Rain water harvesting is the practice of collecting rain water from impermeable surfaces, such as rooftops, and storing for future use.
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.
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):
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
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Beneficial Uses | |||||
---|---|---|---|---|---|
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
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.
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. Standards for various categories of stormwater management practices must be applied in all projects in which at least one acre of new impervious area is being created.
Rainwater harvest and use/reuse systems are not defined and discussed in the CGP. Because water captured by these systems is typically used for irrigation, the Infiltration systems category described in the CGP is generally applicable. Water Quality Volume requirements are applied to the volume of water stored in a tank or pond, which is used to meet the instantaneous volume requirement in the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the bioretention practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
There are situations, particularly retrofit projects, in which a harvest and use/reuse practice is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular practice, depending on where it is situated both jurisdictionally and within the surrounding landscape.
The ability to use harvest and use/reuse as a retrofit practice depends on the development situation. In areas where above-ground storage is limited, ponds and wetlands are not feasible. In new developments, ponds and wetlands may be desirable due to increased volume of storage. Because harvest and use/reuse systems utilize irrigation, soil infiltration rates are generally not limiting.
The table below provides guidance regarding the use of harvest and use/reuse practices in areas upstream of special receiving waters. Note that most harvest and use/reuse practices will fall under the infiltration category.
Infiltration and filtration bmp1 design restrictions for special waters and watersheds. See also Sensitive waters and other receiving waters.
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BMP Group | receiving water | ||||
---|---|---|---|---|---|
A Lakes | B Trout Waters | C Drinking Water2 | D Wetlands | E Impaired Waters | |
Infiltration | RECOMMENDED | RECOMMENDED | NOT RECOMMENDED if potential stormwater pollution sources evident | RECOMMENDED | RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride |
Filtration | Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments | RECOMMENDED | RECOMMENDED | ACCEPTABLE | RECOMMENDED for non-nutrient impairments |
1Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
2 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies
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.
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 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 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 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.
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 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 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.
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).
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.
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.
Considerations for fire suppression sprinkler systems are similar to those for firefighting. Systems must be compatible with IFC and indoor plumbing codes.
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.
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.
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.
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).
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.
Current codes and standards for water harvesting and use systems are described below:
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
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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.
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.
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.
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.
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.
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.
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.
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.
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:
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:
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.
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.
More information on the design process and design sequencing can be found in the following resources:
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.
The first step in the project is to define objectives. Objectives are typically driven by regional or organizational goals, including the following:
Specific, quantifiable goals which address project objectives should be used to evaluate the feasibility of the proposed system. Examples include the following:
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.
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
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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? |
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
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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.
Potential uses of harvested stormwater include the following.
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
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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.
Key operation and maintenance (O&M) questions to consider during the feasibility phase include:
See the section on operation and maintenance for detailed information pertaining to these questions.
If the goals and objectives are compatible with the identified site constraints, source areas, and appropriate uses, proceed to Pre-design.
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).
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 |
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.
Required permits and applicable codes include the following: (note that not all may apply to every situation)
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
See the section on water quality considerations.
See the section on environmental concerns.
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
The design phase involves identifying the specific components of the harvest and use system.
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.
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.
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
For underground storage tanks
For aboveground storage tanks
For indoor tanks
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) |
|
|
Below-ground, closed systems |
|
|
Above-ground, closed systems |
|
|
General design considerations include the following.
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 |
|
|
Polyethylene, polypropylene |
|
|
Barrels and trash cans |
|
|
Metal | ||
Galvanized steel tanks |
|
|
Steel drums (55-gallon) |
|
|
Concrete and masonry | ||
Ferroconcrete |
|
|
Monolithic/poured-in-place |
|
|
Stone, concrete block |
|
|
Wood | ||
Pine, redwood, cedar, cypress |
|
|
Design considerations for above and below-ground tanks are summarized below.
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:
The Metropolitan Council Reuse Guide includes these additional design considerations.
See the section on operation and maintenance for more detailed guidance on developing an operations and maintenance plan.
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:
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 |
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).
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.
Rainwater harvesting overflow discharge location - methods
Link to this table
Overflow Discharge Locations / Methods | Advantages | Disadvantages |
---|---|---|
Discharge to grade via gravity flow (most recommended |
|
|
Discharge to storm sewer via gravity flow |
|
|
Discharge to soakaway pit via gravity flow |
|
|
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.
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:
Design resources:
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
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.
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.
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 | 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 |
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 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. |
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:
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
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.
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:
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 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
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 |
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).
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
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
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 |
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
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.
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.
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. |
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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. |
Required Storage Capacity
Storage Unit
Collection System
Treatment System
Distribution System
Makeup Water Supply System & Backflow Prevention
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.
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.
Below is a list of additional considerations that are not specifically addressed above.
Maximum Detention Time - Average Daily Temperature
Link to this table
Maximum Detention Time (days) to limit algae blooms: | Average Daily Temperature (F) |
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50 | 59 |
30 | 68 |
20 | 77 |
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.
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.
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.
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.
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.
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).
You can access an example inspection report by clicking on the following link:
File:MPCA Stormwater Harvest and Use System Example Inspection Report.docx
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.
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:
In general, stormwater harvesting and use O&M plans should include the following items.
A. Site plans showing
B. As-built drawings showing
C. Operation and troubleshooting guidelines for system controls including:
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
F. Description and schedule of maintenance activities for all system components including
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
I. Monitoring Plan
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 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 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 .
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.
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.
Special consideration may be needed for underground systems
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.
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.
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.
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.
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.
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
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 |
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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
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 |
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Source Area/Collection Surface |
|
Collection System |
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Storage System |
|
Treatment System |
|
Distribution System |
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Overflow/Bypass Systems |
|
Record keeping is part of regular operation and maintenance. Record keeping is important for
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:
Maintenance logs should include, at a minimum, the following information:
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.
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
</noinclude>
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 |
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Nutrients
|
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Organic Matter |
|
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Suspended Sediment |
|
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Chlorides |
|
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Pathogens |
|
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Metals |
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Organic Chemicals
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|
|
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 | |||||
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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
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
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 |
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Hard Roofs | ○ | ○ | ○ | ● | ○ | |||||
Green and Brown Roofs | ○ | ○ | ● | ○ | ○ | ○ | ||||
Paved Surfaces | ○ | ● | ○ | ○ | ○ | ● | ● | ○ | ||
Green Spaces | ○ | ● | ● | ● | ● | ● | ○ | |||
Sedimentation Basins and Detention Ponds | ○ | ○ | ○ | ● | ○ | ○ | ○ | ○ |
● = relatively high concentrations
○ = relatively low concentrations
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 |
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Metal Roofs |
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Sheet Roofing (PVC) |
|
Tile roofs (clay, ceramic, cement, fiberglass) |
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Shingles
|
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Shingles – cedar shakes/wood shingles |
|
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 |
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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) |
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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)
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 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 | ||||
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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 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.
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.
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 |
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Low exposure | No direct physical contact | None | |||
Medium exposure | Direct physical contact | Signage | |||
High exposure | Ingestion or inhalation | Restricted access |
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.
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 |
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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
Stormwater harvest and use systems require some level of pretreatment, similar to other stormwater BMPs, such as:
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
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 |
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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 |
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Med | Med | Med |
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Micro-filtration |
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Med | Med | Med |
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Nano-filtration |
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Med | Med | High |
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Reverse-osmosis |
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High | High | High |
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X | X |
Ion-exchange filter |
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High | High | High |
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Disinfection | Disinfection processes kill, remove, or deactivate pathogenic microorganisms in harvested water. | Chlorination – injects chlorine into stormwater |
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Low | Low | Low |
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Ultra-violet light (UV) radiation – stormwater is passed over an ultraviolet lamp |
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Med | High | High |
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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 |
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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 |
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Low | Low | Low |
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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:
The following factors should be considered when assessing human health and ecological risks of stormwater harvesting and use systems (NAS, 2016):
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 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:
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:
Examples of potential exposure types and pathways
Link to this table
Exposure Types | Exposure Pathways |
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Skin contact |
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Direct ingestion |
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Indirect ingestion |
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Inhalation |
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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.
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 |
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Human Health Risks | ||
Source area pollutants |
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Hazardous spills in the source area, including sudden air releases of hazardous substances that could deposit in the collection and storage systems |
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Metals and other chemicals from roofing materials (link to table 4 in WQ considerations) |
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Bacteria, viruses |
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Mosquito and other vector-borne illnesses |
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Ecological Risks | ||
Plant communities |
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Soils |
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Aquatic ecosystems |
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Local hydrology |
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Equipment degradation |
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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:
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.
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).
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 |
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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
|
Varies | ||
Residual Suspended Solids Removal Systems
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Varies | ||
Residual Colloidal Solids Removal Systems
|
Varies | ||
Dissolved Solids Removal Systems
|
Varies | ||
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 |
Due to the large variability in harvest and use system costs, construction bid estimates are provided as examples of itemized costs.
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 |
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 |
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Watershed Organization Implementation and Cost-Share Programs |
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State Agency Grants and Loans | |
County Funds |
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Municipal Funds and Utility Fees |
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Other Public Financing |
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Private Financing |
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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:
To see a database of reuse projects, link here.
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.
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.
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 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.
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.
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.
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):
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:
Below are Minnesota examples of outdoor and indoor reuse systems.
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. If captured water is routed to a non-infiltrating bmp, removal is determined by that bmp. |
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.
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 best management practice (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.
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.
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.
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.
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
In the following discussion, the Water Quality Volume (VWQ) is delivered as an instantaneous volume 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.
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 * 1556 + V_{nonirrigation}]</math>
Where
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.
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
The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on TP emcs link here. 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
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
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
The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator.
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:
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:
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 | ||||||
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 is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of event mean concentration 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 (EPA 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 MPCA Estimator, this tool was developed specifically for complying with the 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, 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.
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.
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.
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
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.
Field monitoring may be made 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.
This manual contains the following guidance for monitoring.
The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement 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:
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 chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.
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.
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.
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:
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 infiltration 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
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.
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.
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.
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
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.
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
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.
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
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.
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.
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.
This example was completed using Version 2 of the Calculator.
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:
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.
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:
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.
This page provides links to various information related to rainwater and stormwater 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.
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.
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
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
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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).
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.
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.
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
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.
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:
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.
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.
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Hydrologic Soil Group (HSG) | Watershed Area (acre) | Watershed Imperviousness (%) | Average Annual Runoff Volume1 (acre-ft) | Range of Average Annual Runoff Volume Reduction2 (%) | |
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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
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
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Hydrologic Soil Group (HSG) | Watershed Area (acre) | Watershed Imperviousness (%) | Average Storm Event Runoff Volume (acre-ft)1 | Average Annual Runoff Volume Reduction2 |
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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(%) |
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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 edited on 15 May 2017, at 18:52.