Stormwater harvest and use can be implemented at a variety of scales, from individual parcels to regional scales, and in a variety of contexts, from ultra-urban settings to new development. The scale and complexity of any harvest and use project depends on several factors, including source water quality, intended application, and water quality recommendations and/or regulations. The design process for each project must be flexible and rigorous enough to address these factors.
Although there is a wealth of information on selection and design of system components (collection, storage, treatment, and distribution systems), guidance offering a concept-to-finish perspective is limited (Met Council, 2011; NC DEQ, 2014; DPLG, 2010). The design process described in this report uses four broad phases: feasibility, pre-design, design, and implementation. In practice the distinction between phases is not strict. Design is an iterative process which should include several rounds of review beginning in the feasibility phase.
The following discussion includes a description of each design phase, a list of activities typically included in each phase, design guidance, and key resources for individual activities.
The four broad phases of design include feasibility, pre-design, design, and implementation. This section provides a summary list of components of each design phase. These are discussed in greater detail in the following section. Implementation is discussed only briefly in this section and is covered in greater detail in the construction section.
During the feasibility phase, opportunities for stormwater harvest and use are identified and evaluated at a basic level, taking into consideration site characteristics, expected source water quality, and constraints imposed by various use applications. The goal of this phase is to determine if a project is feasible and, if it is, what constraints might exist. Typical feasibility phase activities include the following:
During the pre-design phase, stormwater harvest and use opportunities that were identified in the feasibility phase are evaluated at greater depth to determine the most feasible option or best application. Typical pre-design phase activities include the following:
The design phase includes the selection, sizing, siting, and design of stormwater harvest and use system components. Generalized design steps are listed below. In practice, design is an iterative process and steps will likely be revisited as the system design is refined in conjunction with site, cost, regulatory, or other considerations.
The implementation phase includes all post-design work including construction and installation; operations, maintenance, and monitoring; and additional activities which are included in project objectives. Typical implementation phase activities include the following
The implementation phase is discussed in detail in the sections on construction and on operation and maintenance.
More information on the design process and design sequencing can be found in the following resources:
During the feasibility phase, project objectives are defined and opportunities for stormwater harvest and use are evaluated at a preliminary level. Preliminary evaluations should take into consideration site characteristics, expected source water quality, and constraints imposed by various harvest and use applications. The goal of the feasibility phase is to identify stormwater harvest and use opportunities that are suitable for in-depth feasibility analysis or determine that harvest and use is not feasible.
The first step in the project is to define objectives. Objectives are typically driven by regional or organizational goals, including the following:
Specific, quantifiable goals which address project objectives should be used to evaluate the feasibility of the proposed system. Examples include the following:
In Minnesota, capture and use of stormwater irrigation use is increasingly being used to meet runoff and phosphorus reductions goals. Irrigation practices utilize evapotranspiration losses of water and plant uptake and soil adsorption of the dissolved fraction of phosphorus. The determination of project objectives and goals may be an iterative process in which project objectives are modified based on feasibility, cost, or other concerns. Identifying project objectives which align with broad regional or organizational goals may be important in garnering stake-holder support or procuring funding for the project.
At the feasibility phase, a preliminary assessment of site conditions provides an understanding of stormwater harvest and use constraints and opportunities. More in-depth analysis of site conditions and constraints will be required during pre-design and design phases. The following table describes some site considerations and identifies potential constraints.
Water reuse key site considerations
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|Catchment boundaries||Identify the area(s) from which water can be captured. This is necessary to compute capture volumes, identify site constraints, and determine water quality.||Does the amount of water that can be captured affect the type of possible end use and the size of storage unit?|
|Existing drainage patterns||Identify how water will drain from the catchment area(s).||Do any additional drainage features need to be constructed to centralize runoff collection?|
|Buffers and Setbacks||Identify wetlands, streams, shorelines, and buildings/structures in the potential receiving area and determine setbacks from these, including buffer zones. Identify setback distances for infiltration practices described in the Minnesota Stormwater Manual and determine if these are applicable to the project.||Are there any portions of the irrigation area that fall within a setback?|
|Proximity to water supply wells or sensitive water bodies||Identify setback distances for infiltration practices described in the Minnesota Stormwater Manual and determine if these are applicable to the project.||Are there any setback distances from supply wells or sensitive water bodies to consider?|
|Existing and adjacent land use/ stormwater hot spots||Water quality varies with land use and will impact the level of treatment needed. Runoff from confirmed stormwater hotspots should not be used for harvest and use unless treated to appropriate standards. May need appropriate bypass systems.||Are there any portions of the catchment area that need to be bypassed from the collection system or that require additional treatment prior to use?|
|Future land use changes||Changes in the land use of the source area or irrigation areas may change opportunities and hurdles to implementation, such as level of treatment needed or availability of green space for irrigation.||Are there any future land use changes that may prohibit or alter the proposed stormwater harvest and use system? (e.g. stormwater hotspots)|
|Existing infrastructure||Maintain easements/appropriate distance from sewer lines, fiber optic cables, etc.||Is there any infrastructure that needs to be avoided?|
|Site slopes||Slope may affect runoff calculations and affect the system design (e.g. location of storage tank).||Can sufficient runoff be collected from the source area? Can the cistern be placed at the lowest point on site? If not, what modifications need to be made to the collection system to route stormwater to the cistern?|
|Soil type||Infiltration capacity affects runoff calculations and irrigation demand assumptions.||Can sufficient runoff be collected from the source area? How do the irrigation area soils affect irrigation demand?|
|Proximity to building foundations & utilities||Leakage from storage tanks could damage building foundations or utilities. Geotechnical engineering may be needed to ensure sufficient support for tanks and to prevent damage to adjacent foundations.||Are there any setback distances from building foundations or utilities to consider?|
|Seasonal high water table||Underground components may be constrained by the seasonal high water table. Above-ground systems are generally unaffected by the seasonal high water table.||Will the seasonal high water table level affect any underground components?|
|Maintenance access||Need space for maintenance access.||Is there sufficient space for maintenance access?|
|Code or System Expansion||Configure the system to allow for expansion if space is available. Follow national standards and simple steps of including a pipe stub in or designed plumbing system to allow for future incorporation of indoor uses in a phased fashion||Should we stub in a pipe in case a code or owner changes to allow other uses? Can we configure the plumbing to adapt for indoor use in a second phase or when funding is available?|
Expected stormwater quality from various source areas is described in detail in the section on water quality considerations. Snowmelt from ground surfaces tends to have higher pollutant concentrations compared to runoff at other times of the year (EOR and CWP, 2005). Oberts (2000) discussed the dynamics of snowmelt runoff quality, indicating the initial melt (snow and ice from pavement) can have very high concentrations of soluble pollutants, which can be difficult to remove in traditional stormwater BMPs. Depending on the use of harvested stormwater, it may be desirable to provide a bypass or use a pre-tank filter for spring snowmelt, particularly the initial phase of the melt. Typical pollutants found in stormwater collected from different source areas are summarized in the table below.
Typical pollutants found in stormwater collected from different source areas
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|Source Area||Solids||Total suspended solids||Nutrients||Bacteria||Metals||Chloride||Grease, Oil||Pesticides||Other chemicals|
|Green and Brown Roofs2|
|Paved Surfaces (parking lots, sidewalks, driveways and roadways)|
|Green Spaces (lawns and park areas)|
● = relatively high concentrations
○ = relatively low concentrations
1Metals are highly dependent on roof type-only certified membrane roofs are currently approved for potable through NSF.
2Vegetated roofs and pervious pavers have had poor performance due to discoloration of water and difficulty filtering the water to levels that the UV treatment is effective.
Potential uses of harvested stormwater include the following.
General considerations for the use of harvested stormwater are described here. See also the Source and Use Worksheet, C.1w, from the 2011 Met Council Reuse Guide. In addition, if the harvest and use system is required to follow the Minnesota Plumbing Code, check Section 1702.9.3 for prohibited collection surfaces and discharges.
Harvested stormwater beneficial uses
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|Beneficial Uses||Stormwater from rooftops only (rainwater)||Stormwater|
|Health Criteria Level||Expected level of effort||Health Criteria Level||Expected level of effort|
|Outdoor||Sanitary sewer flushing||Limited human exposure at point of use and limited exposure to pathogens upstream of point of use||No treatment needed||Limited human exposure at point of use and limited exposure to pathogens upstream of point of use||Minimal (pretreatment)|
|Irrigation – low exposure risk||Limited human exposure at point of use and limited exposure to pathogens upstream of point of use.||Minimal (pretreatment)1||Limited human exposure at point of use and limited exposure to pathogens upstream of point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Irrigation – high exposure risk||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.1||High (pretreatment + treatment + disinfection)|
|Vehicle/building washing||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Fire fighting||Limited human contact and controlled access at point of use.1||Minimal (pretreatment)||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Water features (uncontrolled access)||Limited human contact and controlled access at point of use.||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Street cleaning/ dust control||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Indoor||Fire suppression||Limited human contact and controlled access at point of use.||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.||Medium (pretreatment + disinfection OR pretreatment + treatment)|
|Cooling||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.1||High (pretreatment + treatment + disinfection)|
|Process /Boiler Water||Limited human contact and controlled access at point of use.1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Limited human contact and controlled access at point of use.1||High (pretreatment + treatment + disinfection)|
|Flushing||Uncontrolled access at point of use1||Medium (pretreatment + disinfection OR pretreatment + treatment)||Uncontrolled access at point of use1||High (pretreatment + treatment + disinfection)|
|Washing||Uncontrolled access at point of use||Medium (pretreatment + disinfection OR pretreatment + treatment)||Uncontrolled access at point of use1||High (pretreatment + treatment + disinfection)|
|Drinking water||Drinking water standards||Drinking water standards||Drinking water standards||Drinking water standards|
1A higher level of treatment that may be required depending on context
Note: In practice, all indoor non potable uses are typically treated to water quality requirements in code and currently require disinfection.
Key operation and maintenance (O&M) questions to consider during the feasibility phase include:
See the section on operation and maintenance for detailed information pertaining to these questions.
If the goals and objectives are compatible with the identified site constraints, source areas, and appropriate uses, proceed to Pre-design.
The following information may be useful or necessary when designing a harvest and use system. Some of the information is general and should be used for initial screening. The information should be recorded in a form (see link; Data Collection Worksheet, C.2w, items 1-3, from the 2011 Met Council Reuse Guide).
Harvested stormwater site survey and information worksheet
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|Item||Comments / notes||Used|
|Storm sewer maps|
|Hazardous waste site maps|
|Runoff monitoring data|
|Soil borings/geotechnical analyses|
|Gopher State One call|
|Storm sewer surveys|
|Presence of wells|
|Existing information on stormwater ponds|
|Existing information on storage tanks|
A water balance must be calculated to determine if the desired capture volumes can be achieved and to properly size the system. A water balance consists of estimating the amount of water that can be captured and the amount of water that is used. Key considerations include balancing the amount of storage unit overflow with the size of the storage unit, and limiting or eliminating the need for a secondary water supply. For stormwater management, it is recommended to evaluate source areas (harvest and use supply) first and then identify candidate use applications. Calculations of water captured are a function of rainfall and runoff potential from the site. For system design, we recommend using a calculator that uses a daily time step and summarizes the volume in the storage unit on a daily basis. The Minnehaha Creek Watershed District calculator can be used for these calculations. This calculator was developed and funded by the Mississippi Watershed Management Organization and Minnehaha Creek Watershed District. The calculator utilizes an Excel spreadsheet that allows the user to enter information on irrigation use, catchment (watershed) area, and storage. Inputs can be manipulated to determine catchment size, amount used, or needed storage. Guidance on using this calculator, along with example calculations and other methodologies for completing water balance, are found here.
A summary of available stormwater harvest and use calculators and models can be found in the section on calculating credits for stormwater and rainwater harvest and use. The Met Council Water Balance Tool and the VDCR Cistern Design Spreadsheet are other useful calculators for calculating a water balance. The Met Council calculator provides output that must be copied into a spreadsheet or other tool, where sizing calculations can be made. The Virginia calculator utilizes Virginia rainfall data, which limits its applicability in Minnesota, but the calculator incorporates a wide range of indoor use applications.
Some calculators, such as North Carolina’s calculator, utilize mean monthly rainfall. While monthly Minnesota rainfall data is easy to obtain and using monthly data provides an easy method of calculation, this approach can introduce significant errors since rainfall is not uniform throughout a month. Because stormwater harvest and use systems are designed to store a certain size rainfall event for use prior to the next rainfall event, the timing and intensity of rainfall on a daily basis is just as important to the monthly rainfall total. For example, less runoff volume reduction is possible if two large rainfall events occur back to back compared to if they occur further apart.
Additional information on calculating water balance is provided in the 2011 Met Council Reuse Guide (see Quantity and Storage Worksheet, A.2w for a list of steps to balance the volume of water that can be captured and stored (supply) with the volume of water to be used (demand)).
Required permits and applicable codes include the following: (note that not all may apply to every situation)
In general, if water is pumped out of a stormwater basin, an appropriation permit is required. If the water is temporarily drained out of the basin via an operable outlet structure, an appropriation permit is not required. However, Minnesota law now provides an incentive for stormwater capture and reuse. Legislation adopted in 2014 directs the Department of Natural Resources to “waive the water use permit fee for installations and projects that use storm water runoff or where public entities are diverting water to treat a water quality issue and returning the water to its source without using the water for any other purpose, unless the commissioner determines that the proposed use adversely affects surface water or groundwater.” (MS §103G.271, subd. 6(g); emphasis added). Therefore, stormwater reuse is exempt from the annual water use 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 would act as the permittee and coordinate the application and reporting with landowners and managers of each site. For further information, contact Dan Miller at Dan.W.Miller@state.mn.us
See the section on water quality considerations.
See the section on environmental concerns.
Use the Pre-design phase checklist to determine whether to proceed to the design phase. In general, if
you can proceed to the Design Phase.
The design phase involves identifying the specific components of the harvest and use system.
Storage is the central and often most expensive component in a stormwater harvesting and use system. Several factors must be considered, including size, type, siting and materials certification (e.g. NSF 61) of the storage system.
Decision-making on storage size can have a strong impact on the economic feasibility of stormwater harvesting (VA DCR, 2009). During the pre-design phase, a water balance should have been completed. This water balance should be refined during the Design Phase to incorporate other factors such as cost, site constraints, aesthetic concerns, or water quality criteria. For these reasons, the selection and sizing of storage units may be an iterative process. Work with an engineer, system supplier or accredited professional with experience using stormwater harvest and use calculators to appropriately size storage units for your stormwater harvest and use system. Methods for sizing tanks were discussed in the section on water balance. Guidance on methods for sizing, including examples, are found here.
Storage options for stormwater harvesting systems include above- or below-ground tanks or cisterns which are closed to the environment, and above-ground open storage, such as a stormwater retention or detention pond. Depending on the type of storage system, considerations for siting storage units include the following.
For all storage systems
For underground storage tanks
For aboveground storage tanks
For indoor tanks
Storage options for stormwater harvesting systems include above- or below-ground tanks or cisterns which are closed to the environment, and above-ground open storage, such as a stormwater retention or detention pond. Storage tanks or cisterns are commonly used in rainwater harvesting systems and are used for both indoor and outdoor stormwater use applications. Tanks or cisterns will likely provide the best option for indoor stormwater use applications due to ease of siting storage close to point of use. Stormwater ponds are commonly used for outdoor irrigation and in retrofit applications with existing stormwater ponds.
Advantages and disadvantages of above- or below-ground tanks and stormwater ponds are summarized below. The City of Bellingham, WA Rainwater Harvesting Guidance provides a comparison of characteristics of aboveground and belowground tanks (See Table 2 in Appendix A).
Advantages and disadvantages of above- or below-ground tanks and stormwater ponds
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|Type of storage system||Advantages||Disadvantages|
|Open systems (e.g. ponds)||
|Below-ground, closed systems||
|Above-ground, closed systems||
General design considerations include the following.
The suitability of different storage tank options is evaluated based on three main criteria: required capacity, potential location, and choice of materials (VA DEQ, 2009). Certain storage tank types may not provide adequate storage volume or structural integrity for large capacity systems. Site constraints may limit the size of above- or below-ground storage units, and tank materials can influence water quality. The following table summarizes comparisons of tank materials.
Comparison of properties of different types of storage tanks
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|Barrels and trash cans||
|Galvanized steel tanks||
|Steel drums (55-gallon)||
|Concrete and masonry|
|Stone, concrete block||
|Pine, redwood, cedar, cypress||
If planned for potable water, tank material should be certified to NSF Standard 61.
Design considerations for above and below-ground tanks are summarized below.
For additional information, see Section 2.3 of the Ontario Guidelines and Procedure Tool I.2 from the Metropolitan Council Stormwater Reuse Guide. Section 2.4 Of the Ontario Guidelines for Residential Rainwater Harvesting provides a detailed summary of design and installation guidelines for above- and below-ground storage tanks. The guidelines can be summarized as follows:
The Metropolitan Council Reuse Guide includes these additional design considerations.
See the section on operation and maintenance for more detailed guidance on developing an operations and maintenance plan.
Stormwater ponds can be used as the storage component of a stormwater harvest and use system. These ponds are multi-purpose, providing stormwater retention, sedimentation, and storage for later use. In this way, stormwater harvest and use systems can be part of a treatment train approach for stormwater management. Existing ponds can be retrofitted to serve as a water source for a harvest and use system.
Maximum Detention Time - Average Daily Temperature
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|Maximum Detention Time (days) to limit algae blooms:||Average Daily Temperature (F)|
Footing (above and below ground tanks) and buoyancy (partly submerged below ground tanks) calculations are required for safe design of stormwater harvesting systems that use tanks. All below ground tank companies will provide information on buoyancy. The designer must consult appropriate standards in designing footings and determining tank buoyancy. The civil engineers will confirm that the weight of the specified backfill counteracts the buoyant force of the tank. Simple tank buoyancy calculations can be found in the literature (US EPA; FEMA (example C7); Oregon DEQ: ExcelCalcs; ARCSA/ASPE 63).
All harvest and use systems have a limited capacity to convey and store harvested stormwater. For this reason, stormwater harvesting systems must include a bypass valve or overflow structure to safely convey excess stormwater to downstream stormwater flow paths and networks when runoff exceeds design capacity.
Rainwater harvesting overflow discharge location - methods
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|Overflow Discharge Locations / Methods||Advantages||Disadvantages|
|Discharge to grade via gravity flow (most recommended||
|Discharge to storm sewer via gravity flow||
|Discharge to soakaway pit via gravity flow||
In a stormwater harvest and use system, flowing stormwater must be intercepted and conveyed to storage through a collection system: gutters, downspouts, pipes, drains, channels, or swales. The character of the collection system is determined in large part by the source area - rooftop runoff is typically conveyed via roof drains, gutters and downspouts which may be internal or external to the building structure; ground surface runoff is typically conveyed via pipes and channels. Collection system design is also influenced by the size, type, and location of the storage unit – pipes to underground storage tanks must be water tight to prevent leakage that results in saturation of soils and damage to building foundations. Site topography also affects the storage system. The elevation drops associated with the various components of a stormwater harvesting system and the resulting invert elevations should be considered early in the design, in order to ensure that the stormwater harvesting system is feasible for the particular site. The collection area may also affect treatment requirements if there are locations in the treatment area that will contribute larger quantities of pollutants. The collection system may include multiple treatment practices.
For the harvesting system to perform as intended, the sizing of conveyances must take into account both the volume and flow rate of intercepted stormwater. Intense rain storms may cause damage or pose a safety risk if conveyances and contingency overflows are undersized or otherwise deficient. Designers must follow appropriate design protocols and standards of practice in the sizing and design of all conveyances and fittings. Guidance on sizing and design of conveyances and fittings can be found here.
The design of ground surface conveyance systems should follow established stormwater management design standards and protocols. Water harvested from ground surface source areas is typically stored in stormwater BMPs (stormwater ponds) or underground tanks rather than aboveground tanks. The conveyance of stormwater to the storage system is via standard stormwater practices (stormsewers, ditches, and swales) and treatment is provided by the conveyance system (e.g. swales) or storage system (e.g. constructed stormwater ponds).
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; and the number of downspouts. 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.
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.
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.
While designing the controls system, specify if the system is to serve a public education objective. Controls packages can be delivered to tie to the internet and show the system operation in real time
Stormwater harvesting and use systems will typically contain one or more water quality treatment components to protect equipment, meet stormwater treatment objectives, minimize risk of exposure to stormwater pollutants, or meet end use water quality criteria. Terminology on treatment can be confusing and there is no consistent terminology in the literature. The following discussion is based on the location where treatment occurs relative to storage.
The type and number of treatment practices included in the system will depend both on the end use and the quality of runoff entering the system. The quality of harvested stormwater prior to any treatment is influenced by many factors including the catchment surface, surrounding land use, and drainage area activities (e.g. amount of road salt applied, presence of vehicle fueling areas, etc.). Information on typical pollutants found in stormwater can be found here. The following table summarizes typical pollutant concentrations in stormwater from different land uses. Harvesting stormwater from confirmed stormwater hotspots is not recommended.
In addition to source area considerations, harvesting and storing stormwater in a storage pond or basin may attract unwanted organisms which can degrade the quality of harvested water or the surrounding environment. Harvesting systems should be designed to minimize the potential for water quality to degrade during collection and storage. Some additional BMPs and maintenance guidelines for minimizing the risk of water quality degradation in harvesting systems are summarized in Table 3.4 (page 19) of the Ontario Residential Rainwater Harvesting Guidelines.
The Toolbox R.4: Treatment in the 2011 Metropolitan Council Stormwater Reuse Guide includes a comprehensive summary of treatment practices including target pollutants, treatment alternatives, pros and cons of treatment options, and considerations for the design, operation, and maintenance of treatment systems. The Minnesota Stormwater Manual contains a discussion of the applicability of several traditional stormwater BMPs and includes design information for these BMPs. Additional resources may need to be consulted for the proper design and sizing of treatment components not covered in the Minnesota Stormwater Manual. Source water quality, environmental concerns in harvesting and use systems, and operation and maintenance considerations are discussed in greater detail in the Water Quality Concerns, Environmental Concerns, and [Operation and Maintenance] sections, respectively.
Generalized steps in the design of water quality treatment systems for stormwater harvest and use (Ontario guide) include the following.
Pre-storage treatment practices are used upstream of the storage unit. These practices reduce particulate and particulate-bound pollutant loads, and remove gross solids from stormwater. By reducing particulate and gross solids loads, these practices also preserve the function and extend the maintenance life of downstream components in the system (tank, water treatment, distribution). For some outdoor use applications, pre-storage treatment will be sufficient to meet water quality regulations (Metropolitan Council, 2011). Treatment may be achieved using a single practice or more than one practice in series. Common treatment practices used in stormwater harvest and use systems are described in the table below.
Common pre-storage practices used in stormwater harvesting and use systems
Link to this table
|Swales||A swale is a wide, shallow, vegetated depression in the ground designed to channel drainage of water. Swales can reduce the velocity of flowing stormwater which allows larger particle to drop out of suspension in the water column. Swales are considered permanent primary treatment practices. Information on design, construction, maintenance, and performance assessment can be found here.|
|BMPs with an underdrain
||BMPs having an underdrain are designed to filter water prior to being discharged to the underdrain. These BMPs include bioretention, permeable pavement, sand filters, and tree trenches/boxes. Most of the water entering these BMPs is passed to the underdrain. In a harvest and use system, water passing through the underdrain must be discharged back to the harvest and use system. As water passes through the filter media, particulates are trapped and nutrients are removed via plant uptake. Note that media mixes with high organic matter may contribute nutrients and impart a dark color to the discharge water. Information on design, construction, maintenance, and performance assessment can be found in the appropriate section of this manual.|
|Manufactured screens and filters
||Filtration practices make use of porous media, mesh, or screens to trap pollutant as water flows through the filter. Filtration practices utilize sand filters or bio-filters, and include a number of proprietary devices. These devices are typically classified as pretreatment screen practices. Information on design, construction, maintenance, and performance assessment can be found here.|
||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 are flow-through structures with settling chamber or sediment traps that remove particulates and gross solids from stormwater. Some devices also remove floatable or include grease traps. These devices are typically classified as pretreatment settling practices. Information on design, construction, maintenance, and performance assessment can be found here.|
|Settling||Settling is the simple practice of slowing the flow rate of water so that particulates will fall out of the water column.
Settling practices include pretreatment settling practices such as settling basins, catch basins with sumps, and settling chambers. Information on design, construction, maintenance, and performance assessment of pretreatment practices can be found here.
Post-storage treatment is used to remove fine particulates (clay), dissolved pollutants (nutrient, metals, organics), and microorganisms (algae, bacteria, viruses) from harvested stormwater. This additional treatment is needed to comply with plumbing codes for all indoor uses and to comply with health criteria for most indoor uses and some outdoor uses. Common post-storage treatment practices used in stormwater harvest and use systems are listed in the table below. A schematic of treatment sequencing is provided in Figure R.4a: Stormwater Treatment Processes for Multiple End Users in the 2011 Met Council Reuse guide.
Summary of treatment practices used in stormwater harvest and use systems
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|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.|
|Activated Charcoal1||Remove chlorine, reduce odor, remove organics||After sediment filtration, before use and before or after UV filter|
|Boiling/distilling1||Kill microorganisms||Before use|
|Chemical treatments (chlorine) 1, a||Kill microorganisms||During storage or at distribution. Most commonly injected post filtration at point of distribution or in day tank. Should not be required in tank if adequate prefiltration is implemented|
|Electrodialysis2||Remove dissolved ions (salts)||Between storage and end use
Not required in rainwater systems
|In-line filters||Removes sediment, reduces turbidity, increases ultraviolet treatment||Between storage and end use and prior to UV treatment. Recommend 1 micron to meet current indoor water quality criteria of less than 1 ntu or sample the water and send to the UV manufacturer.|
|Microfiltration2||Remove suspended solids and microorganisms||Between storage and end use|
|Ultrafiltration||Remove suspended solids,organics||Between storage and end use|
|Nano-filtration2||Remove multivalent ions, organics microorganisms, viruses and proteins||Between storage and end use|
|Reverse Osmosis2||Remove monovalent ions, organics, microorganisms and viruses. May remove beneficial nutrients and produce highly corrosive water that needs to be reconditioned.||Between storage and end use|
|Sand filtration||Between storage and end use|
|Ozonizationb||Inactivate microorganisms||Before use typically applied through a venture and injected into the tank or a smaller contact tank.|
|Ultraviolet Disinfectionb||Inactivate microorganisms||Before use|
1Georgia DCA, 2009
2Met Council, 2011
aBefore activated charcoal (if included)
bAfter activated charcoal (if included). Refer to specific UV and carbon filter manufacturer for guidance,br> 3 VA Rainwater Harvesting Manual, Cabell Brand center, 2009
If the harvest and use system is being used for drip irrigation by gravity flow, the distribution system may simply be a length of drip tubing. However, if the system operates under pressure, there are additional components.
Some design considerations for pressurized distribution systems are similar to considerations for harvested stormwater conveyance systems (material properties, hydraulic calculations). Others, such as backflow prevention and signage, take into account the role of the distribution system as the link between harvested water and end users. The discussion in this section is limited to general considerations for pressurized distribution systems. Codes, regulations, and design standards that apply to distribution systems may vary considerably depending on the use application. Distribution systems for indoor uses, such as toilet flushing and laundry, must follow appropriate plumbing and building codes. Distribution systems for commercial or industrial uses may be subject to additional, industry specific standards. Designers should consult with a licensed plumbing professional for design of indoor and specialized distributions systems.
Harvest and use systems typically require a pump to deliver water from the storage unit to a point of use at a higher elevation. The type and size of pump used depends both on the energy required to transfer water to the point of use and the pressure required at the point of use. In most cases, a pressure pump, which can deliver low flows at high pressure, will be needed to meet end use water pressure requirements. Larger systems with back-up or secondary storage may also require transfer pumps to move water from one area of storage to another at low pressure.
Cost, system configuration, space constraints, ease of operation, and anticipated maintenance are additional factors that may influence the choice of pump type. Submersible pumps, which are placed at the bottom of water tanks, may have greater efficiency than surface pumps, which are located outside the water storage unit; however, pump maintenance may be more costly since pump units must be physically removed to perform maintenance. Provide a union disconnect to allow pump removal without entering tank to reduce maintenance costs. Additional advantages and disadvantages of submersible and surface pumps are summarized in Table 5.2 of the Ontario Rainwater Harvesting Guidelines.
In addition to point of use water pressure requirements, important design elements for pump systems include the following:
Improper installation and operation of pumps can result in serious damage to pumps or catastrophic failure. Qualified professionals should be consulted in the design of pumping systems and manufacturer’s guidelines should be followed in the installation, operation and maintenance of all pumps.
Irrigation is the most common use application of harvested stormwater (EOR, 2013). Rainwater in particular is desirable for irrigation since it is generally low in chlorides and is slightly acidic. At small scales, hose watering can be used to passively transfer water from a storage tank to garden or lawn plots. This simply requires a valve at a low tank elevation and a hose nozzle to control flow. At larger scales, hose watering becomes impractical, and irrigation systems design can become complex. Designers should consult trained professionals in designing irrigation systems. Irrigation system plans should be reviewed by a licensed plumbing professional, certified irrigation designer or professional engineer as part of the permitting process.
Most harvest and use systems will require a secondary water supply, or ‘makeup supply’ (also called backup supply) to meet demand when harvested water is not available or insufficient to meet demand (e.g. during summer). For small, decentralized irrigation systems or other outdoor uses, it may be acceptable to access the secondary supply by manually disconnecting from the harvest system storage and connecting the secondary supply. For other applications an automated makeup supply will be required. For example, a toilet flushing system cannot be turned off when harvested stormwater is unavailable. In most cases, the secondary supply will be either municipal drinking water or a private well.
The primary concern with makeup systems is that typically they require potable water to be brought into close proximity with harvested stormwater, which introduces a risk of a cross-connection between the two supplies (Despins, 2012). Cross connection of the two supplies can lead to harvested water being drawn into the potable water supply if there is a drop in pressure in the potable water distribution system. Whenever there is a risk of cross-connection between the two supplies, adequate backflow prevention must be installed to prevent contamination of the potable water supply (EPA, 2012). Designers are responsible for following applicable plumbing and electrical codes.
For automated systems, the design must also take into account stormwater management objectives and pump intake elevation. The makeup system must be triggered before [the pump head drops below operating levels], but makeup water should not displace storage volume required [for water quality or volume control objectives]. Design considerations for automated makeup supply systems are outlined in the Design Considerations for Automated Makeup Water Supply Systems table below.
Design Considerations for Automated Makeup Water Supply Systems
Link to this table
|Field Observations||For some practices, field observations may be required to determine the design demand. For example, a washing station may require field observation of the time spent washing each vehicle.|
|Air-gap||An air gap and other cross-connection requirements included in relevant building codes must be followed when combining potable and non-potable waters. An air gap physically separates two sections of pipe and is open to the atmosphere. The air gap must be located higher than the overflow drainage piping from the tank and the overflow drainage piping must remain free of blockage so that excess rainwater flows to the overflow system and does not back up and overflow at the air gap. Air gaps are not generally utilized or recommended in large outdoor storage tank applications due to freeze protection and loss of volume storage due to required air space.|
|Minimal make-up water storage||Stormwater management objectives must not be compromised if a secondary or makeup water supply is used. The design must provide adequate storage for the next design storm. Make-up systems must place a minimal amount of volume in the storage at any one time. Utilizations of an air gap in a day tank is one option to address this issue.|
|Level Indicators||Automatic make-up is typically triggered by a float switch, pressure transducer, or level indicator. The water elevation that triggers the make-up must be high enough to avoid running the pumps dry and must be lower than the passive draw-down orifice (if applicable). Float switches are preferred for critical operations including dry run protection and as fail safe cut off when transferring water to a day tank. Level indicators can be used for informational levels and non critical devices and are prone to failure in moist environments.|
|Wet wells||An option for larger, underground storage tanks is to have a separate wet well at a lower elevation. This eliminates the storage of make-up water in the harvest storage tank.|
Required Storage Capacity
Makeup Water Supply System & Backflow Prevention
The performance of stormwater harvest and use systems depends not only on sound design, but also on appropriate installation, operation, and maintenance. Considerations for implementation phase activities are discussed briefly below.
See Construction Sequence. Construction oversight should include a professional familiar with installation of rainwater or stormwater harvesting systems and installation of all manufactured components should follow manufacturer’s specifications. Construction should be sequenced to ensure that stormwater is managed appropriately while the harvest and use system is off-line.
Depending on the context of the harvest and use application, designers may be required to submit O&M plans for approval by municipal, regional, or state agencies. This should be done prior to installation and in cooperation with parties who will hold ownership of the system during construction, site development, or post-construction. See Operation and maintenance for stormwater and rainwater harvest and use/reuse.
Harvest and use projects may provide an opportunity to offer public education and outreach on stormwater management or integrated water management issues - particularly in commercial, municipal, or institutional settings which are generally accessible to the public. In such cases, it may be advantageous to seek local partners who can provide support for outreach goals. Partnerships should be developed early in the design process to maximize potential education and outreach opportunities. The benefits of public participation in stormwater harvest and use projects are discussed in Chapter 8, Public Outreach, Participation and Consultation of the EPA Water Reuse Guide.