Many parts of the United States face chronic or episodic water shortages. In the Colorado River Basin and California, recent multi-year droughts have resulted in reservoirs at near record low levels (Figure 1-1), forcing state drought declarations and decreased water allocations for many users. Climate change is anticipated to further impact water supplies by altering the timing and amounts of precipitation, increasing evapotranspiration, and altering snowmelt and the timing of runoff in the western states (Barnett and Pennell, 2004). Also, longer-term droughts are expected to intensify in the southwest, the Great Plains, and the southeast (Melillo et al., 2014). Meanwhile, population growth in the more water scarce regions of the United States (Figure 1-2) compounds the issue, placing additional strain on water supplies and infrastructure. For example, the states of California, Nevada, Arizona, Texas, and Florida saw their populations increase between 85 percent and 400 percent between 1970 and 2009, while the overall population of the United States increased by less than 50 percent during that same timespan (NRC, 2012a). The current population in the United States (321 million) is expected to grow by 30 percent by 2060, mostly in cities (Colby and Ortman, 2014).
To help alleviate these water shortage problems, alternative water sources such as stormwater and graywater are increasingly being viewed as resources to supplement scarce water supplies rather than as waste or nuisance water. Harvesting stormwater has many potential benefits including water conservation, energy savings, and reduced impacts of urban development on the environment. Even in the more humid areas of the United States, stormwater capture and use are growing in popularity as a means to enhance water supply and reduce nutrient loads to receiving waters. Similarly, the reuse of graywater for residential and building landscape irrigation or other nonpotable uses can reduce the year-round demand on public water supplies treated to drinking water standards (hereafter called potable water).
Stormwater and graywater use are two options among many in a diverse water supply portfolio, including conservation, desalination, managed aquifer recharge, and wastewater reuse (see NRC, 2008a, 2009a, 2012a). Conservation and water use efficiency are generally the best ways to address water supply problems on a broad scale. In some water-challenged areas, numerous water conservation initiatives have already been implemented, including installing
low-flow devices in buildings and incentivizing less-water-intensive landscaping, but water utility managers need additional strategies to address current and future challenges of water supply reliability and water demand. For example, an analysis of the water supply portfolio for the City of Los Angeles shows that continued conservation efforts over the next 20 years (based on utility projections) may accommodate future population growth but are not likely to significantly reduce the present demand for imported water (Luthy and Sedlak, 2015).
Brackish and seawater desalination and wastewater reuse have received substantial attention as means to augment public water supplies (NRC, 2008a, 2012a), but less information is available on the beneficial use of graywater and stormwater. The process through which graywater and urban stormwater are captured, stored, and used has mostly developed in an ad hoc manner (Grebel et al., 2013). Because of the absence of ample documentation of costs, performance, and risks, many utilities are hesitant to integrate the practices into their long-term water resource plans. Potential public health risks from microbial or chemical contamination associated with graywater or stormwater use also raise concerns and subsequent debate over the appropriate regulatory framework to protect public health without adding excessive cost and permitting burdens to these projects (EBMUD, 2009). To better address these challenges, the National Academies of Sciences, Engineering, and Medicine (Academies) formed a committee to conduct a study on the risks, costs, and benefits of stormwater and graywater use to augment and conserve existing water supplies.
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 (NRC, 2009a). What constitutes “shortly” depends on the size of the watershed and the efficiency of the drainage system. From a practical perspective, stormwater runoff is water that may be in an engineered feature, running over the ground surface, or seeping into the shallow subsurface and soon reemerges as seeps.
Stormwater runoff is distinct from deeper percolation of precipitation that moves slowly through the ground and sustains the base flow of streams and rivers and recharges groundwater. Urbanization results in an increase in the amount of land covered with impervious surfaces, resulting in a greater percentage of precipitation appearing as stormwater runoff.
Rainfall that is captured directly from rooftops and stored on site in barrels or cisterns is frequently called rainwater harvesting. For the purposes of this report, the term “stormwater” is used broadly to include runoff captured directly from rooftops, and the term “roof runoff” is used when flows from the ground surface are not included.
Graywater is the wastewater produced from bathroom sinks, showers, bathtubs, clothes washers, and laundry sinks and is derived from residential buildings or commercial establishments. Graywater is mainly a byproduct of washing and does not include toilet water (sometimes called “blackwater”). This report and most recent scientific papers also exclude water from kitchen sinks and dishwashers from the definition of graywater (Sharvelle et al., 2013), because kitchen water contains high levels of organic matter and solids along with foodborne pathogens (Eriksson et al., 2002), necessitating more extensive treatment (with unit processes similar to wastewater reuse). Nevertheless, some states regulations (e.g., Wash. Chap. 246-274) include water from kitchen sinks and dishwashers in the definition of graywater. Graywater, as defined in this report, accounts for about one-half of a typical indoor home wastewater flow (Sheikh, 2010). The definition of graywater could conceivably include condensate from air conditioning units, which represents an additional on-site water resource, although condensate is not typically included in the definition of graywater.
Stormwater and graywater can be captured and used at various scales using engineered conveyance, treatment, and storage systems of varying complexity. For stormwater, this may include the household, neighborhood (or multi-residential building), or regional scales. Accordingly, the annual capture can vary from several hundred gallons for rain barrels to millions of gallons in large subsurface tanks to billions of gallons for large, regional surface reservoirs or groundwater infiltration systems. Graywater systems also can be applied at varying scales, from the individual residence to the multi-residential building to a large residential development with a semi-centralized graywater treatment system serving as many as 10,000 people.
Many different drivers exist for local water capture and use, and these drivers vary by region as to their relative importance. Current interest in the beneficial use of stormwater and graywater is driven by water scarcity in regions of the country that experience chronic or episodic droughts. Additional drivers are flood control, pollution prevention, nutrient management, reduced hydromodification, and energy savings associated with locally sourced water supplies. Although water scarcity, flood control, and pollution prevention are primary drivers depending on local conditions, other co-benefits such as green space, community amenities, and public education may be equally important to decision making. The relative importance of each of these drivers determines the graywater and stormwater strategies that might be appropriate for a particular site or region.
Many areas in the West, Southwest, and Southeast face water shortages. Chronic water shortages exist from California and the desert Sun Belt to the Colorado Front Range and the Great Plains. A 14-year drought has lowered the water levels in the Colorado River basin to historic levels, forcing communities in the southwest that rely on water from the Colorado River to look for alternative supplies. In northern California, communities that have traditionally taken groundwater from the Carmel River basin (Monterey County) are under state mandate to preserve water for the river ecosystem, withdraw less, and find other supplies (MPWMD, 2014). In California, 2013 was the driest calendar year since the Gold Rush when record keeping began, and by the winter of 2015, the state had the lowest snowpack in recorded history. California has been under a drought state of emergency since 2014 (Governor of the State of California, 2014) with substantial water use and water delivery restrictions (CA DWR, 2014).
Such problems, however, are not confined to the West. The southeastern United States experiences extended periods of low rainfall as a normal component of the climate system (Figure 1-3), resulting in conflicts between Georgia and Florida over water supply for the city of Atlanta versus water releases to the Chattahoochee River and Apalachicola Bay (NRC, 2009b). Water shortages and the over-pumping of groundwater resulted in Tampa becoming the first large U.S. city to substantially augment its water supply via seawater desalination (Tampa Bay Water, 2008).
Population growth and redistribution to water scarce regions has exacerbated these challenges. Figure 1-2 shows high levels of projected population growth in the Southwest and Southeast in areas already facing water challenges.
Climate change may add further stresses to water scarce regions that are already exceeding the limits of imported surface water supplies and sustainable yields from groundwater basins to meet the needs of urban users. These conditions
are leading communities to conserve existing potable water supplies and seek out alternative sources, such as stormwater and graywater (see Box 1-1).
Water Supply Reliability and Diversification
The beneficial use of stormwater and graywater also provides ways to augment and diversify local water supplies and reduce reliance on imported water supplies. In Los Angeles, for example, 88 percent of the current water supply is imported, and the city seeks to diversify its water portfolio and increase the use of local water supply sources, such as stormwater (Box 1-2). In 2010, Los Angeles announced plans to meet at least 4 percent of its water supply through new stormwater capture systems by 2035 (Figure 1-4; LADWP, 2010), although this could grow to become even larger as a result of recent stormwater capture planning (see Box 1-2). More aggressive timelines for reducing dependence on imported water are presented in the 2015 sustainability plan for Los Angeles (City of Los Angeles, 2015). California’s State Water Resources Control Board has ambitious goals for increasing stormwater capture and use by an additional 500,000 AF/yr (620 million m3/yr) by 2020 and by 1 million AF/yr (1.2 billion m3/yr) by 2030 to reduce the state’s reliance on imported water (SWRCB, 2013). For comparison, the Metropolitan Water District of Southern California (MWDSC, 2010) estimated that as of 2007 approximately 470,000 AF/yr (580 million m3/yr) of stormwater was being captured in the coastal plain of southern California, with significantly less urban stormwater captured in northern California (e.g., the San Francisco Bay Area). The Pacific Institute concluded that additional urban stormwater capture in southern California and the San Francisco Bay Area could potentially increase water supplies by 420,000 to 630,000 AF/yr (520 to 780 million m3/yr) (Pacific Institute and NRDC, 2014).
Graywater systems offer a reliable, year-round source of water to irrigate landscaping or flush toilets that can help conserve existing water supply sources. This reliability offers a major benefit in areas that face frequent outdoor water use restrictions during times of drought.
Urban stormwater contains a number of contaminants and is a major source of nonpoint pollution to surface waters for chemicals and pathogens (EPA, 1994). Chemical contaminants include those derived from paving materials, automobile tires, and urban biocides (Grebel et al., 2013). Stormwater runoff also contributes substantial loads of nitrogen, phosphorus, and sediment, which can cause algal blooms, low dissolved oxygen, and reduced water clarity and significantly impact aquatic life in inland water bodies and coastal estuaries. Nutrient discharges from urban and agricultural runoff, wastewater discharges, and air pollution have created a “dead zone” with low oxygen in the Chesapeake Bay (Figure 1-5), which has motivated a multi-state pact to reduce pollution loads.1
In numerous U.S. cities, particularly in the Midwest and Northeast, stormwater and wastewater infrastructure were constructed together (termed combined sewer systems), such that stormwater runoff drains into sewers and passes through the wastewater treatment plant (Figure 1-6). In areas with advanced wastewater treatment, such construction can be a benefit under low to normal hydrologic conditions, because the treatment plant can remove nutrients and sediment from both stormwater and wastewater. However, combined sewer systems were constructed with overflows that would prevent the wastewater treatment plant from being overloaded after storm events. Under extreme wet weather conditions or when blockages or mechanical failures occur, combined sewer overflows (CSOs) discharge untreated wastewater, polluting the surface waters with pathogens, organic matter, and nutrients. Improved stormwater capture and use provide a means to reduce CSOs and the associated pollution loads.
In coastal regions, stormwater may be a significant contributor to pathogens that pollute recreational waters. Stormwater runoff is the most frequently identified source of beach closings and advisory days, and the U.S. Environmental Protection Agency (EPA) estimates that more than 10 trillion gallons (38 trillion liters) of untreated stormwater make their way into our surface waters each year (EPA, 2004a). In 2012
there were more than 20,000 beach closing and advisory days, with more than 80 percent caused by bacteria levels in recreational waters that exceeded public health standards (Dorfman and Haren, 2013).
Discharges to impaired (i.e., degraded) water bodies are typically governed by “total maximum daily load” (TMDL) requirements, which outline the maximum amount of a pollutant that a water body may receive and still meet water quality standards (with a factor of safety). TMDLs have been approved for pathogens, nutrients, mercury, other metals, and sediment, among others.2 Stormwater management efforts are often driven by TMDLs (discussed in more detail in Chapter 7), and meeting TMDLs can be an expensive and uncertain proposition. However, stormwater capture and use provides a strategy to reduce pollution while providing additional water supply benefits (see Box 1-3).
Hydromodification and Flood Management
Stormwater runoff in urban areas is characterized by increased volumes of runoff and more intense peak flows compared to the more natural state. This change in runoff regime, called “hydromodification,” is caused by land use and altered landscapes in the watershed that destabilize stream-beds and impair stream condition and function (Goodman and Austin, 2011). As watersheds urbanize and are covered with impervious surfaces, runoff is conveyed directly to streams via the conventional storm drain system. Infiltration into soil is reduced and overland flow increases. As a result, the magnitude and duration of flows entering receiving streams increase, which contributes to more erosive energy within the channel. Unless managed, hydromodification can cause channel erosion, unstable stream banks, altered base flow, change in bed material composition, and biological impacts to stream systems (Figure 1-7; OEHHA and SWRCB, 2009; Paul and Meyer, 2001).
Hydromodification management in new developments seeks to mimic natural hydrologic conditions by retaining stormwater and subsequently releasing it to match pre-development flow volumes, durations, and frequencies. The theory is that if the pre-development distribution of in-stream flows is maintained over a broad range of critical flow rates for long periods, then the baseline capacity to transport sediment, a proxy for the geomorphic condition, will be maintained as well. Stormwater infiltration or capture alters the runoff hydrograph of a site through the changes to the timing of discharges, and stormwater capture also reduces the overall volume. Evaluation of onsite stormwater use for hydromodification management typically involves sizing such strategies based on continuous simulation of both the pre-development and post-development conditions and incorporating demand for onsite use and iterative design of the facility until flow duration control is achieved. Hydromodification management can provide a flood control benefit by holding up and releasing water more slowly to waterways.
Since the early 1900s, water suppliers and flood control agencies in the southwestern United States have been capturing floodwaters behind dams and/or diverting stormwater into large-scale spreading basins to replenish groundwater basins and manage flood risk. These facilities can be combined with flow control (e.g., constructed wetlands) to benefit hydromodification control strategies (Santa Clara Valley, 2005). The development of multi-purpose flood control and stormwater capture facilities to enhance percolation of stormwater (or historically called flood water) supplies along the river channel or alongside the river banks into recharge percolation ponds has developed into a more sophisticated water resources management strategy in recent decades (see Box 1-4). Los Angeles County’s Department of Public Works alone operates 27 spreading basins to enhance local water supplies.3
Energy Savings and Greenhouse Gas Reductions
Beneficial use of stormwater and graywater can save energy compared to conventional sources under certain conditions. In the western and southwestern United States, there is a mismatch between population centers and areas of precipitation, and massive systems have been constructed to convey water over long distances to urban areas. Many of the conventional water supplies are derived from surface water that is pumped long distances, with significant energy and infrastructure costs. A study commissioned by the California State Water Resources Control Board concluded that capturing 1 acre-foot (1,200 m3) of southern California stormwater and storing it in the ground saves roughly a metric ton of carbon dioxide (CO2) compared to imported water supplies (Spencer, 2013). Although energy savings from stormwater and graywater are highly variable (see Chapter 6), it may be feasible to reduce energy use and greenhouse gas emissions through the increased use of stormwater or graywater, particularly when these local water sources require minimal treatment or pumping.
The implementation of stormwater and graywater beneficial use projects may also be driven by a sense of environmental stewardship, even in the absence of specific water conservation or pollution prevention goals. Individuals, businesses,
and municipalities may be driven toward “green” practices out of a motivation to be good stewards to the earth. Individuals and communities may be motivated by the positive emotional return from making investments in green infrastructure, and businesses and municipalities may aim to enhance their public image through such initiatives.
There is a growing trend of environmental practices that embrace the concepts of sustainable urban water management and “green design” (Allen et al., 2010; WERF, 2009). For example, low-impact development projects, including green roofs and enhanced stormwater capture and infiltration, help manage the quantity and quality of stormwater runoff and better mimic the undeveloped landscape. These projects can reduce pollution, provide habitat, and contribute to the creation of a greener, more aesthetically pleasing city. Once considered pioneering, these practices now are widely implemented (e.g., Prince George’s County, Maryland, 1999) and are required in some cities for development or redevelopment (e.g., San Francisco [SFPUC, 2009]).
The U.S. Green Building Council’s trademark Leadership in Energy and Environmental Design (LEED) certification program recognizes environmental practices in building design.4 The LEED program credits graywater irrigation to reduce water consumption and wastewater discharges, rainwater capture systems, pervious pavements, and on-site infiltration to reduce stormwater runoff.5 Innovative water management strategies, such as on-site use of graywater or stormwater to reduce indoor and outdoor water demand and stormwater discharge, can earn builders up to one-half of the points required for basic LEED certification,6 which can be a motivator even in areas where water scarcity is not a primary driver (Box 1-3).
Likewise, the Sustainable Sites Initiative is a voluntary effort to transform land development and landscaping in ways that offset impacts and use less energy and water.7
Extending the Capacity of Existing Infrastructure
America’s urban water infrastructure was largely developed in the middle of the twentieth century, a time of inexpensive energy, smaller urban populations, and less appreciation of environmental impacts such as damage to aquatic habitat and consequences of greenhouse gas emissions. Massive dams, aqueducts, and pipelines were built to supply water to metropolitan areas such as the Colorado Front Range, the San Francisco Bay Area, the Los Angeles basin, Phoenix, and Dallas. Many of these water systems were characterized by a linear pattern of taking, treating, and discharging, using capital- and energy-intensive technologies with high costs for maintenance and operation (Daigger, 2009). In many urban areas today, this water infrastructure is reaching the end of its design life (ASCE, 2009).
Alternative water supplies, such as stormwater and graywater, can provide a means to prolong the life of existing infrastructure and avoid or postpone costly upgrades to centralized infrastructure. For example, New York City launched a plan to address the city’s pollution problems from combined sewer overflows that included $2.4 billion in spending on green infrastructure, including incentives for graywater use and stormwater capture. If realized over a 20-year period, then it is estimated to save the city $1.4 billion compared to the conventional infrastructure approach (NYSDEC, 2013). At the household scale, graywater reuse may prolong the operating life of septic systems.8
In dense urban areas, population growth and rising real estate prices often spur even denser use and taller buildings. If water use patterns remain the same, then this would require that additional water supply capacity be provided, along with a commensurate increase in wastewater collection and conveyance capacity. Because water supply and wastewater collection facilities are generally located in existing streets and adjacent rights-of-way, constructing these facilities is quite costly both monetarily and in terms of the traffic and business disruption caused by construction in existing roadways. Engineers in Tokyo have found that localized wastewater or graywater reuse for nonpotable purposes can be quite cost-effective because it can eliminate the need to construct new
water supply and wastewater collection facilities to serve the more dense areas. Implementing nonpotable water reuse can reduce the net water supply requirement and wastewater production volume by about one-half, meaning that even if the total demand is doubled, existing water supply and wastewater collection infrastructure can serve the new facility. Tokyo requires graywater reuse in all new buildings larger than 32,000-54,000 ft2 (3,000-5,000 m2) and in existing buildings larger than 320,000 ft2 (30,000 m2) or with the capacity to reuse 26,000 gpd (100 m3/d) (Ogoshi et al., 2001; CSBE, 2003).
Financial Incentives and Business Opportunities
Builders and developers may also implement stormwater capture and graywater reuse systems to take advantage of financial incentives or even to permit development in some water scarce regions. For example, the California legislature passed two bills in 2003 (SB 221 and SB 610) to advance water supply planning for growing communities. These laws require future water reliability assessments for all development projects subject to the California Environmental Quality Act and written verification by the water agency serving that project prior to approval of the project. The result of these requirements has been that new urban development in California has to go through a process of ensuring that new development has a reliable supply. Developers are thus motivated to have a minimal impact on local/regional supply reliability and typically incorporate state-of-art water use efficiency and conservation practices for indoor and outdoor water use, including on-site stormwater capture with low-water-use landscapes. Some cities also offer tax incentives or zoning allowances to LEED-certified buildings.
Increasingly, regulated municipal separate storm sewer system operators are looking for effective strategies to reduce the cost of compliance. To this end, Washington, DC, was the first entity to establish a true stormwater credit trading system with the intent of using a market-based approach to improve the efficiency of implementation of stormwater controls. The District of Columbia Stormwater Retention Credit Trading Program9 allows private and public developers the ability to both buy and sell “stormwater retention credits” (SRCs). Properties can generate SRCs by building, operating, and maintaining green infrastructure that reduces stormwater runoff, including stormwater capture and use systems. Owners can sell their SRCs in an open market to developers or other private or public entities who can use them to meet regulatory requirements for retaining stormwater within the District. The first SRC trades occurred in the fall 2014 at $2.27/gallon per year ($0.60/liter per year) for a total value of about $25,000 (Brian VanWye, DDOE, personal communication, 2015).
Balancing Multiple, Sometimes Conflicting Drivers
The many drivers discussed here can lead to a wide variety of different water management strategies, depending on which drivers are given the highest priority. A stormwater capture system that maximizes pollution prevention could be designed quite differently from one that maximizes capture and use, although systems can be designed to optimize multiple drivers. Likewise, a household graywater system could be designed in concert with native plantings to maximize water conservation, while a similar graywater system to provide a supplemental low-cost, reliable water supply for new nonnative plantings in an arid climate could actually increase overall water use.
Because urban water systems typically provide social and environmental benefits in addition to financial benefits and costs, it is important to fully account for the broad array of benefits and costs that may be associated with a graywater or stormwater beneficial use project, in the context of overall objectives (see Chapters 7 and 9). Multi-criteria decision analysis or broadly defined benefit-cost analysis are two important tools that may be useful for evaluating future management strategies that create such a broad spectrum of valuable outcomes. These decision support methodologies may include both monetized and non-monetized objectives such as water reliability and resiliency, locally sourced water and reduced imports, energy savings and conservation, financial incentives and cost sharing, environmental outcomes, community acceptance, and support of nongovernmental organizations (see Chapter 9).
The combined drivers of water scarcity, pollution prevention, infrastructure replacement costs, and concerns about energy and the environment have expanded efforts to capture and use stormwater and graywater.
Historically stormwater management meant flood control, but since passage of new sections in the Clean Water Act in 1987, attention has focused on control of pollutants from runoff under stormwater programs. Today, stormwater management can mean many different things, including controlling pollution and improving urban waterways, improving aquatic habitat, creating green space, and recharging
local groundwater. Urban stormwater control measures are also a vital part of managing flooding and drainage in a city (NRC, 2009a).
There has been a recent increase in the use of stormwater practices that recharge groundwater. In wetter climates infiltration can raise groundwater levels, increase base flows, and sustain wetlands and lakes. Stormwater infiltration has been practiced in scattered locations for a long time. On Long Island, New York, infiltration basins were built in the 1930s to reduce the need for a storm sewer system. In Los Angeles, managed aquifer recharge with stormwater has been practiced since 1938, when the Rio Hondo and San Gabriel Coastal Spreading Grounds were opened by the Los Angeles County Flood Control District (Johnson, 2011; see Box 1-4). Within the area served by the Metropolitan Water District in the greater Los Angeles area, an annual average of about 477,000 AF/yr (588 million m3/yr) of stormwater runoff is captured for recharge (MWD IRP Technical Work Group, 2009). In the 1980s, Maryland took the lead on the East Coast in developing statewide infiltration practices, and the number of states embracing low-impact development infiltration practices has increased (NRC, 2009a). These facilities, and low-impact development systems, were originally managed for fast infiltration. Today there is interest in understanding how these infiltration systems can provide water treatment in addition to improved aesthetics and new habitats that enhance community acceptance.
Evidence shows increasing trends and interest for stormwater use spreading to other parts of the country. A number of states are viewing stormwater as a resource for development of additional, local water supplies and reductions in demands on an aquifer. With increasing population and greater extent of impervious surfaces in urban areas, the Texas Water Development Board is assessing stormwater runoff to augment water supplies. The Board’s report (Alan Plummer Associates, 2010) goes beyond small-scale rainwater harvesting and provides guidance on intermediate-scale stormwater harvesting to capture larger amounts of overland flow. Even temperate regions that normally experience adequate rainfall are exploring stormwater use (see Box 1-3). The Minnesota Pollution Control Agency began a pilot program for beneficial use of stormwater in industrial processes that otherwise would use groundwater (MPCA, 2012).
National data on the use of urban stormwater for water supply are not currently available. Rather, the literature describes many examples of systems at different scales, ranging from household to regional scales, and in different U.S. climatic zones, from the humid East Coast to the dry Southwest. Evidence of increased interest in stormwater harvesting is illustrated by signature projects in the past dozen years designed to comply with LEED certification, help reduce stormwater discharges to combined sewer systems, or develop sustainable urban environments in water-stressed cities. As part of its planning, the San Francisco Public Utilities Commission (SFPUC, 2013) documented numerous neighborhood-scale case studies such as the Solaire in New York City, where stormwater is collected in a 10,000 gallon (38,000 liters) tank for irrigation, and the Olympic Village in Sydney, Australia, where stormwater runoff from the 1,580-acre (640 ha) Olympic Park is harvested and reused for irrigation, washing, and other uses. Singapore harvests stormwater on a large-scale for its drinking water supply (PUB, 2015). Within the service district of the Metropolitan Water District of southern California there are 34 stormwater projects anticipated for completion between 2009 and 2020 that could increase regional stormwater capture by about 50,000 AF/yr (60 million m3/ yr) (MWD IRP Technical Work Group, 2009), and even more new projects will be developed as part of the Los Angeles Stormwater Master Plan (see Box 1-2).
In recent years, interest in graywater reuse has greatly increased. Documenting this trend are state laws that promote graywater reuse. As of 2013, 20 states allow some form of graywater reuse, and Arizona and California are considered leaders in promoting graywater reuse (Sharvelle et al., 2013). Arizona allows graywater reuse without a permit for systems less than 400 gpd (1,500 lpd), while California allows household laundry-to-landscape systems without a permit as long as they follow specific design guidelines (EBMUD, 2009; California Plumbing Code Ch. 16A, Sec. 108.4.1). The heightened interest in graywater reuse is also documented by an increase in the number of national conferences and workshops on graywater reuse and ways to promote the practice, such as an EPA workshop in Atlanta in 2010 on graywater practices and regulations (EPA, 2010). Professional societies have sponsored studies on graywater practices and effects (e.g., Sheikh, 2010) and offer official policy statements on water use efficiency including appropriate on-site graywater reuse (e.g., Olson, 2014).
Interest in safe and effective graywater use has prompted some limited surveys on the practice. A 1999 survey by the Soap and Detergent Association reported that 7 percent of U.S. households reuse graywater, mainly for irrigation, with the largest concentration residing in the West and Southwest (NPD Group, 1999). A 2000 survey in Pima County, Arizona, found about 8 percent of respondents employed some form of graywater reuse but this was highly variable depending on water district and ranged from 2 to 25 percent (Little, 2000).
Recent trends suggest that beneficial use of graywater and stormwater are small but increasing parts of the nation’s water supply portfolio. To meet future water demands amidst challenges from aging infrastructure, population growth and redistribution, and climate change, an array of water supply and conservation alternatives and innovative water management strategies will be necessary. The nature of this re-invention of urban water management is difficult to predict, but the nation’s water infrastructure will likely look quite different in 50 years than it does today. Future water infrastructure designs could create more cost-effective opportunities for the use of local or on-site water sources, such as stormwater and graywater. Currently, few buildings contain dual plumbing to take advantage of nonpotable water use, but as buildings are redeveloped in the future, major changes in water distribution and use become possible. Thus, consideration of the potential for graywater and stormwater to augment the nation’s water supply should not be limited by current infrastructure constraints. An example of a recent, rapid change in water infrastructure can be found in adoption of low-flow toilets (see Box 1-5).
Drivers of water management evolve over time with changing infrastructure, water availability, prices, and societal and individual values. Today’s infrastructure investments may shape water management practices for decades and should ideally support future water management priorities rather than maintaining outdated or inefficient practices that are anticipated to decline over time. For example, early water reuse efforts in the Southwest provided dual delivery of nonpotable reclaimed wastewater for landscape irrigation in parks and highway medians; today, such infrastructure may be seen as wasteful, when native landscaping strategies can significantly reduce irrigation demand. Thus, graywater and stormwater infrastructure investment decisions ideally include the anticipation of the role of alternative water supplies in the future, rather than simply solving today’s water management challenges.
Further advances in water use efficiency and conservation could impact the demand for alternative water sources, such as stormwater and graywater, as well as the supply of graywater available for reuse. Graywater relies upon the reuse of laundry, sink, and shower water, and additional improvements in water efficiency in these applications, such as further advances in the water efficiency of washing machines, would impact the amount and quality of graywater available for reuse. Developments in source separation (separating urine from solid waste) could reduce water use, increase the cost-effectiveness of water reuse, and allow the recovery of energy and nutrients to provide important sources of revenue in the future (Daigger, 2009; Guest et al., 2009). Similarly, climate change could increase the intensity of precipitation, which in some regions could result in less capture and/or recharge of stormwater with existing systems. Thus, the potential contributions of graywater and stormwater to the nation’s water supply will evolve over time based on demand and supply and evolving requirements for urban water, wastewater, and stormwater utilities in the United States (Hering et al., 2013).
In the United States, despite steady population growth, total water use has declined since its peak in 1980 (Figure 1-8) because of improvements in industrial and domestic water efficiency, conservation, and recent declines in the use of once-through thermoelectric cooling. Even public water supply use, which supplies domestic, commercial, and industrial uses, declined 5 percent between 2005 and 2010 after increasing steadily between 1950 and 2005, although these declines may be related to the economic recession (Figure 1-9). Per capita domestic use declined at an even steeper rate, from 100 gpcd (380 lpcd) in 2005 to 89 gpcd (340 lpcd) in 2010 (Maupin et al., 2014). However, domestic water use varies widely across the country. At the city level, an analysis of data from 2005 to 2010 for 21 U.S. cities found median domestic
water use ranged from 43 to 177 gpcd (160 to 670 lpcd) with some correlation with a city’s precipitation and temperature (Kenny and Juracek, 2012). In drier climates outdoor residential water use is typically much greater than indoor water use. For example, the Water Research Foundation (Coomes et al., 2010) reported three times or more residential water use in Dallas and Phoenix compared to Seattle; similarly, a U.S. Geological Survey (USGS) state-by-state survey of public water supplies showed a threefold difference in residential water use from 55 gpcd (210 lpcd) in Maine to 168 gpcd (636 lpcd) in Idaho (Maupin et al., 2014). By contrast, in Australia residential water use is less at about 39 gpcd (150 lpcd) in Melbourne on the wetter east coast (Gan and Redhead, 2013; Melbourne Water, 2013/2014) and about 76 gpcd (290 lpcd) in arid Perth on the west coast with about 39 percent for irrigation (The Water Corporation, 2010).
At a household level, substantial improvements in outdoor water use efficiency are possible through the use of native landscaping, but once high-efficiency appliances and plumbing fixtures are installed, indoor water use is unlikely to see substantial additional declines without the use/reuse of on-site sources such as stormwater or graywater. Meanwhile, population growth and redistribution will continue to increase urban water demand.
Domestic water conservation and trends in water use in agricultural and industrial sectors will influence regional water availability and the benefits of graywater and stormwater use in the future. Today water professionals and urban designers understand that more efficient use of water and resources is possible. The “Cities of the Future” initiative by the International Water Association seeks to integrate water and city planning much more closely and support innovation and strategic thinking (Daigger, 2011). So-called water-centric urban design can lead to both better use of resources and an enhanced urban environment. In the same fashion, the concept of the “water sensitive city” embraces sustainable urban water planning and management practices in which cities serve as water supply catchments, provide ecosystem services and prioritize livability, sustainability and resilience (Ferguson et al., 2013). These concepts are examples of where the future could be going and the role that graywater and stormwater could play.
Despite several drivers supporting increased use of local alternative water supplies to address water demands, many questions remain that have limited the broader application of graywater and stormwater capture and use. In particular, limited information has been 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 the timeliness of a severe drought in the West and periodic water shortages elsewhere and funding support from the EPA Office of Water, EPA Office of Research and Development, EPA Region 9, National Science Foundation, Water Research, Water Environment Research
Foundation, Los Angeles Department of Water and Power, WateReuse Foundation, City of Madison, Wisconsin, National Water Research Institute, and the Academies’ President’s fund, the Academies formed a Committee on the Beneficial Use of Graywater and Stormwater. This study builds on previous work of the Academies to assess the augmentation of urban water supplies by desalination (NRC, 2008a) and reuse of municipal wastewater (NRC, 2012a), with a new focus on two on-site water sources—graywater and stormwater. The goals of the committee are to be forward looking and to conduct a study and prepare a report on the risks, costs, and benefits of various uses of stormwater and graywater and the approaches needed for their safe use. The committee’s work considers multiple scales for these approaches—from the household scale to multi-residential or neighborhood scales to large municipal systems. The study will address both technology and policy questions:
- Quantity and suitability. How much stormwater capture and graywater reuse occurs in the United States and for what applications? What is the suitability—in terms of water quality and quantity—of captured stormwater and graywater to significantly increase in the United States, and where regionally would increases in these practices have the most benefit? How would significant increases in the beneficial use of stormwater and graywater affect water demand, downstream water availability, aquifer recharge, and ecological stream flows? What research should be pursued to understand these issues?
- Treatment and storage. What are typical levels and methods of treatment and storage for stormwater capture and graywater reuse for various end uses? What types of treatment are available to address contaminants, odors, and pathogens, and how do these treatment methods compare in terms of cost and energy use? What research opportunities should be pursued to produce improved technologies and delivery and ensure adequate safeguards to protect public health and the environment?
- Risks. What are the human health and environmental risks of using captured stormwater and graywater for various purposes? What existing state and regulatory frameworks address the beneficial use of stormwater and graywater, and how effective are they in assuring the safety and reliability of these practices? What lessons can be learned from experiences using captured stormwater and graywater both within and outside the United States that shed light on appropriate uses with varying levels of treatment? What local measures can be taken to reduce risk?
- Costs and benefits. What are the costs and benefits of the beneficial use of stormwater and graywater (including non-monetized costs and benefits, such as ef-
fects on water and energy conservation, environmental impacts, and wastewater infrastructure)? How do the economic costs and benefits generally compare with other supply alternatives? Can cost improvements be achieved through research?
- Implementation. What are the legal and regulatory constraints on the use of captured stormwater and graywater? What are the policy implications regarding the potential increased use of stormwater and graywater as significant alternative sources of water for human consumption and use?
The committee’s report and its conclusions and recommendations are based on a review of relevant technical literature, briefings, discussions, and field trips at its six meetings, and the experience and knowledge of the committee members in their fields of expertise. The committee received briefings from a range of experts, including water utilities, practitioners, government and public health officials, nongovernmental organizations (including public interest and industry groups), and academics.
The report focuses on nonpotable uses of graywater and stormwater, such as irrigation and toilet flushing (see Chapter 2), which can be met with little or no additional treatment, and recharge of groundwater that eventually may be used for drinking water supplies. The committee did not examine technologies for direct on-site use of graywater or stormwater for drinking water, because of the associated unique safety issues and treatment requirements and the unlikely potential for expanding such uses in the United States in ways that significantly augment existing water supplies. The committee recognizes that roof runoff is used to meet potable needs in many rural areas in the United States and around the world, and other reports are available that recommend specific capture and treatment practices (e.g., Macomber, 2010; TWDB, 2005).10 In light of this report’s focus on the potential water supply contributions of graywater and stormwater projects, the report’s stormwater components focus on stormwater capture and infiltration efforts that have intentional or inadvertent water supply benefits. A full discussion of stormwater management strategies, including those designed primarily for water quality benefits, are described in NRC (2009a).
Following this introduction, the statement of task is addressed in eight subsequent chapters of this report:
- Chapter 2 describes potential uses of graywater and stormwater and specific water quality constraints on these uses.
- Chapter 3 describes the potential water savings provided by stormwater or graywater, and provides the results of an original scenario analysis in six different locations of the country.
- Chapter 4 describes what is known about the water qualities of various sources of stormwater and graywater before treatment.
- Chapter 5 describes approaches for characterizing the risk of on-site nonpotable uses of graywater and stormwater and summarizes the research to characterize these risks.
- Chapter 6 examines the state of the technology of graywater and stormwater system components.
- Chapter 7 analyzes the costs and benefits (including both financial and non-monetized costs and benefits) of stormwater and graywater projects.
- Chapter 8 outlines the legal and regulatory controls on graywater and stormwater projects and identifies the largest impediments to expanding the use of such sources.
- Chapter 9 synthesizes the major report findings, as they are relevant to the perspective of a local decision maker, and presents major steps to consider in the development of such projects.
- Chapter 10 summarizes major research needs to enhance the implementation of graywater and stormwater to meet current and future water demands.