for facility construction
Operators: Mississippi Watershed Management Organization
Cost (additional plumbing would be needed if plumbed into the building to be used as grey water):

  • Concrete Slab - $5,200
  • Cistern (including piping) – $23,700
  • Rain Leader- $8,400


Type and size of system: 4,000 gallon capacity CorGal Water Tank, volume annual dependent on rainfall, filled by roof runoff only
Year of Completion: 2012
Drivers/Stormwater Goals: To water trees, reduce stormwater runoff that reaches the river, infiltrate rainwater. Future use as grey water in facility.
Funding sources: Public – MWMO funding
Monitoring: Total Suspended Solids, VSS, bacteria, once every 3 months
Web links: http://mwmo.org/
Lessons Learned: Design flaws – the plumbing was not correct and all the water drained out, the rain leader diverter operation was not intuitive. Additionally plumbing code hindered process of connecting to facility to be used as grey water. Staff opted to do this at a later time once plumbing codes were updated and other projects set precedence.
For more information contact:

Doug Snyder, Mississippi Watershed Management Organization Executive Director
E-mail: Dnsyder@mwmo.org
Phone: 612-746-4971

Links to Other Case Studies in Minnesota

Below are Minnesota examples of outdoor and indoor reuse systems.

Outdoor use

  • The City of St. Anthony Village water reuse facility collects stormwater runoff from a county road, city hall, local streets and backwash water from the City's water treatment plant in a half million gallon reservoir located underground. Water stored in the reservoir is recycled to irrigate a 20 acre site that includes a municipal park and St. Anthony's City Hall campus. For more information, watch the City of St. Anthony video or go to the Mississippi Watershed Management Organization's web site.
  • The Oneka Ridge Golf Course project in the City of Hugo, collects and stores stormwater runoff from nearly 1,000 acres of land upstream of Bald Eagle Lake and uses it, instead of pumped groundwater, to irrigate 116 acres with the golf course. For more information, go to the Rice Creek Watershed District web page
  • Target Field, home of the Minnesota Twins, collects stormwater in a 200,000 gallon cistern. This water is treated and used to irrigate the ball field, reducing city water use by 2 millions gallons per year. Read Greening the Ballpark for more information.
  • The Eagle Valley and Prestwick Golf Courses in Woodbury are installing two large scale water re-use systems that will capture urban runoff and excess nutrients that would otherwise flow into Colby and Bailey Lakes and use it for irrigation. This project was funded in 2013 with a Clean Water Fund grant. The *City of Woodbury has additional reuse and irrigation projects in the city-Windwood Park, Bielenberg Sports Center, Views at City Walk, Bielenberg Gardens and St. Therese. For more information on these projects, contact Sharon Doucette, Environmental Resources Coordinator at sdoucette@ci.woodbury.mn.us
  • The Minnesota National Guard Facility in Arden Hills has an extensive water-collection system that stores rainwater in a 25,000-gallon underground cistern for reuse in wash bays with a second 20,000-gallon tank to filter rainwater for irrigation.
  • Maplewood Mall has a 5,700 gallon cistern that catches roof runoff.
  • Carver County has five sites where stormwater is being to irrigate balllfields and turf. They are Beise Addition, Chevalle, Club West, Copper Hills and Waconia School District. Carver County Watershed Management Organization also has stormwater Reuse Guidelines.
  • The City of Cottage Grove installed a rainwater harvesting system that captures, stores and cleans City Hall rooftop runoff, which is then used to irrigate planting areas throughout the site. The system reduces the City's annual water usage by 570,000 gallons. This project was partially funded with a grant from the South Washington Watershed District. For more information, contact Jennifer Levit, City of Cottage Grove at jlevitt@cottage-grove.org
  • The City of Lakeville has a stormwater pond irrigation system located in King Park. The project was completed cooperatively by the City of Lakeville and the Vermillion River Watershed Joint Powers Organization.
  • Edison High School in Northeast Minneapolis will be irrigating the athletic field using stormwater runoff from the gymnasium roof and new plaza. For more information, go to the Mississippi Watershed Management Organization's web site.
  • The City of Roseville and the Capitol Region Watershed District are constructing a project to treat stormwater and store it for use as irrigation at the softball field at Upper Villa Park.
  • File:New Hope Rainwater Harvesting and Water Quality System Sign High Res.pdf

Indoor use

  • The City of St. Paul's Lowertown Ball Park (CHS Field) is using rainwater for toilet flushing and for irrigation of the ballpark. The cistern was installed in February, 2015 and is now operational. For more information, contact Wes Saunders-Pearce/Water Resources Coordinator at 651- 266-9112 or Wes.Saunders-Pearce@ci.stpaul.mn.us. Information about the project (including an informative video) is available at the Metropolitan Council's web site .
  • Great River Energy in Maple Grove: Rainwater from the rooftop is collected in a 20,000-gallon cistern, treated with ultraviolet light, and used for toilet and urinal flushing.
  • The new Target Field Station (The Interchange) features the first-ever, year-round snowmelt and stormwater runoff capture and reuse system in Minnesota. Snowmelt or stormwater is collected in cisterns and then pumped to the Hennepin Energy Recovery Center (HERC), a nearby waste-to-energy facility that burns municipal waste to generate energy. The HERC facility uses the water in a variety of industrial processes, thereby reducing the facility's dependence on the municipal water supply. In total, the stormwater system will direct approximately 1 million gallons of stormwater runoff per year toward the HERC facility for reuse. For more information, go to the Mississippi Watershed Management Organization's web site.
  • The City of Shoreview's maintenance facility captures rain water runoff from the roof in an underground tank that is used for toilet flushing and vehicle washing. For more information contact Mark Maloney at the City of Shoreview: mmaloney@shoreviewmn.gov
  • The University of Minnesota's 17th Avenue Residence Hall is collecting rainwater from the roof, holding it in a 35,000 gallon cistern and using the water to flush 200 toilets. For more details about this project, visit the Metropolitan Council's web site.
  • Schaar's Bluff Gathering Center in Dakota County utilizes rainwater harvesting for toilet flushing.


Calculating credits for stormwater and rainwater harvest and use/reuse

image
Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption
Information: This manual currently does not contain information on models and calculators developed specifically for rainwater/stormwater harvest and use/reuse systems. Once that information is developed it will be incorporated into this page.
Recommended pollutant removal efficiencies, in percent, for infiltration BMPs. Sources.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
Pollutant removal is 100 percent for the volume that is captured and infiltrated

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how harvest and use/reuse practices can achieve stormwater credits. It is assumed that captured water is applied as irrigation and that all irrigation water infiltrates. To view the credit articles for other BMPs, see the Related pages section.

Overview

This schematic shows Example Stormwater Harvesting and Use System Schematic
Schematic illustrating the basic concepts for a harvest and use-reuse system. Stormwater or rainwater is stored in a cistern or pond and delivered outdoors or indoors through a distribution system.

Stormwater and rainwater harvest and use/reuse systems capture and store runoff. The stored water is typically utilized for irrigation. This water is assumed to infiltrate. Credits for these BMPs are therefore similar to credits for other infiltration practices in that all water applied for irrigation and pollutants in that water are credited. The methodology differs, however, in that the water is captured instantaneously, but use of the water is dependent on the irrigation rate rather than the soil infiltration rate, as is the case with infiltration BMPs. The period of use is also during the growing season, meaning the generated credits only apply at that time. If harvested water is used indoors, it may be discharged to a sewer system, to a septic drainfield, or to another stormwater BMP. Credits for these vary and are discussed below.

Pollutant removal mechanisms

Harvest and use/reuse practices reduce stormwater volume and pollutant loads through infiltration of the captured and stored stormwater runoff into the native soil. All pollutants in infiltrated water are considered to be removed from the stormwater conveyance system. Because infiltration typically occurs on turf or other vegetated media, a wide variety of stormwater pollutants will be retained through secondary removal mechanisms including filtration, biological uptake, and soil adsorption through plantings and soil media (WEF Design of Urban Stormwater Controls, 2012). See Other Pollutants, for a complete list of other pollutants retained by filtration practices.

Location in the treatment train

Stormwater Treatment Trains are comprised of multiple Best Management Practices that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. The position of a harvest and use/reuse system in a treatment train is a function of the surface from which the water is being collected. Rainwater harvest systems, which are designed to collect water from rooftops, will generally be located near the beginning of the treatment train, while systems that store water in ponds will be located near the end of treatment trains.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article. If harvest water is being infiltrated, this practice is also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and approach

In developing the credit calculations, it is assumed the harvest and use/reuse system is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see Stormwater and rainwater harvest and use/reuse

Warning: Pretreatment is required for all infiltration practices

In the following discussion, the water quality volume (VWQ) is delivered instantaneously to the BMP. VWQ is stored in a cistern or a pond. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch off new impervious surface. For MIDS, VWQ is 1.1 inches.

The approach in the following sections is based on the following general design considerations:

  • Credit calculations presented in this article are for both event and annual volume and pollutant load removals.
  • Stormwater volume credit equates to the volume of runoff that will ultimately be infiltrated into the soil.
  • TSS and TP credits are achieved for the volume of runoff that is infiltrated.

Volume Credit Calculations

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil from the existing stormwater collection system. These credits are assumed to be instantaneous values. However, unlike other stormwater infiltration practices, for an irrigation system, the volume credit is a function of both the water available for storage, the rate at which water is applied, and the area over which the water is applied.

If we assume that on average there are 3 days between rain events, the volume reduction capacity of the BMP (V) that counts toward a performance goal is equal to either the storage capacity of the storage device or the amount of water that is used for irrigation and non-irrigation over a three day period, whichever value is lowest.

<math>V=min[S; A_I * R_I * 3 days * 1556 + V_{nonirrigation}]</math>

Where

S is the storage volume of the storage container in ft3;
AI is the irrigation application area in acres;
RI is the calculated average achieved weekly irrigation rate May-August (inches per week);
1556 is a conversion factor; and
Vnonirrigation is the volume used for non-irrigation purposes, in cubic feet.

This credit can only be applied during the time of year when the irrigation system is in practice. To determine compliance with a performance goal throughout the year, we need to know the annual volume of runoff and the volume of water applied as irrigation. The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator and the Simple Method. Example values are shown below for a scenario using the MIDS calculator. For example, if a harvest and use/reuse system captures and uses 68 percent of the annual runoff volume on B soils, the system is capturing the equivalent of 0.5 inches of runoff annually, even though it may be capturing considerably more during the time of year when the system is operating.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and water quality volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


The above calculations may include nonirrigated uses. The nonirrigated uses will need to be translated into the correct units.

Total phosphorus credit calculations

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

<math> M_{TP_i} = 0.0000624\ V_{inf_b}\ EMC_{TP} </math>

where

EMCTP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter).

The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. The above calculation may be applied on an annual basis and is given by

<math> M_{TP_f} = 2.72\ V_{annual}\ EMC_{TP} </math>

where

Vannual is the annual volume treated by the BMP, in acre-feet.

Total suspended solid (TSS) calculations

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, MTSSi in pounds, is given by

<math> M_{TSS_i} = 0.0000624\ V_{inf_b}\ EMC_{TSS} </math>

where

  • Vinfb is the volume of water infiltrated, in cubic feet; and
  • EMCTSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on EMC values for TSS, link here. The above calculation may be applied on an annual basis and is given by

<math> M_{TSS_f} = 2.72\ F\ V_{annual}\ EMC_{TSS} </math>

where

Vannual is the annual volume treated by the BMP, in acre-feet.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator.

Methods for calculating credits

This section provides specific information on generating and calculating credits from infiltration practices for volume, TSS and TP. Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

  1. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
  2. The Simple Method and MPCA Estimator
  3. MIDS Calculator
  4. Quantifying volume and pollution reductions based on values reported in literature
  5. Quantifying volume and pollution reductions based on field monitoring

Credits based on models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for infiltration practices. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

  1. Model name and version
  2. Date of analysis
  3. Person or organization conducting analysis
  4. Detailed summary of input data
  5. Calibration and verification information
  6. Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for biofiltration BMPs. Sort the table by Infiltrator BMPs to identify BMPs that may include infiltration practices.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Link to this table
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SELECT X X X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.


The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of mean pollutant concentrations and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin/trench, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Warning: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

Download MPCA Estimator here: File:MPCA Estimator.xlsx

A quick guide for the estimator is available Quick Guide: MPCA Estimator tab.

MIDS Calculator

mids logo
Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including infiltration practices. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits Based on Reported Literature Values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the pond or wetland device. A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page here. Designers may use the pollutant reduction values or may research values from other databases and published literature. Designers who opt for this approach should

  • select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
  • select a pollutant removal reduction from literature that studied a bioretention device with site characteristics and climate similar to the device being considered for credits;
  • review the article to determine that the design principles of the studied bioretention are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
  • give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed stormwater pond, considering such conditions as watershed characteristics, pond sizing, and climate factors.

  • International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals.
    • Compilation of BMP performance studies published through 2011.
    • Provides values for TSS, Bacteria, Nutrients, and Metals
    • Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels.
  • Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon.
    • Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations.
    • Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics.
    • Applicable to Filters, Wet Ponds, Porous Pavements, Soakage Trenches, Flow through Stormwater Planters, Infiltration Stormwater Planters, Vegetated Infiltration Basins, Swales, and Treatment Wetlands.
  • The Illinois Green Infrastructure Study.
    • Figure ES-1 summarizes BMP effectiveness
    • Provides values for TN, TSS, peak flows / runoff volumes
    • Applicable to Permeable Pavements, Constructed Wetlands, Infiltration, Detention, Filtration, and Green Roofs
  • New Hampshire Stormwater Manual.
    • Volume 2, Appendix B summarizes BMP effectiveness
    • Provides values for TSS, TN, and TP removal
    • Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
  • BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
    • Appendix B provides pollutant removal performance curves
    • Provides values for TP, TSS, and Zn.
    • Pollutant removal broken down according to land use.
    • Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement.

Credits based on field monitoring

Field monitoring may be used to calculate stormwater credits in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

  1. Establish the objectives and goals of the monitoring.
    1. Which pollutants will be measured?
    2. Will the monitoring study the performance of a single BMP or multiple BMPs?
    3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    4. Will the results be compared to other BMP performance studies?
    5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
  2. Plan the field activities. Field considerations include:
    1. Equipment selection and placement
    2. Sampling protocols including selection, storage, delivery to the laboratory
    3. Laboratory services
    4. Health and Safety plans for field personnel
    5. Record keeping protocols and forms
    6. Quality control and quality assurance protocols
  3. Execute the field monitoring
  4. Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

  • Chapter 2: Designing the Program
  • Chapters 3 & 4: Methods and Equipment
  • Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
  • Chapter 7: BMP Performance Analysis
  • Chapters 8, 9, & 10: LID Monitoring
Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes the following chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP:

  • Chapter 4: Stormwater Characterization
    • 4.2: General Characteristics and Pollutant Sources
    • 4.3: Sources of Stormwater Quality data
  • Chapter 8: Performance Evaluation
    • 8.1: Methodology Options
    • 8.5: Evaluation of Quality Performance for Individual BMPs
    • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
  • Chapter 10: Hydrologic Evaluation
    • 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices.

In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The information contained in this White Paper would be of use to those considering the monitoring of a manufactured BMP. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include:

  • Chapter 4: Monitoring Methods and Equipment
  • Chapter 5: Analytical Methods and Laboratory Selection
  • Chapter 6: Monitoring Site Selection
  • Chapter 8: Equipment Installation and Maintenance
  • Chapter 10: Pre-Storm Preparation
  • Chapter 11: Sample Collection and Handling
  • Chapter 12: Quality Assurance / Quality Control
  • Chapter 13: Laboratory Reports and Data Review
  • Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice, involving:

  • Level 1: Visual Inspection
  • Level 2: Capacity Testing
  • Level 3: Synthetic Runoff Testing
  • Level 4: Monitoring
  • Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other Pollutants

In addition to TSS and phosphorus, infiltration practices can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that infiltration practices are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD). A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Relative pollutant reduction from bioretention systems for metals, nitrogen, bacteria, and organics.
Link to this table

Pollutant Category Constituent Treatment Capabilities

(Low = < 30%; Medium = 30-65%;

High = 65 -100%)
Metals1 Cr, Cu, Zn High2
Ni, Pb
Nutrients Total Nitrogen, TKN Medium/High
Bacteria Fecal Coliform, E. coli High
Organics High

1 Results are for total metals only
2 Treatment capabilities are based mainly on information from sources that referenced only metals as a category and did not provide individual efficiency for specific metals


References and suggested reading


Definitions for stormwater and rainwater harvest and use/reuse

  • Backwash: Water that is pumped in reverse through filters, removing trapped sediment and other collected material
  • Beneficial use of stormwater: Use of stormwater to meet water demands, including but not limited to: irrigation, drinking, washing, bathing, cooling, and flushing. Commonly referred to as “reuse.”
  • Cistern: A tank that stores water.
  • Collection efficiency: Harvested water volume as a percent of the total rainfall on the source area during a certain period of time.
  • Disinfection: Reduction of viable micro-organisms to a level that is deemed suitable for the intended use.
  • Dry running protection: System for protecting a water pump against running when no water is present.
  • First flush device: A device that diverts runoff generated during the beginning of a rainfall event, which carries higher levels of debris and contaminants from collection surfaces, from entry into storage components. Only suited for warm weather applications. Extreme caution should be observed in cold climates.
  • Green Roof: a rooftop treatment practice where a thin planting media is established on roof surfaces and then planted with hardy, low–growing vegetation
  • Harvested water: Water that is collected from impermeable surfaces, such as rooftops and parking lots, and stored for future use.
  • Make-up water supply: Municipal water or other reliable water source that is used to supply water for beneficial use in the event that harvested water is not available.
  • Non-potable water: Water that does not meet drinking water standards.
  • Overflow Siphon: Functions as a trap to keep out vermin and also to evacuate floating pollen and debris in the tank during rain events.
  • Potable water: Water that meets drinking water standards.
  • Pre-storage treatment: Best management practices that are used upstream of the storage unit.
  • Pretreatment: Treatment of harvested stormwater prior to entering the storage unit; typically processes that remove trash, gross solids, and particulate matter.
  • Post-storage treatment: Practices that are used to remove fine particulates, dissolved pollutants and microorganisms from harvested rainwater.
  • Rainwater: A form of stormwater that is collected directly from roof surfaces which can have lower levels of pollutants than other sources of stormwater.
  • Recycled Water: Water which, as a result of treatment of waste, is suitable for direct beneficial use or a controlled use that would not otherwise occur and is therefore considered a valuable resource (Current California Water Code 13050-13051)
  • Reuse: Use of stormwater, greywater, or blackwater to meet water demands, including but not limited to: irrigation, drinking, washing, cooling, and flushing.
  • Runoff: The portion of rainfall or snowmelt not infiltrated, evaporated, or transpired that drains or flows over the land and becomes surface flow.
  • Source area: Surface area from which water is harvested (e.g., rooftop, roadway, green space).
  • Stormwater: Rainfall or snowmelt that runs off surfaces.
  • Sub-surface irrigation: Water that is applied below ground level for plants and is not directly exposed to above ground surface and/or air.
  • Surface irrigation: Water that is applied above ground level for plants and is directly exposed to the above ground surface and/or air.
  • System pressure: Pressure needed to deliver water to the designated fixtures.
  • Treatment: Treatment of harvested stormwater after storage but prior to distribution; typically processes that remove dissolved pollutants and bacteria.
  • Wastewater: Used or spent water containing pollutants or solids and discharged from homes, commercial establishments, farms, or industries.
  • Water harvesting: The process of capturing and retaining water for beneficial uses at a different time or place than when or where the water was generated.
  • Water yield: The volume of harvested water over a certain period of time.


Requirements, recommendations and information for using Harvest and re-use/cistern as a BMP in the MIDS calculator

Volume retention is achieved in the harvest and re-use/cistern BMP through re-use of the water for irrigation or other acceptable purposes (e.g. toilet flushing). The type of storage device is not considered in the calculator (e.g. pond or cistern). Any pollutant associated with the water used for irrigation or non-irrigation uses is removed from the system. Pollutants in water that bypasses the storage device or not used for irrigation or non-irrigation uses are not removed.

MIDS calculator user inputs for harvest and re-use/cistern

symbol for harvest and reuse in the MIDS calculator
MIDS calculator symbol for harvest and reuse.
watershed tab for harvest reuse in MIDS calculator
Screen shot of the Watershed tab for the MIDS calculator harvest and reuse.
screen shot BMP Parameters tab harvest reuse
Screen shot of the BMP Parameters tab for MIDS calculator harvest and reuse.

For a harvest and reuse/cistern system, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.

  • Watershed tab
    • BMP Name: This cell is auto-filled, but can be changed by the user.
    • Routing/downstream BMP: If this BMP is part of a treatment train and overflow water that bypasses the storage device is being routed from this BMP to another BMP, the user selects the name of the BMP from the dropdown box to which water is being routed. All water must be routed to a single downstream BMP. Note that the user must include the BMP receiving the routed water in the Schematic tab or the BMP will not appear in the dropdown box.
    • BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP. Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for three Land Cover types (Forest/Open Space, Managed Turf and impervious). The surface area of the storage device should not be included in the BMP watershed area. If the storage device is enclosed it is assumed that runoff from the surface area of the container will not be routed into the unit. If the storage container is open (i.e. pond) it is assumed that precipitation and evapotranspiration at the surface of these systems are neutralizing. Units are in acres.
  • BMP Parameters tab
    • Reuse storage volume (S): This is the storage volume of the storage device used to collect runoff to be used for irrigation or non-irrigation purposes. This can be a single storage container or a combined volume from multiple storage containers onsite. Units are in cubic feet.
    • Irrigation application area (AI): This is the area that will be irrigated during the irrigation season. This can be a single area or a combined area from multiple irrigation sites. Units are in acres.
    • Provide user defined maximum weekly irrigation rate?: A dropdown menu asks the user if they are setting a maximum value to the amount of irrigation applied each week. If the user answers "No", then the irrigation rate is determined based on plant water demand (PET or potential evapotranspiration).
    • User defined maximum irrigation application rate: This defines the maximum irrigation rate applied to the irrigation area during the irrigation season. If the rate exceeds the PET, the irrigation rate is the PET. Units are in inches/week. Guidance on watering lawns and turf, including recommended irrigation rates, are found at [4].
    • Irrigation season start month: This is the starting month of the irrigation season (i.e. the first month during which water stored in the storage container is used for irrigation).
    • Irrigation season end month: This is the ending month of the irrigation season. After this month runoff is either collected and stored in the storage container until the next irrigation season or bypasses the system.
    • Does the system go offline during the off season?: This is a YES/NO question. Answering YES means that the system goes off line during months outside of the irrigation season. The reuse storage device is emptied at the end of the irrigation season and any runoff generated bypasses the system. Answering NO means that the storage device will continue to collect runoff during the off season until it fills. This runoff will then be available at the start of the next irrigation season. Typically, the answer will be "Yes" for a cistern and "No" for a pond.
    • Is water retained on-site for non-irrigation uses?: This is a YES/NO question. Answering "Yes" allows the user to specify a volume of water taken from storage and used for non-irrigation purposes, such as toilet flushing.
    • Weekly water volume retained for non-irrigation uses: This is the weekly volume of water retained for uses other than irrigation, such as toilet flushing. Units are cubic feet per week.
  • BMP Summary Tab: The BMP Summary tab summarizes the volume and pollutant reductions provided by the specific BMP. It details the performance goal volume reductions and annual average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary are the total volume and pollutant loads received by the BMP from its direct watershed, from upstream BMPs and a combined value of the two. Also included in the summary, are the volume and pollutant load reductions provided by the BMP, in addition to the volume and pollutant loads that exit the BMP through the outflow. This outflow load and volume is what is routed to the downstream BMP if one is defined in the Watershed tab. Finally, percent reductions are provided for the percent of the performance goal achieved, percent annual runoff volume retained, total percent annual particulate phosphorus reduction, total percent annual dissolved phosphorus reduction, total percent annual TP reduction, and total percent annual TSS reduction.

Methodology

Required Treatment Volume

Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches for MIDS or user-specified performance goal) times the impervious area draining to the BMP. This value also includes any left over water routed to this BMP from upstream BMPs. This stormwater is delivered to the BMP instantaneously.

Caution: Note that if a first flush diverter is used, the volume diverted must be removed from the calculations.

Volume Reduction

The volume reduction achieved by a BMP compares the capacity of the BMP to the required treatment volume. The Volume reduction capacity of BMP [V] is calculated using BMP inputs provided by the user. The harvest and re-use/cistern BMP achieves volume reduction through the capture of stormwater runoff into a storage device and the subsequent release of the stored water over an irrigation area or for a non-irrigation use such as toilet flushing. The volume reduction capacity of the BMP (V) that counts toward the performance goal is equal to either the storage capacity of the storage device or the amount of water that is used for irrigation and non-irrigation over a three day period, whichever value is lowest.

<math>V=min[S; A_I * R_I * 1556 + V_{nonirrigation}]</math>

Where

S is the storage volume of the storage container in ft3;
AI is the irrigation application area in acres;
RI is the calculated average achieved weekly irrigation rate May-August (inches per week);
1556 is a conversion factor; and
Vnonirrigation is the volume used for non-irrigation purposes, in cubic feet.

May to September was chosen to take into count the peak growing season and provides a higher credit then using the average throughout the irrigation period. The average achieved irrigation application rate is adjusted to a 3 day period between expected rain events and multiplied by the application area.

Analysis of rainfall patterns indicates that a typical time period between precipitation events is 72 hours in Minnesota. Therefore, the irrigation and non-irrigation use is applied over a 3 day time period. Derivation of RI is discussed below.

The Volume of retention provided by BMP is the amount of volume credit the BMP provides toward the performance goal. This value is equal to Volume reduction capacity of BMP [V], calculated using the above method, as long as the volume reduction capacity is less than or equal to Required treatment volume. If Volume reduction capacity [V] is greater than Required treatment volume, then the BMP volume credit is equal to Required treatment volume. This check makes sure that the BMP is not getting more credit than the amount of water it receives. For example, if the BMP is oversized, the user will only receive credit for Required treatment volume routed to the BMP.

Calculating irrigation rates

plot of PET over time
Statewide average weekly potential evapotranspiration (PET) from May 1 through September 30. See the discussion for information on data used to compile this plot.

Two methods are used to calculate irrigation rate. The first is based on plant water demand, called potential evapotranspiration (PET). A daily rate was calculated using data collected from the University of Wisconsin Extension. Daily PET rates between January 1, 2000, and December, 2015 were compiled in an Excel spreadsheet. Data for 11 locations in Minnesota was compiled individually for each location. The locations were Bemidji, Thief River Falls, Moorhead, Marshall, Morris, St. Cloud, Rochester, Hibbing, Minneapolis, Brainerd, and Duluth. Data for January, February, and December were discarded. The average daily PET was calculated for each location using the long-term data. State zip codes were then associated with one of these locations. Thus, when a user enters a zip code in the MIDS calculator, the PET record for that location is associated with the PET data for one of the ten cities. The Excel spreadsheet containing this information is File:ET data.xlsx. The weekly irrigation rate is based on the average achieved irrigation application rate from May through September.

The second method is based on the user defined maximum daily irrigation rate. This is input as a constant value during the specified irrigation period.

Irrigation rates from the two methods are compared on a daily basis. The lower of the two values is chosen as the irrigation rate for that day.

<math>((R_I/7) * 3 * A_i *43560)/12</math>

Where

RI is the calculated average achieved weekly irrigation rate May-August (inches per week) and
Ai is the application area (acres)

May to September was chosen to take into count the peak growing season and provides a higher credit then using the average throughout the irrigation period. The average achieved irrigation application rate is adjusted to a 3 day period between expected rain events and multiplied by the application area.

On a day in which it rains, irrigation can occur such that rainfall plus irrigation for that day equals RI. For example, if PET is 0.90 inches per week, this would be 0.129 inches per day. If the user defined maximum was 1 inch per week, this would be 0.143 inches per day. The lower value is the PET and that would be used to compute irrigation applied that day. If 0.1 inches of rain fell that day, then (0.129 - 0.100) = 0.029 inches of water could be applied as irrigation on that day.

Finally, irrigation rates are adjusted for crop type (1, 2, 3). The irrigation rate is multiplied by the following coefficients to give the final irrigation rate.

  • Turf = 0.95
  • Vegetables = 0.95
  • Forages = 0.85
  • Cereals = 0.75
  • Trees = 1.0

Pollutant Reduction

Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is to determine an annual volume reduction based on the design parameters. This is accomplished through a daily time step water balance imbedded within the MIDS calculator. The water balance tracks watershed runoff, the reuse storage volume that is available, the amount of watershed runoff that bypasses the storage system, and the application volume. Runoff is routed to the storage device until it is filled. Once filled, overflow water bypasses the system. Water is released from the storage device during the irrigation period at a flow equal to the irrigation rate times the irrigation area, during days that precipitation does not happen. Irrigation only occurs if water is present in the storage device. If the volume of water present in the storage device on a particular day is less than the irrigation rate only the volume of water present in the storage device will be removed from the storage device via irrigation. At the end of the irrigation season, water either remains in the storage device while runoff is continually collected or the storage device is emptied and all runoff bypasses the system. This is dependent on the answer to the question Does the system go offline during the off season.

Daily watershed runoff volumes were generated for a 10 acre site using the P8 model, during precipitation and snowmelt events over a 30 year period (years 1974 – 2004). The average annual precipitation throughout the 30-year period used for the P8 modeling was 28.8 inches. The modeled daily runoff values are adjusted to match site conditions based on the size of the contributing watershed and the annual average precipitation for the site. The adjustment factor is given by

<math>adjustment= A_W / (10 acres) * P_A / (28.8 inches)</math>

Where

AW is the contributing watershed area, in acres, to the harvest and re-use/cistern BMP; and
PA is the annual average precipitation amount, in inches, based on the zip code of the site.
  • The resulting daily runoff values adjusted for the site watershed area and annual precipitation are used in the water balance. The water balance is used to calculate the annual volume reduction, which is equal to the annual average volume of water used for irrigation divided by the total annual average runoff volume.
  • All pollutants in the stormwater used for irrigation are 100 percent removed. Any water that bypasses the re-use system or is not used for irrigation results in a 0 percent pollutant removal.

NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.

Routing

Overflow from a harvest and re-use/cistern BMP can be routed to any other BMP, except for a green roof, a swale side slope, and any BMP in a stormwater treatment sequence that would cause stormwater to be rerouted back to the harvest and re-use/cistern BMP already in the treatment sequence. All BMPs can be routed to a harvest and re-use/cistern BMP.

Assumptions for harvest and re-use/cistern

The following general assumptions apply in calculating the credit for harvest and re-use/cistern BMP. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.

  • The re-use system is constructed and design according to design criteria.
  • The irrigation area can accept water at the irrigation rate defined.

Harvest and re-use/Cistern Example (Version 2)

schematic used for reuse example
Schematic for MIDS calculator example for harvest and re-use. In this example, a 0.5 acre roof drains to a cistern, where the water is stored until used to irrigate a 0.5 acre area. See Step 1.
screen shot site information tab reuse example
Screen shot of the MIDS calculator Site Information tab for the harvest and reuse example. See Step 2.
screen shot schematic tab for harvest reuse
Screen shot of the MIDS calculator Schematic tab for the harvest and reuse example. See Step 3.

A cistern is going to be constructed to collect runoff from a 0.5 acre roof. The cistern will be constructed to hold 1995 cubic feet of water and will irrigate a 0.5 acre lawn area at an average irrigation rate of 1 inch per week. Irrigation will occur from May through September after which the cistern will drain completely and not collect runoff during the off season. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have a 0.5 acre site with all 0.5 acres being impervious. The entire roof area is draining to a cistern. The cistern area and irrigation area are not included in the watershed because runoff from these two areas do not drain into the cistern BMP.

Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the Harvest and re-use/Cistern icon into the Schematic Window.

Step 4: Open the BMP properties for the Cistern by right clicking on the Harvest and re-use/Cistern icon and selecting Edit BMP properties, or by double clicking on the Harvest and re-use/Cistern icon.

Step 5: If help is needed, click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculation specific to the Harvest and re-use/Cistern BMP.

Step 6: Determine the watershed characteristic for the cistern. For this example the impervious area is draining to the cistern. The watershed parameters therefore include a 0.5 acre site, all of which is impervious. Fill in the BMP specific watershed information (0.5 acres on impervious cover).

Step 7: Enter in the BMP design parameters into the BMP parameters tab. Harvest and re-use/Cistern requires the following entries:

  • Reuse storage volume equal to the cistern volume of 1995
  • Irrigation application area equal to 0.5 acres
  • Irrigation application rate of 1 inch/week
  • Irrigation start month equal to May
  • Irrigation end month equal to September
  • Does the system go offline during off season – Yes

Step 8: Click on BMP Summary tab to view results for this BMP.

Step 9: Click on the OK button to exit the BMP properties screen.

Step 10: Click on Results tab to see overall results for the site.

Harvest and re-use/pond Example (Version 3)

schematic used for reuse example
Schematic for MIDS calculator example for harvest and re-use. In this example, 5 acres of impervious surface and 3 acres of managed turf drain to a 1 acre pond that will be used to irrigate a 10 acre ballfield and provide water for toilet flushing. See Step 1.
screen shot site information tab reuse example
Screen shot of the MIDS calculator Site Information tab for the harvest and reuse example. See Step 2.
screen shot schematic tab for harvest reuse
Screen shot of the MIDS calculator Schematic tab for the harvest and reuse example. See Step 3.

A 1-acre stormwater pond is used to irrigate a 10 acre ballfield and provide water for toilet flushing for a recreation building located at the ballfield. The pond collects runoff from 5 acres of impervious surface and 3 acres of turf on B soils. The pond has a 5-foot deep permanent pool, resulting in 2 feet of water that can be withdrawn for irrigation in order to maintain a minimum pool depth of 3 feet. The maximum irrigation rate is 1 inch per week. Irrigation will occur from May through September. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have an 8 acre acre site draining to a 1 acre pond. The contributing area to the pond consists of 5 acres of impervious surface and 3 acres of turf on B soils.

Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55105 for this example) and the watershed information from Step 1. Zip code and impervious area must be filled in or an error message will be generated. The user must also indicate whether the calculator is being used to determine compliance with the Construction Stormwater permit. In this case the answer to this question is no. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the Constructed stormwater pond icon into the Schematic Window. Go to the Schematic tab and drag and drop the Harvest and re-use/Cistern icon into the Schematic Window.

Step 4: Open the BMP properties for the pond by right clicking on the Constructed stormwater pond icon and selecting Edit BMP properties, or by double clicking on the Constructed stormwater pond icon.

Step 5: If help is needed, click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculation specific to the Constructed stormwater pond BMP.

Step 6: Determine the watershed characteristic for the pond. For this example the entire impervious and pervious area is draining to the pond. The watershed parameters therefore include 5 acres of impervious surface and 3 acres of pervious turf on B soils. Fill in the BMP specific watershed information (5 acres on impervious cover and 3 acres on Managed turf and B soils). Route the pond to the Harvest and re-use/Cistern. Click on the BMP Parameters tab and select Pond Design Level 2 from the dropdown menu. For information on pond level designs, link here.

Step 7: Open the BMP properties for the harvest and use/reuse BMP by right clicking on the Harvest and re-use/Cistern icon and selecting Edit BMP properties, or by double clicking on the Harvest and re-use/Cistern icon.

Step 8: Since the stormwater runoff has already been routed to the pond (Step 6), do not enter anything on the Watershed tab. Enter in the BMP design parameters into the BMP parameters tab. Harvest and re-use/Cistern requires the following entries:

  • Reuse storage volume. Storage occurs in a 1 acre pond with 2 feet of available water depth, for a volume of 87120 cubic feet
  • Irrigation application area equal to 10 acres
  • Irrigation maximum application rate of 1 inch/week
  • Irrigated vegetation type is turf
  • Irrigation start month equal to May
  • Irrigation end month equal to September
  • Does the system go offline during off season – No (pond is in use throughout the year)
  • Water retained on site for non-irrigation uses equal the volume of water used for toilet flushing equal 15 cubic feet per week

Step 9: Click on BMP Summary tab to view results for this BMP.

Step 10: Click on the OK button to exit the BMP properties screen.

Step 11: Click on Results tab to see overall results for the site.

Links to MIDS pages



Links for stormwater and rainwater harvest and use/reuse

This page provides links to various information related to rainwater and stormwater harvest and use/reuse.

Manuals

Facts sheets, general information pages

Other links

References for stormwater and rainwater harvest and use/reuse

Because of the large number of references on the topic of harvest and use, this list of references correspond with the different sections of the harvest and use section of the manual.

Overview

Design process and sequencing

Design feasibility phase

Design phase storage siting

Design

Required Storage Capacity

Storage Unit

Collection System

Treatment System

Distribution System

Makeup Water Supply System & Backflow Prevention

Operation and maintenance

Environmental concerns

Calculating credits

Technical support for stormwater and rainwater harvest and use/reuse

The following documents were submitted by the contractor as final drafts. The information from these was used to update the manual. The documents may have undergone slight formatting, but the content is essentially what was submitted by the contractor.


Stormwater reuse for irrigation - preliminary modeling analysis

Preliminary modeling analysis was performed to begin understanding the potential impact of stormwater reuse for irrigation on annual volume and pollutant load reductions. This section includes a discussion of

  • general considerations impacting irrigation in the state of Minnesota, with a focus being on irrigation of turf and landscaping areas (non-agricultural applications);
  • modeling methodology used to evaluate the impact of stormwater reuse on annual volume and pollutant load reductions; and
  • preliminary results of this analysis.

Landscape and turf irrigation use rates in Minnesota

Although the state of Minnesota has abundant water, there are times when the demand for water exceeds the supply (UMN Extension, 2009). As previously noted, as much as 50 percent of potable water supply is used for outdoor, non-potable uses in Minnesota during the summer. During hot weather and extended periods of drought, Twin Cities’ property owners will use 45 to 120 gallons of treated drinking water per person per day for outdoor uses, with peak usage on large lots and new turf reaching as much as 200 gallons per person per day (Metropolitan Council, 2011).

Exactly how much water is required for irrigation is dependent on many factors. These factors include the amount of precipitation and the potential evapotranspiration (PET). The PET is the amount of water that could be evaporated from the land, water, and plant sources if soil water were in unlimited supply. PET is a function of soil moisture, daily available sunlight, and air temperature (EOR, 2011 (draft)). The University of Wisconsin (UW) Agriculture Extension Service has estimated PET based on satellite derived measurements of solar radiation and air temperatures at regional airports using the UW Soil Science model.

Potential evapotranspiration for Minneapolis-St. Paul from 2001-2010
Link to this table

Year Mean Potential Evapotranspiration (in/d) Maximum Potential Evapotranspiration (in/d) Standard Deviation Potential Evapotranspiration (in/d)
2001 0.17 0.31 0.07
2002 0.18 0.33 0.06
2003 0.16 0.29 0.07
2004 0.14 0.28 0.06
2005 0.16 0.29 0.07
2006 0.16 0.30 0.07
2007 0.17 0.29 0.06
2008 0.17 0.28 0.06
2009 0.16 0.32 0.05
2010 0.16 0.29 0.07
Avg (2001-2010) 0.16 0.30 0.06

Source: EOR, 2011 (draft) based on Bland and Diak, 2011


Plots of monthly average rainfall and the average potential evapotranspiration can be used to identify periods when the potential evapotranspiration is greater than the average precipitation. These are periods of the year when irrigation may be necessary and they typically fall within the period from May through September. However, because of the geographic variation in climate, the expected irrigation periods vary across the state (UMN Extension, 2009).

Average monthly evapotranspiration and precipitation values for four sites in Minnesota
Average monthly evapotranspiration and precipitation values for four sites in Minnesota. Shaded area indicates time when rainfall needs to be supplemented by irrigation. Evapotranspiration values were determined using the method by C.W. Thornthwalte.

The amount of water that needs to be applied during irrigation depends on the soil type and wetness of the soil. Typical irrigation depths for turf and landscaping in Minnesota range from 1 to 1.5 inches (minus any rainfall received) per week (UMN Extension, 2009). Additionally, the frequency of watering can be highly variable and is affected by the plant species, soil texture, climate, exposure, and intensity of use (UMN Extension, 2009). There is also variability based on the season. In the spring and summer, plants only require 40 to 60 percent of available water for evapotranspiration, while in mid-summer, turfgrass can require up to 100 percent (EOR, 2011 (draft)). Because of this variability in the supply and demand of water for irrigation, stormwater reuse systems for irrigation typically require a potable water back-up supply as well. Transpiration by urban trees can also vary by type of tree. Peters et al. (2010) found that water use by evergreens was greater than deciduous trees because they transpire more water and have a longer growing season. For example, transpiration from an evergreen can be on the order of 0.075 inches per per square meter of canopy, while for deciduous trees, the transpiration is on the order of 0.044 inches per day per square meter of canopy (EOR, 2011 (draft)).

When utilizing stormwater for irrigation as a means of managing stormwater for volume reduction or water quality improvements, the approach to the application of the irrigation water will likely vary from typical irrigation methods. The goal of the reuse is to maximize the saturation of the soil without limiting plant growth (EOR, 2011 (draft)). In addition to human health concerns related to the potential exposure to pathogens there may be concerns with the effect of stormwater pollutants on plants (e.g., nutrients, sediments, heavy metals, hydrocarbons, chlorides, and pathogens). Therefore, if possible, the plants selected for a stormwater irrigation system should have high tolerance to both water logging and pollutant concentrations.

Stormwater reuse for irrigation modeling methodology

P8 (Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds, IEP, Inc., 1990) is a computer model used for predicting the generation and transport of stormwater runoff and pollutants in urban watersheds. Barr used the P8 model, Version 3.4, in this analysis to simulate the stormwater runoff and phosphorus loads generated from hypothetical development sites with varying levels of imperviousness to represent variation in typical development density. The model requires user input for watershed characteristics, local precipitation and temperature, and other parameters relating to water quality and BMP pollutant removal performances. Barr then utilized a daily water balance spreadsheet model of the runoff water and pollutant loads generated by the P8 model to estimate the potential impact of stormwater reuse for irrigation on runoff volume reduction, and, ultimately, the pollutant load reduction.

P8 watershed modeling

The P8 analysis evaluated runoff from several hypothetical 10-acre development scenarios with varying levels of imperviousness. Twenty hypothetical watersheds were included in the P8 modeling nalysis, including

  1. A soils with 10% imperviousness;
  2. A soils with 30% imperviousness;
  3. A soils with 50% imperviousness;
  4. A soils with 70% imperviousness;
  5. A soils with 90% imperviousness;
  6. B soils with 10% imperviousness;
  7. B soils with 30% imperviousness;
  8. B soils with 50% imperviousness;
  9. B soils with 70% imperviousness;
  10. B soils with 90% imperviousness;
  11. C soils with 10% imperviousness;
  12. C soils with 30% imperviousness;
  13. C soils with 50% imperviousness;
  14. C soils with 70% imperviousness;
  15. C soils with 90% imperviousness;
  16. D soils with 10% imperviousness;
  17. D soils with 30% imperviousness;
  18. D soils with 50% imperviousness;
  19. D soils with 70% imperviousness; and
  20. D soils with 90% imperviousness.

Watershed runoff volumes from pervious areas were computed in P8 using the SCS Curve Number method. Pervious curve numbers were selected for each hypothetical watershed based on soil type and an assumption that the pervious areas within the hypothetical development would be open space areas in fair to good condition. References on SCS curve numbers provide a range of curve numbers that would apply to pervious areas in fair to good condition. Pervious curve numbers of 39, 65, 74, and 80 were used for hydrologic soil groups A, B, C, and D, respectively.

Depression storage represents the initial loss caused by such things as surface ponding, surface wetting, and interception. As previously discussed, the P8 model utilizes the SCS Curve Number method to estimate runoff from pervious areas. For impervious areas, runoff begins once the cumulative storm rainfall exceeds the specified impervious depression storage, with the runoff rate equal to the rainfall intensity. An impervious depression storage value of 0.06 inches was used for the P8 simulation.

The P8 model requires hourly precipitation and daily temperature data; long-term data was used so that watersheds and BMPs can be evaluated for varying hydrologic conditions. The hourly precipitation and average daily temperature data were obtained from the National Weather Service site at the Minneapolis-St. Paul International Airport. The simulation period used for the P8 analysis was January 1, 1955 through December 31, 2004 (50 years).

For the P8 analysis, the 50-year hourly dataset was modified to exclude the July 23-24, 1987 “super storm” event, in which 10 inches of rainfall fell in 6 hours. This storm event was excluded because of its extreme nature and the resulting skew on the pollutant loading and removal predictions. Excluding the July 23-24, 1987 “super storm”, the average annual precipitation throughout the 50-year period used for the P8 modeling was 27.7 inches.

The NURP50.PAR particle file was used for the P8 model. The NURP 50 particle file represents typical concentrations and the distribution of particle settling velocities for a number of stormwater pollutants. The component concentrations in the NURP 50 file were calibrated to the 50th percentile (median) values compiled in the EPA’s Nationwide Urban Runoff Program (NURP).

There are numerous additional input parameters that can be adjusted in the P8 model. Several of the parameters related to simulation of snowmelt and runoff are summarized below.

  • Minimum inter-event time (hours) = 10. P8 summarizes results in a series of discrete events. The minimum inter-event time is equals the minimum number of consecutive dry hours which must occur before a new storm event is initiated. This parameter influences event-based model output, but will not impact overall mass balance or load reductions.
  • Snowmelt factors — melt coefficient (Inches/Day-Deg-F) = 0.06. The rate of snowmelt is governed in P8 by the SCS degree-day equation, in which the snowmelt (inches/day) is a product of the melt coefficient and the difference between the observed daily mean temperature and the specified melt temperature (32 degrees Fahrenheit).
  • Snowmelt factors — scale factor for max abstraction = 1. This factor controls the quantity of snowmelt runoff from pervious areas by adjusting the maximum abstraction used with the SCS Curve Number method (i.e., controls losses due to infiltration). With a scale factor of 1 (P8 default), the maximum abstraction is unmodified during snowmelt or frozen ground conditions.
  • Snowmelt factors — soil freeze temperature (degrees Fahrenheit) = 32. This temperature setting can be adjusted to control the rate of runoff from pervious areas when the soil is likely to be frozen. At the start of each precipitation or snowmelt event, if the 5-day-average antecedent air temperature is below the soil freeze temperature, the pervious curve number will be modified to reflect Antecedent Moisture Condition (AMC) III and the Maximum Abstraction scale factor will be applied.
  • Runoff factors - 5-day antecedent rainfall and snowmelt (inches): Growing Season AMC-II = 1.4 and AMC-III = 2.1 (P8 defaults), Non-growing Season AMC-II = 0.5 and AMC-III = 1.1 (P8 defaults). These input parameters allow the model to make curve number adjustments based on antecedent moisture conditions.

Daily mass balance/volume reduction modeling

A daily spreadsheet mass balance model was developed to estimate the expected annual stormwater runoff volume reduction due to reuse of stormwater for irrigation. We assumed that the annual pollutant reduction would be equivalent to the estimated annual volume reduction as the result of stormwater reuse for irrigation.

There are a variety of parameters that can influence the annual stormwater runoff reduction due to stormwater collection and reuse for irrigation. These parameters include:

  • watershed characteristics: area, imperviousness, soil type;
  • volume for reuse storage;
  • available application area for irrigation;
  • irrigation rate; and
  • irrigation period.

To begin understanding the range in the potential impact of stormwater reuse for irrigation on stormwater runoff volume and pollutant removal, we ran a variety of reuse scenarios for each of the twenty watershed conditions. The daily mass balance modeling for the stormwater reuse was evaluated for the same period that was run in the P8 analysis, from January 1, 1955 through December 31, 2004 (50 years). The following is a summary of the assumptions used in the preliminary mass balance modeling analysis to develop the range in the potential impact of stormwater reuse for irrigation.

As previously mentioned, to generate the watershed runoff loads, the P8 model was used, evaluating 20 different 10-acre development scenarios, varying the imperviousness and soil types. A variety of potential application areas for irrigation were evaluated, ranging from 1 percent of the hypothetical watershed area (0.1 acre) to three (3) times the watershed area (30 acres). For the preliminary model runs, we assumed that the irrigation use rate was equivalent to 1 inch per week (or 0.14 inches per day) over the application area during the irrigation season and that the storage volume for stormwater (for irrigation) was equivalent to the one week demand for irrigation water. For the preliminary model runs, we assumed irrigation would begin in May and continue through the end of September and that irrigation would occur at a rate of 0.14 inches per day during that period, regardless of the amount of precipitation.

On a daily time step for the 50-year period that was evaluated in P8, the mass balance model tracked the available storage volume in the stormwater reuse system at the beginning of the day, the volume of watershed runoff (as generated by P8), the amount of watershed runoff that would bypass the system (due to storage not being available), the irrigation volume (if applicable and available), and the storage volume remaining in the stormwater reuse system at the end of the day. The estimated annual volume reduction (and equivalent pollutant reduction) was determined based on comparison of the average annual volume used for irrigation with the average annual watershed runoff volume.

Estimated impact of stormwater reuse for irrigation

The results of the long-term continuous simulation P8 and daily mass balance modeling analyses are presented below.

The table below summarizes the average annual watershed runoff volumes from the hypothetical 10-acre watersheds along with the range of average annual runoff volume reduction based on reuse for irrigation. There is a significant amount of variability in the estimated average annual removal efficiencies related to stormwater reuse for irrigation, based on the sizing of the reuse storage, application area, the irrigation rate and irrigation period. The modeled average annual removals are based on the assumption that the stormwater reuse storage volume is equivalent to the one week demand for irrigation water with an application rate of 1 inch per week over the application area for an irrigation period from May through September. Depending on the combinations of variables, the estimated annual volume reduction (and equivalent pollutant removal) can range from 1 to 98 percent.

Estimated impact of stormwater reuse for irrigation on average annual runoff reduction. The table shows average annual runoff volumes (acre-feet) for 10 acre wastersheds as a function of soil group and percent imperviousness. The range of average annual volume reduction from reuse is shown in the last two columns.
Link to this table

Hydrologic Soil Group (HSG) Watershed Area (acre) Watershed Imperviousness (%) Average Annual Runoff Volume1 (acre-ft) Range of Average Annual Runoff Volume Reduction2 (%)
A 10 10 2.3 6.0 98.3
10 30 6.2 2.2 93.6
10 50 10.2 1.4 86.2
10 70 14.1 1.0 80.6
10 90 18.1 0.8 76.7
B 10 10 3.2 4.3 91.7
10 30 6.9 2.0 88.2
10 50 10.7 1.3 83.3
10 70 14.4 1.0 79.3
10 90 18.2 0.8 76.4
C 10 10 3.9 3.5 87.6
10 30 7.5 1.9 85.4
10 50 11.1 1.3 81.8
10 70 14.7 1.0 78.6
10 90 18.3 0.8 76.2
D 10 10 4.7 2.9 84.6
10 30 8.1 1.7 83.2
10 50 11.5 1.2 80.5
10 70 14.9 0.9 78.1
10 90 18.4 0.8 76.0

1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data
2 - Assumes stormwater storage volume for reuse equivalent to the one week demand for irrigation water assuming an application rate of 1"/week over the application area & irrigation period from May through September


Stormwater reuse for irrigation performance curve – watershed 0 percent impervious
Stormwater reuse for irrigation performance curve – watershed 0 percent impervious
Stormwater reuse for irrigation performance curve – watershed 10 percent impervious
Stormwater reuse for irrigation performance curve – watershed 10 percent impervious
Stormwater reuse for irrigation performance curve – watershed 30 percent impervious
Stormwater reuse for irrigation performance curve – watershed 30 percent impervious
Stormwater reuse for irrigation performance curve – watershed 50 percent impervious
Stormwater reuse for irrigation performance curve – watershed 50 percent impervious
Stormwater reuse for irrigation performance curve – watershed 70 percent impervious
Stormwater reuse for irrigation performance curve – watershed 70 percent impervious
Stormwater reuse for irrigation performance curve – watershed 90 percent impervious
Stormwater reuse for irrigation performance curve – watershed 90 percent impervious
Stormwater reuse for irrigation performance curve – watershed 100 percent impervious
Stormwater reuse for irrigation performance curve – watershed 100 percent impervious

Curves from modeling analysis for the Twin Cities region show estimated average annual runoff volume reductions due to irrigation for a range of soil types (HSG A through D). The curves are based on a relationship between the estimated stormwater reuse storage volume and watershed area. However, exactly how stormwater reuse for irrigation can be incorporated into a site design or retrofitted into an existing system will vary. In some cases, the application area available may control the sizing of the reuse storage. In other situations, there is sufficient application area but a limited watershed area to generate the runoff for reuse resulting in a smaller storage volume for reuse water. Spatial constraints on the site may limit the amount of storage volume that could be included into the site design and layout. Additionally, depending on the geographic location within the state, the current climate, and/or the type of plant species that are being irrigated, the application rate may vary (0.5 to 2.0 inches per week) along with the irrigation period throughout the year. Currently, the set of curves developed from the preliminary modeling does not capture the potential variability in all these parameters (and resulting removal efficiencies).

Also, it is important to look at the relationship between all of these variables when designing a stormwater reuse system to optimize the sizing of the system for a specific site to maximize the cost-benefit of the system. For example, for a hypothetical 10-acre watershed at 50 percent impervious and B type soils, the average storm event runoff (based on the 50-year climatic period evaluated in P8) is 0.16 acre-feet. Assuming the stormwater reuse volume is sized to store the average event runoff from the watershed (and that there is sufficient application area to reuse the stormwater), this would result in an average annual reduction in stormwater runoff volumes of approximately 24 percent. Average annual TSS and total phosphorus reductions are also 24 percent for this example. Using a NURP pond after reuse leads to more than an 87 percent reduction in TSS and more than a 60 percent reduction in total phosphorus.


Examples of the estimated runoff volume reduction for a stormwater reuse storage system sized for the average storm event runoff volume
Link to this table

Hydrologic Soil Group (HSG) Watershed Area (acre) Watershed Imperviousness (%) Average Storm Event Runoff Volume (acre-ft)1 Average Annual Runoff Volume Reduction2
B 10 10 0.05 21.4
30 0.10 26.4
50 0.16 24.0
70 0.21 21.9
90 0.26 21.3

1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data 2 - Assumes stormwater storage volume for reuse equivalent to the average storm event runoff volume and that the application area is sufficient to handle an irrigation application rate of 1"/week over the application area & irrigation period from May through September


Examples of the estimated pollutant load reductions for a stormwater reuse storage system sized for the average storm event runoff volume
Link to this table

Hydrologic Soil Group (HSG) Watershed Area (acre) Watershed Imperviousness (%) Average Annual Runoff Volume Reduction1,2(%) Average Annual TSS Reduction Reuse Only3(%) Average Annual TP Reduction Reuse Only3(%) Average Annual TSS Reduction NURP & Reuse3,4(%) Average Annual TP Reduction NURP & Reuse3,4(%)
B 10 10 21.4 21.4 21.4 87.4 60.7
30 26.4 26.4 26.4 88.2 63.2
50 24.0 24.0 24.0 87.8 62.0
70 21.9 21.9 21.9 87.5 61.0
90 21.3 21.3 21.3 87.4 60.7

1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data
2 - Assumes stormwater storage volume for reuse equivalent to the average storm event runoff volume and that the application area is sufficient to handle an irrigation application rate of 1"/week over the application area & irrigation period from May through September
3 – Assumes pollutant removal equivalent to volume reduction for reuse only
4 - Assumes average annual NURP pond removal: TP=50%, TSS=84%
.


When considering stormwater reuse only, it may be possible to increase the reuse storage volume for the same watershed and application area to maximize the expected annual removal efficiency. For example, to achieve an 80 percent average annual removal efficiency for the same hypothetical watershed, the stormwater reuse volume would need to be increased to 2.5 acre-feet (more than 15 times the average event runoff volume) and would also need sufficient application area to reuse the stormwater within a week at an application rate of one (1) inch per week. This increase in storage volume means that for most storm events, the reuse storage system will remain nearly empty. Additionally, this increase in storage volume typically comes with an associated cost that may be prohibitive, unless the volume is associated with an existing pond that can be utilized for storage and reuse. All of these factors must be weighed when evaluating and optimizing a stormwater reuse system in the context of a specific site.

This page was last modified on 15 May 2017, at 13:52.

Minnesota Pollution Control Agency | 651-296-6300, 800-657-3864 | Assistance | Web site policy