Overview for stormwater and rainwater harvest and use/reuse

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

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

Introduction

Example Stormwater Harvesting and Use System Schematic

A stormwater harvest and reuse system is a constructed system that captures and retains stormwater for beneficial use at a different time or place than when or where the stormwater was generated. A stormwater harvesting and use system potentially has four components (See Example Stormwater Harvesting and Use System Schematic at right):

• collection system (which could include the catchment area and stormwater infrastructure such as curb, gutters, and stormsewers),
• storage unit (such as a cistern or pond)
• treatment system: pre and post (that removes solids, pollutants and microorganisms, including any necessary control systems), if needed, and the
• distribution system (such as pumps, pipes, and control systems).

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

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

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

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

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

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

Water harvesting and use system matrix
Link to this table

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

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

Function within stormwater treatment train

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

MPCA permit applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. Standards for various categories of stormwater management practices must be applied in all projects in which at least one acre of new impervious area is being created.

Rainwater harvest and use/reuse systems are not defined and discussed in the CGP. Because water captured by these systems is typically used for irrigation, the Infiltration systems category described in the CGP is generally applicable. Water Quality Volume requirements are applied to the volume of water stored in a tank or pond, which is used to meet the instantaneous volume requirement in the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the bioretention practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.

There are situations, particularly retrofit projects, in which a harvest and use/reuse practice is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular practice, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Retrofit suitability

The ability to use harvest and use/reuse as a retrofit practice depends on the development situation. In areas where above-ground storage is limited, ponds and wetlands are not feasible. In new developments, ponds and wetlands may be desirable due to increased volume of storage. Because harvest and use/reuse systems utilize irrigation, soil infiltration rates are generally not limiting.

Special receiving waters suitability

The table below provides guidance regarding the use of harvest and use/reuse practices in areas upstream of special receiving waters. Note that most harvest and use/reuse practices will fall under the infiltration category.

Infiltration and filtration bmp¹ design restrictions for special waters and watersheds. See also Sensitive waters and other receiving waters.
Link to this table

BMP Group	receiving water
	A Lakes	B Trout Waters	C Drinking Water2	D Wetlands	E Impaired Waters
Infiltration	RECOMMENDED	RECOMMENDED	NOT RECOMMENDED if potential stormwater pollution sources evident	RECOMMENDED	RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride
Filtration	Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments	RECOMMENDED	RECOMMENDED	ACCEPTABLE	RECOMMENDED for non-nutrient impairments

¹Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
² Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies

Cold climate suitability

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

Water quantity treatment

Water quantity considerations for harvest and reuse systems are typically not driven by performance goals, such as the Minimal Impact Design Standards performance goal of 1.1 inches or the Construction Stormwater General permit requirement of 1 inch. Instead, water quantity considerations are more often driven by meeting a specific site goal, such as an irrigation need, and/or by available storage. On permeable soils (hydrologic soil group (HSG) A and B soils), other practices such as bioinfiltration are more effective at retaining water. On HSG C and D soils harvest and reuse systems can be utilized to retain water and are more effective than traditional infiltration practices since the water can be applied across larger areas (e.g. for irrigation) or used indoors.

When designing a harvest and reuse system, water quantity considerations include the following.

• Determining the appropriate storage size for a stormwater and rainwater harvest and use/reuse system
• Estimating the water balance for a stormwater and rainwater harvest and use/reuse site
• Correctly calculating the volume reduction

Water quantity treatment

Harvest and reuse systems are an excellent stormwater treatment practice due to the variety of pollutant removal mechanisms including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. They are particularly effective when used for irrigation on C and D soils where traditional infiltration practices are less effective. Pollutant removal and effluent concentration data for select parameters are provided in the adjacent table.

Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.
Link to this table

Practice	TSS	TP	PP	DP	TN	Metals1	Bacteria	Hydrocarbons
Infiltration2	3	3	3	3	3	3	3	3
Biofiltration and Tree trench/tree box with underdrain	80	link to table	link to table	link to table	50	35	95	80
Sand filter	85	50	85	0	35	80	50	80
Iron enhanced sand filter	85	65 or 746	85	40 or 606	35	80	50	80
Dry swale (no check dams)	68	link to table	link to table	link to table	35	80	0	80
Wet swale (no check dams)	35	0	0	0	15	35	35	NSD
Constructed wet ponds4, 5	84	50 or 685	84	8 or 485	30	60	70	80
Constructed wetlands	73	38	69	0	30	60	70	80
Permeable pavement (with underdrain)	74	41	74	0	NSD	NSD	NSD	NSD
Green roofs	85	0	0	0	NSD	NSD	NSD	NSD
Vegetated (grass) filter	68	0	0	0	NSD	NSD	NSD	NSD
Harvest and reuse	Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.

TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
¹Data for metals is based on the average of data for zinc and copper
²BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
³Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
⁴Dry ponds do not receive credit for volume or pollutant removal
⁵Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
⁶Lower values are for Tier 1 design. Higher values are for Tier 2 design.

Beneficial use of stormwater key considerations

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

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

Outdoor uses

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

Irrigation

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

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

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

Water features

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

Sanitary sewer flushing

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

Street cleaning and dust control

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

Vehicle/building washing

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

Firefighting

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

Indoor uses

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

Flushing

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

Fire suppression

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

Washing

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

Cooling

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

Process and boiler water

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

Drinking water

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

Water harvesting and use benefits

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

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

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

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

Water harvesting: use of codes and standards

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

Plumbing codes

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

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

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

Minimum water quality
Link to this table

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

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

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

Water harvesting and use considerations

Treatment

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

Storage versus supply

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

Cold climates

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

Site constraints

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

Public acceptance

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

References

• Alan Plummer Associates, Inc. 2010. Stormwater Harvesting Guidance Document for Texas Water Development Board.
• Hatt, B. E., A. Deletic, and T. D. Fletcher. 2006. Integrated Treatment and Recycling of Stormwater: A Review of Australian Practice. Journal of Environmental Management 79, no. 1: 102–113.
• Metropolitan Council. 2011. Stormwater Reuse Guide. Prepared by CDM Smith, Short Elliott Hendrickson (SEH), and Hoisington Koegler Group.
• National Academies of Sciences, Engineering, and Medicine (NAS). 2015. Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits. Washington, DC: The National Academies Press. Prepublication Copy.
• Natural Resource Management Ministerial Council, Environment Protection and Heritage Council, and National Health and Medical Research Council. 2009. Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2) - Stormwater Harvesting and Reuse.
• North Carolina Department of Environmental Quality (NCDEQ). April 2014. North Carolina Stormwater BMP Manual. Chapter 25. Rainwater Harvesting. (Draft document).
• United States Environmental Protection Agency. 2004. Guidelines for Water Reuse. EPA 625/R-04/108. USEPA, Cincinnati, OH.
• United States Environmental Protection Agency (USEPA). June 2008. Indoor Water Use in the US. EPA Water Sense. Doc ID: EPA-832-F-06-004.
• United State Environmental Protection Agency (USEPA). September 2012. 2012 Guideline for Water Reuse. Document ID EPA/600/R-12/618.

Related pages

• Overview for stormwater and rainwater harvest and use/reuse
• Design criteria for stormwater and rainwater harvest and use/reuse
• Construction specifications for stormwater and rainwater harvest and use/reuse
• Operation and maintenance for stormwater and rainwater harvest and use/reuse
• Water quality considerations for stormwater and rainwater harvest and use/reuse
• Environmental concerns for stormwater and rainwater harvest and use/reuse
• Cost-benefit considerations for stormwater and rainwater harvest and use/reuse
• Case studies for stormwater and rainwater harvest and use/reuse
• Calculating credits for stormwater and rainwater harvest and use/reuse
• Definitions for stormwater and rainwater harvest and use/reuse
• Requirements, recommendations and information for using Harvest and re-use/cistern as a BMP in the MIDS calculator
• Links for stormwater and rainwater harvest and use/reuse
• References for stormwater and rainwater harvest and use/reuse
• Technical support for stormwater and rainwater harvest and use/reuse