Chronic and episodic water shortages are becoming common in many regions of the United States, and urban population growth in water scarce regions further compounds the challenges. In mid-2015, much of California faced an “exceptional drought” within an already moderate to severe drought throughout much of the western United States, but non-arid regions, such as the Southeast, have not been exempt from major water shortages. Increasingly, alternative water sources such as stormwater and graywater are being viewed as resources to supplement scarce water supplies, particularly in urban areas experiencing large population growth.
Stormwater runoff is the water from rainfall or snow that can be measured downstream in a pipe, culvert, or stream shortly after the precipitation event. For the purposes of this report, the term “stormwater” is used broadly to include runoff from rooftops, as well as other runoff from small to large source areas. Graywater is untreated wastewater that does not include water from the toilet or kitchen, and may include water from bathroom sinks, showers, bathtubs, clothes washers, and laundry sinks. Both can offer on-site alternative water supplies to a household or building, although stormwater can be captured and used at neighborhood and regional scales and graywater can be reused in neighborhoods and large multi-residential developments.
Stormwater and graywater can serve a range of nonpotable uses, including irrigation, toilet flushing, washing, and cooling, although treatment may be needed. Stormwater may also be used to recharge groundwater, which may ultimately be tapped for potable use. In addition to increasing of local water supply, harvesting stormwater has many potential benefits, including saving energy, preventing pollution, reducing the impacts of development on urban streams, and enhancing the livability of cities. Similarly, the reuse of graywater can enhance water supply reliability and extend the capacity of existing wastewater systems in growing cities.
Despite the benefits of using local alternative water sources to address water demands, many questions remain that have limited the broader application of graywater and stormwater capture and use. In particular, limited information is available on the costs, benefits, and risks of these projects, and beyond the simplest applications, many state and local public health agencies have not developed regulatory frameworks for full use of these local water resources. With funding support from the Environmental Protection Agency (EPA); National Science Foundation; Water Research Foundation; Water Environment Research Foundation; Los Angeles Department of Water and Power; WateReuse; City of Madison, Wisconsin; National Water Research Institute; and the National Academies of Sciences, Engineering, and Medicine’s President’s Fund, the Academies formed the Committee on the Beneficial Use of Graywater and Stormwater to analyze the risks, costs, and benefits on various uses of stormwater and graywater, as described in Box S-1. This study addresses technical, economic, regulatory, and social issues associated with graywater and stormwater capture and use across a range of uses and scales.
Graywater and stormwater capture and use can expand local water availability while providing additional financial, environmental, and social benefits, such as reduced water pollution and combined sewer overflow discharges (for stormwater), drought-resistant year-round water availability (for graywater), and diversification of water supplies. For stormwater, neighborhood- and regional-scale stormwater capture projects can contribute significantly to urban water supplies. In most cases, the technology is mature, and treatment can be provided to address contaminants to meet “fit-for-purpose” water quality objectives. However, broader implementation is hindered by the absence of risk-based guidelines for stormwater and graywater use across a range of applications, as well as water quality data (particularly for human pathogens) necessary to assess these risks.
There is no single best way to use graywater or stormwater to address local water needs because project drivers and objectives, legal and regulatory constraints, potential applications, local site and climatic conditions, source water availability, and project scales all vary widely. The report instead recommends clear objectives and provides a decision framework in Chapter 9 that can be used when considering the use of graywater or stormwater, with supporting information for each of the decision steps. Major report findings are highlighted below along with recommended research needs to improve support for decision making.
Potential potable water savings from graywater and stormwater will vary based on factors such as local climatic conditions, approaches, and scales. Chapter 3 details lessons learned regarding water saving potential largely based on an original scenario analysis in six U.S. locations. The scenarios considered medium-density residential development using graywater or stormwater for conservation irrigation of turfgrass, toilet flushing, or both.
Water savings from stormwater capture and use are dependent on tank size and the amount and timing of precipitation relative to water demand. Substantial potential household-scale water savings (24 to 28 percent) from the capture and use of roof runoff were calculated for scenario analyses in four of the six cities analyzed using one moderately sized (2,200-gallon [8,300-liter]) storage tank per house. These cities—Lincoln, Nebraska; Madison, Wisconsin; Birmingham, Alabama; and Newark, New Jersey (all located in the Midwest or East Coast)—have year-round rainfall closely matching irrigation demands. In contrast, the scenario analysis showed lower potential potable water savings for Los Angeles and Seattle (5 and 15 percent, respectively). In much of the arid West, the timing and intensity of rainfall limits the capacity of household-scale stormwater collection to reduce potable water use. Very small stormwa-
ter water storage volumes provide much lower water savings benefits (less than 2 percent in Los Angeles to up to 10 percent in Newark using two 35-gallon [130 liter] rain barrels per house, for example).
Neighborhood- and regional-scale stormwater capture projects can contribute significantly to urban water supplies. This is especially important for arid climates in which stormwater can be stored in aquifers for use during drought or the dry season. Based on 1995-1999 data for Los Angeles, average stormwater runoff from medium-density residential developments, if captured and stored, would be roughly sufficient to meet indoor residential water needs in those areas.
Graywater reuse offers the potential for substantial potable water savings and could provide a reliable source of water for arid regions. Based on the committee’s scenario analyses, graywater reuse in Los Angeles and Seattle provides greater potential potable water savings than does household-scale stormwater capture, because graywater provides a steady water source during summer months with little or no rainfall. Additionally, the analyses showed that graywater can more effectively meet toilet flushing demand compared to stormwater in all cities analyzed. Graywater use for toilet flushing has been demonstrated to achieve potable water savings as theoretically expected without impacting water availability to downstream users, but water savings associated with graywater irrigation at the household scale have not been demonstrated with confidence. Little is known about the impact of installing on-site nonpotable water systems on human water use behavior, which points to the need to study behavioral responses to conservation measures.
Beneficial use of graywater is typically more appropriate for residential and multi-residential applications than commercial application. Most commercial facilities do not generate enough graywater to justify use for toilet flushing or irrigation. Even offices that have on-site showers are not likely to generate enough graywater to meet end-use demands (toilet or irrigation). Some commercial applications for which graywater use may be appropriate include fitness facilities, hotels, and laundromats.
If water conservation is the primary objective for stormwater and graywater investments, then strategies that reduce outdoor water use should first be examined. In arid regions, potential potable water savings for residential and multi-residential use of stormwater and/or graywater are significant, but small relative to today’s outdoor water demand. Although use of graywater or roof runoff for toilet flushing can reduce indoor demand by up to 24 percent, the committee’s scenario analysis estimated potential water savings of only 13 percent with graywater use in the Los Angeles area (and significantly less for stormwater capture, even using large tanks). Significantly reducing or eliminating irrigation demand, for example through the use of xeriscaping, would provide much larger reductions in water demand in arid regions. In these circumstances, graywater could be used to supply irrigation water to meet specific small irrigation needs. Otherwise, graywater and stormwater may help facilitate the continued use of landscaping that is not sustainable in the long term and inappropriate for local climate conditions.
Understanding the potential applications of graywater and stormwater as on-site water supplies and associated treatment needs requires a clear understanding of source water quality.
Pathogens and organic matter in graywater impact opportunities for beneficial uses without treatment. Human pathogens are likely to occur in graywater, although the specific types and concentrations vary substantially among sources and their occurrence and fate are not yet well understood. Organic matter is present in high enough concentration in graywater to enhance microbial growth, thus limiting the potential uses of graywater without disinfection. Sodium, chloride, boron, and other chemicals can impact the quality of graywater for irrigation uses. Best management practices exist for source control of microbial and chemical constituents, and such practices can be implemented at the household scale to reduce concentrations of these constituents in graywater.
Stormwater quality is highly variable over space and time and might contain elevated levels of microorganisms, metals, organic chemicals, and sediments, potentially necessitating treatment to facilitate various beneficial uses. Stormwater quality is a direct function of land use, source area, catchment size, and climatic and seasonal factors. Existing data suggest that most stormwater contains elevated levels of organic matter, suspended sediment, and indicator bacteria. Metals are also commonly found in urban stormwater runoff and may pose concerns for some beneficial uses, including irrigation and surface reservoirs or wetland features. Despite the enormous spatial and temporal variability of stormwater quality, the treatment systems required for achieving end uses may be relatively consistent over a wide variety of catchments. Land uses, contributing areas, and collection materials can be selected that minimize contaminants of concern to optimize stormwater quality and minimize treatment requirements for the intended use.
Little is known regarding the occurrence of human pathogens and organic chemicals in stormwater, and additional research is needed to characterize their occurrence and fate. Studies on the presence of microorganisms in stormwater have consistently reported high concentrations
of fecal indicator microorganisms across different source areas. In the few studies that have analyzed for pathogenic microorganisms in stormwater, they have generally been detected, at least in some samples. However, more work is needed to characterize their occurrence and fate, particularly for roof runoff systems where the beneficial use of untreated stormwater is common and raises concerns for uses with the potential for human exposure. More research is also needed to characterize the occurrence of organic chemicals in stormwater and their fate during various uses.
HUMAN HEALTH AND ENVIRONMENTAL RISKS
Although no documented reports of adverse human health effects from the beneficial use of stormwater or graywater have been identified, additional examination of risk is necessary to support safe and appropriate design and implementation of stormwater and graywater use systems.
Risk assessment provides a means to determine “fit-for-purpose” water quality criteria or treatment needs based on human exposures. Risk from graywater or stormwater is a factor of chemical or microbial concentrations and exposure (typically, the amount of water ingested). Thus, unlike drinking water criteria, which are established based on 2 liters of water consumed per day, criteria for applications with minimal human exposures might allow for much higher concentrations of contaminants in graywater or stormwater and still result in acceptably low health risks. Risk assessment tools provide a ready means for developing such criteria for many chemicals and microbes for which drinking water criteria exist. As nonpotable on-site use of graywater and stormwater becomes more common, additional public health risk communication efforts would be beneficial to help the public understand risk-based treatment objectives and appropriate safeguards.
Considering the low exposures in most nonpotable graywater and stormwater applications, pathogens represent the most significant acute risks. Available risk assessments and the committee’s risk calculations using limited, observed pathogen data and various possible exposure scenarios suggest that disinfection is necessary for many uses of graywater, including spray irrigation, food crop irrigation, and toilet flushing, to protect human health. Subsurface landscape irrigation (including drip systems covered by landscape) with graywater does not pose significant risk, if best practices are followed, because human exposure is minimized. These findings are consistent with most regulatory guidance, although the risk of surface drip irrigation (without landscape cover) at the household scale remains unresolved. Limited data on pathogens in roof runoff suggest that treatment may also be needed, even for low levels of human exposure, such as toilet flushing, although more research on pathogens in roof runoff is needed. Chemicals become of concern in groundwater infiltration projects, where drinking water supplies could be impacted.
Extremely limited data are available on the pathogen content in graywater and roof runoff, which precludes a full assessment of microbial risks. Most water quality monitoring assesses microbial indicator data, and microbial risk assessments are conducted using assumed relationships between the concentrations of indicator microorganisms and pathogenic microorganisms. Consistent relationships between surrogates and contaminants have not been established for graywater or stormwater. This is a particular concern for roof runoff, which may include microbial indicator organisms from the waste of animals that do not transmit human pathogens.
Enhanced infiltration of stormwater for groundwater recharge poses risks of groundwater contamination and necessitates careful design to minimize those risks. The risk of groundwater contamination from stormwater recharge is related to the contaminants present, any pretreatment processes installed, the capacity for the subsurface soil and engineered media used in the infiltration basin to remove them, and the proximity to groundwater used as a drinking water supply. Dry wells, which directly inject water into the subsurface, and surface infiltration through sandy soils do not effectively attenuate chemical contaminants.
Environmental impacts from the outdoor use of graywater and stormwater generally appear low, but risks depend upon several factors, including water quality, application rates, and plant or animal species exposed. Effects of irrigation on plant and soil health can occur from salts, boron, and metals, but source control practices and appropriate irrigation rates can reduce these impacts. If not controlled at the source, then long-term build-up of boron or salt can pose risk to plant and soil health, depending on soil and climatic conditions. Constructed stormwater ponds and wetlands typically contain elevated contaminant levels sufficient to impair reproduction among some aquatic species, often leading to a habitat dominated by pollution-tolerant organisms. Such ecological affects may be acceptable, considering the overall environmental benefits provided by such features, including reduced pollution to other surface waters, but the ecological objectives of such projects are often unclear, hindering efforts to limit ecological risks through improved management and design.
STATE OF PRACTICE AND SYSTEM DESIGN
The report also outlines the state of practice for graywater and stormwater system designs at household, neighbor-
hood, and regional scales and treatment that may be used to meet specific quality objectives.
Graywater irrigation at the household scale can be achieved with simple systems that require little energy and maintenance. These simple systems, such as the laundry-to-landscape system or systems that include storage, coarse filtration, and pumps, typically do not include organic matter removal or disinfection, and risk is managed through a series of best management practices. Neighborhood-scale systems typically provide disinfection where access-control is not feasible, which creates more system complexity and requires more energy.
Graywater reuse for toilet flushing requires plumbing components and treatment systems that are most appropriate in multi-residential buildings or neighborhoods. Graywater systems for toilet flushing require dual plumbing with a connection to potable water and backflow preventers that require annual inspection. Treatment systems for toilet flushing should include disinfection to reduce risk and prevent bacterial growth, and existing technologies are available. Even the simplest treatment systems require periodic maintenance that can be a burden at the household level, although such maintenance is more easily managed by contractors or on-site staff at the neighborhood/multi-residential scale. For broader adoption of graywater for toilet flushing at the household scale, treatment systems are needed that are low maintenance and include process automation and control to ensure safe use at a reasonable cost.
Many state graywater treatment standards for toilet flushing are not risk-based or fit-for-purpose. Standards vary widely across states, resulting in an inconsistency in treatment systems that can be applied, and several are based on standards unrelated to residential or multi-residential toilet flushing. Many standards for toilet flushing may be unnecessarily strict in terms of organic content and turbidity removal, resulting in requirements for technologies that are costly, energy-intensive, and require frequent maintenance. Additional research is needed to determine appropriate design standards for dissolved organic carbon and turbidity that prevent aesthetic and maintenance issues while allowing proper function of disinfection systems when using graywater for toilet flushing.
New developments and future urban planning provide opportunities for rethinking the conveyance and use of various water and waste streams for maximum cost, energy, and water savings. Separation of graywater results in blackwater that is more concentrated in solids and organic matter than conventional domestic wastewater and may be amenable to methane biogas production. These systems can also be integrated with urine separation including nutrient capture. Thus, graywater reuse can be a key element of energy-efficient urban water and resource management systems that not only minimize net water abstraction from the environment but also achieve a high level of energy and nutrient recovery.
The state of practice and development of cost-effective and safe stormwater capture systems for roof runoff are hindered by the lack of data on human pathogens and the risk associated with various uses. Design and treatment standards are generally well accepted for nonpotable use of runoff collected from land surfaces, and no treatment other than coarse solids removal is needed for subsurface irrigation where human exposures are minimal. For beneficial uses of roof runoff with low to moderate exposures, additional pathogen data and risk analyses are needed to establish a consistent state of practice for on-site stormwater use. Technologies are mature and can be readily adapted for various scales and uses.
Operations and maintenance of household and neighborhood graywater and stormwater use systems is not well guided or monitored. All systems that capture graywater and stormwater for beneficial use require routine maintenance. For systems where disinfection is not required (e.g., subsurface irrigation), failure to conduct needed maintenance poses operational concerns but does not pose a significant risk for human health or environmental quality. However, for systems with disinfection processes to protect human health (i.e., systems for toilet flushing), ongoing maintenance is critical. Although many states require that installed systems meet certain water quality targets, ongoing monitoring is not required. More guidance is needed to ensure safe operations of graywater and stormwater treatment systems at household and neighborhood scales. Because frequent routine water quality analyses are expensive and impractical even at the neighborhood scale, system operational performance standards and online monitoring of surrogate parameters (e.g., residual chlorine, suspended solids, or turbidity) should be considered.
Stormwater infiltration for aquifer recharge is commonly practiced, but designs and regulations in the United States may not be adequately protective of groundwater quality. Design for large-scale stormwater infiltration projects are still emerging. For many locations, the design and performance standards for stormwater infiltration have been developed to address surface water regulatory drivers rather than the protection of groundwater quality. Of particular concern is the infiltration of organic contaminants and salts from highly urbanized areas into water supply aquifers, although human pathogens may also be of concern depending on the infiltration site characteristics. Thoughtful planning, source area selection, source control, and mechanisms to integrate treatment into the watershed could improve ef-
ficiency of these systems and reduce the amount of treatment required. Treatment systems, such as engineered wetlands and filter media, may also be needed for regional-scale systems where source control is challenging.
COSTS AND BENEFITS
It is important to recognize the full suite of benefits—as well as the full costs—of graywater and stormwater projects, although it may be empirically challenging to do so. Some of these benefits are financial and can be readily estimated and portrayed in monetary terms, such as the value of water savings or the avoided cost of obtaining water from an alternative supply. In addition, important social and environmental benefits may apply but may be difficult to quantify or monetize. Costs for graywater and stormwater projects are highly dependent on scale, system design, and plumbing requirements, and generally are better understood than the benefits, yet there is a lack of well-documented and complete cost information for many of the possible applications. The following findings are based on limited available cost data and some example analyses of potential water savings based on the committee’s scenario analysis.
Simple household-scale graywater reuse or roof runoff capture systems can offer reasonable financial payback periods under certain water use scenarios and appropriate climate conditions. For example, considering the committee’s scenario analysis of potential water savings in medium-density residential development, simple laundry-to-landscape graywater systems can offer payback periods as low as 2.5-6 years (not accounting for the cost of labor), with the shortest payback periods in the Southwest and central United States. These estimates assume graywater for irrigation actually offsets potable use—an assumption that remains to be demonstrated. Longer payback periods were estimated for rain barrels (5-26 years) and cisterns (14 to more than 50 years, not accounting for labor) used for conservation irrigation. The longer payback periods reflect locations where distinct wet and dry seasons do not coordinate well with irrigation demands, as in the arid Southwest. The cost of installation (whether by contracting with a paid professional or valuing homeowner-provided labor) greatly extends the payback period, as do water uses in which additional plumbing and treatment are required.
Economies of scale are evident for large stormwater and graywater use projects. Several regional stormwater capture and recharge projects in Southern California, for example, can pay back large dividends by avoiding the cost of expensive imported water in addition to other social and environmental benefits. Based on available unit cost data, stormwater alternatives designed to recharge groundwater at neighborhood and regional scales tend to be much less expensive than on-site or neighborhood tank capture. Published cost data from larger-scale graywater projects is extremely limited, but some efficiencies of scale would be associated with graywater toilet flushing systems in large, new multi-residential developments (particularly compared to smaller retrofits). Additional incentives may be possible if such investments defer water and wastewater infrastructure expansion in densely populated urban areas.
Depending on the stormwater or graywater system design, energy savings are possible compared with conventional water supplies, but data for a sound assessment are lacking. Conventional water systems in the United States are reported to provide water to customers at an energy cost of between less than 1 kWh/m3 to as much as 5 kWh/m3, depending mostly on pumping costs for conveying the water from the source to the water treatment plant. Rooftop stormwater capture systems have been reported in a limited number of studies to have a greater energy demand (median is 1.4 kWh/m3) in practice than in theoretical studies (0.2 kWh/m3), but many potential variables (e.g., scale, pumping, treatment, material inputs) will drastically affect the life-cycle energy demands of these systems, and the effects of these variables in practice remain poorly understood.
LEGAL AND REGULATORY ISSUES
As technologies and strategies continue to advance, graywater and stormwater use is being incorporated into law in a variety of respects at the federal, state, and local levels. However, as is often the case with innovative technologies, the law has not evolved quickly enough to keep up with the technology and its use. Several legal and regulatory constraints remain that hinder the capacity for graywater and stormwater to significantly expand the nation’s water supplies.
In most western states, acquisition of water rights is a requirement for large-scale stormwater capture and use projects, and water rights may limit widespread implementation of smaller-scale stormwater and graywater projects for consumptive uses. Unless water rights can be acquired or legislative solutions developed, opportunities for large-scale stormwater capture projects to expand existing water supplies would largely be limited to coastal regions with no downstream users or to non-consumptive uses (e.g., toilet flushing). Several states (e.g., California, Kansas, Oregon, Utah, and Washington) have established regulations that allow small-scale roof runoff capture projects to proceed without water rights permits, and only one state (Colorado) has strict limits on stormwater capture and use out of concern for water rights impacts. The right to stormwater and graywater use in most prior-appropriation states has not been firmly resolved
through judicial decisions, leaving an unclear outlook for projects that have not acquired water rights, because they could be vulnerable to legal challenges. New scientific analyses of the impacts to return flows of various on-site water uses in different regions would help clarify these concerns, but additional legal research and guidance could better facilitate the use of on-site water supplies, considering potential legal challenges.
There is substantial variation in on-site graywater and stormwater regulations at the state level with respect to design and water quality for household-scale projects, which leads to varying exposures and risk. As one example, there is lack of consistency among states on whether outdoor graywater use is limited to subsurface irrigation. At least three states allow drip irrigation without landscape cover, which could lead to higher pathogen exposures. In addition, states vary on their regulation of untreated graywater irrigation of food crops. Whether such exposures would lead to unacceptable risks at various scales has not been definitively resolved, but higher risks are likely with increased exposures. Regulations affecting large-scale graywater and stormwater use where public access is not controlled tend to include conservative public health protection measures, such as disinfection.
The lack of authoritative, risk-based guidelines for the design and potential applications of graywater and stormwater in the United States is a major impediment to their expanded use. The wide variability in existing regulations and absence of federal guidance leaves stakeholders and local decision makers uncertain about the safety of these practices and the appropriate level of treatment necessary for particular uses. Development of rigorous, risk-based guidelines for graywater and stormwater across a range of possible uses and exposures could improve safety, build public confidence in the practices, reduce expenditures on unnecessary treatment, and assist communities that lack an existing regulatory framework for on-site water supplies. Such guidelines could be developed by the Environmental Protection Agency (EPA), a collaboration of states, or a collaboration of U.S. water organizations working with the EPA. This guidance could then serve as a basis for developing standards of practice for on-site nonpotable water use. Oversight and enforcement of water quality standards for applications with significant exposures is also important but challenging, and local enforcement agencies would benefit from additional guidance on appropriate, cost-effective maintenance, monitoring, and reporting strategies.
Information is generally available to support water management decision making for simple, household-scale graywater and stormwater systems with minimal human exposures, but additional research would enhance decision making for larger systems or those with significant exposures. Key uncertainties affect the capacity to make fully informed decisions on appropriate and cost-effective designs, particularly for larger or more complex graywater or stormwater beneficial use systems, including
- Fit-for-purpose water quality objectives that are protective of public health;
- The occurrence and fate of pathogens in stormwater and graywater;
- Costs and benefits for neighborhood- and regional-scale systems, including nonmonetized benefits, such as water pollution control and community amenities;
- Energy implications of on-site alternative water supplies; and
- Long-term system performance and maintenance needs.
A summary of research needs to enhance decision making and ensure the safe and reliable use of graywater and stormwater to reduce water demand is provided in Box S-2.