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

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

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

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

Contents

Overview of broad phases of design

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

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

Feasibility phase

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

Pre-design phase

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

Design phase

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

Implementation phase

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

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

Additional resources

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

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

Feasibility phase

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

Identify harvest and use system goals and objectives

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

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

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

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

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

Additional Resources

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

Consider site opportunities and constraints

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

Water reuse key site considerations
Link to this table

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

Additional Resources

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

Identify source areas and expected source water quality

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

Typical pollutants found in stormwater collected from different source areas

Link to this table

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

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

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

Potential uses of harvested stormwater include the following.

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

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

Harvested stormwater beneficial uses
Link to this table

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

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

Consider level of operation and maintenance needed

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

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

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

Decide whether to proceed to pre-design

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

Pre-design phase

Complete site survey & information gathering

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

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

Harvested stormwater site survey and information worksheet
Link to this table

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

Estimate the water balance for the site

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

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

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

Identify required permits and applicable codes

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

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

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

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

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

Identify water treatment requirements

See the section on water quality considerations.

Conduct an environmental and health risk assessment

See the section on environmental concerns.

Decide whether to proceed to design

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

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

Design phase

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

Storage

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

Determining the appropriate storage size

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

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

Determining the siting of the storage system

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

For all storage systems

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

For underground storage tanks

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

For aboveground storage tanks

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

For indoor tanks

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

Storage siting additional resources

Selecting and designing the storage unit

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

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

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

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

Storage tanks and cisterns

General design considerations include the following.

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

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

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

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

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

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

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

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

The Metropolitan Council Reuse Guide includes these additional design considerations.

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

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

Stormwater ponds

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

Design considerations:

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

Maximum Detention Time - Average Daily Temperature
Link to this table

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

Design resources

Footings and buoyancy calculations

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

Overflows and bypass

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

Design considerations

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

Rainwater harvesting overflow discharge location - methods
Link to this table

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

Design resources

Collection

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

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

Ground surface collection

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

Design considerations:

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

Design resources:

Rooftop runoff collection

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

Roofs - design considerations

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

Gutters and downspouts - design considerations

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

Implement cooperative goals (education and outreach, etc.)

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

Treatment

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

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

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

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

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

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

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

Pre-storage treatment

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

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

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

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

Design considerations

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

Design resources

Post-storage treatment

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

Design considerations:

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

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

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

Not required in rainwater systems

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

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

Design resources

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

Distribution

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

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

Design considerations:

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

Pumps

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

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

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

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

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

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

Pump demand

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

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

\( H_{pump} = Required System Pressure Head + TDH \)

where TDH is the total dynamic head, given by

\( TDH = H_L + H_S + H_f \)

where

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

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

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

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

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

Estimating peak demand for non-potable residential/commercial use

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

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

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

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

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

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

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

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

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

Example 2 – Heavy Equipment Washing

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

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

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

Irrigation systems

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

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

Makeup water supply system and backflow prevention

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

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

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

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

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

Key design resources

Required Storage Capacity

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

Storage Unit

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

Collection System

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

Treatment System

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

Distribution System

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

Makeup Water Supply System & Backflow Prevention

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

Design considerations for ponds used for harvest and use

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

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

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

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

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

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

Additional considerations

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

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

Maximum Detention Time - Average Daily Temperature
Link to this table

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


Implementation phase

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

Construction activities

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

Implement operation and maintenance and monitoring plans

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

Implement cooperative goals (education and outreach, etc.)

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

References


Related pages

This page was last modified on 25 October 2017, at 10:23.

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