2
Beneficial Use Options for Graywater and Stormwater
Stormwater and graywater can provide on-site or local sources of water for an array of uses. As discussed in Chapter 1, the practice of capturing graywater and urban stormwater for safe use has many potential benefits, such as reducing the impact of urban development on water quality and stream flow and contributing toward water conservation objectives. This chapter provides an overview of the suitability of graywater and stormwater for various beneficial applications.
GRAYWATER AND STORMWATER USE ACROSS MULTIPLE SCALES
Graywater and stormwater can be captured and used to meet water demands or to conserve conventional water supplies across a range of scales. The scale of the application usually determines which beneficial uses are reasonable, the feasibility of treatment (considering maintenance requirements and cost), and appropriate sizing of cost-effective storage. Both graywater and stormwater use can occur at the household (or small building) scale, at the neighborhood/ multi-residential scale, or at the regional scale (Figure 2-1). The committee’s definitions of these scales are provided in Table 2-1.
At the household (or small building) scale, storage capacities and treatment for captured stormwater and graywater are typically limited because of cost. At this scale, irrigation with untreated graywater is most common and provides a small but steady year-round source of irrigation water. Larger storage and treatment systems at the household scale are possible, but they are expensive and require periodic maintenance. Stormwater capture tanks at the residential scale also tend to be small (rain barrels or cisterns) and generally provide supplemental water for irrigation and outdoor uses near times of rainfall. Although rooftop capture at the household scale can reduce some domestic water use and reduce stormwater discharges, it does not represent a reliable long-term source of water, especially in arid regions. For residential stormwater harvesting, most of the attention is given to roof runoff, because its quality is typically better than other source areas, and it is elevated above the likely use areas, easily captured, and therefore hydraulically simpler to utilize.
At the neighborhood or multi-residential scales (including large commercial buildings and institutions), more complex stormwater and graywater system designs with storage and treatment become feasible. The added treatment expands the array of applications available, particularly those in which human contact and inadvertent ingestion is possible. For stormwater, larger storage extends the capacity to address water supply needs during periods without rain through the use of retention basins or subsurface tanks. Construction of infiltration facilities resulting in managed groundwater recharge is feasible at a neighborhood scale, where blended runoff from several source areas is utilized.
At the regional scale, stormwater capture and infiltration systems can be developed in some areas to collect runoff from multiple neighborhoods or a large drainage basin resulting in managed aquifer recharge for later recovery. Regional stormwater capture systems might also offer opportunities to establish water features in public parks for aesthetics and recreational purposes in addition to water storage. Stormwater runoff collected at a regional scale comes from a wide variety of land uses resulting in a water quality that requires a higher level of treatment prior to use. Regional-scale graywater capture and beneficial reuse have been adopted very recently in large new developments (see Box 2-1). Because of the infrastructure requirements to separate graywater from blackwater at larger scales, regional graywater use might not always be economically feasible, particularly in existing buildings, and instead, reuse of municipal wastewater effluents can be implemented at large scales to make use of this locally available water supply (NRC, 2012a).
Currently, captured stormwater and graywater are used primarily for nonpotable applications, mainly irrigation (i.e.,
lawn, trees, xeriscape) and selected urban applications (e.g., toilet flushing, air conditioning, car washing). In addition, stormwater can be used for wildlife habitat maintenance and recreational uses and to recharge groundwater, which eventually may serve potable uses or maintain stream base flow for aquatic habitat. Common and specific uses for stormwater and graywater, their typical scale, and some limitations of these applications are summarized in Table 2-2. Although it is feasible to treat stormwater or graywater for on-site potable use (Ahmed et al., 2010), such applications are uncommon in the United States, except in remote areas with no reliable groundwater supplies where rainwater may be harvested from roofs for household supplies.1 Given the complexity and unique safety issues associated with on-site potable use of graywater and stormwater at the household and neighborhood scales and the unlikely potential for expanding such uses in the United States to maintain and expand existing water supplies, the committee did not examine issues surrounding on-site potable use in this report.
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1 For example, see http://health.hawaii.gov/sdwb/raincatchment.
TABLE 2-1 Scales of Beneficial Use of Graywater and Stormwater, as Defined by the Committee
Householda | Neighborhoodb | Regional | |
Approximate number of people served | < 10 | 10-8,000 | >8,000 |
Number of land uses | 1 | ≤3c | >3b |
Approximate area d | < 1 acre | 1 acre-1 mi2 | >1 mi2 |
aThe parameters for household scale were identified as those likely to fall under the lowest tier of typical state tiered regulatory systems, which are commonly limited to 400 gpd (1,500 1pd) or less (see Chapter 8).
bMulti-residential units fall under the neighborhood scale.
cAt neighborhood scales, land uses are typically selected to minimize sources of contaminants, such as residential, commercial, or institutional. At regional scales, many urban land uses are typically included within the project footprint and may include industrial uses and high traffic roadways.
d1 acre = 0.4 hr; 1 mi2 = 260 hr.
TABLE 2-2 Current Uses of Graywater and Stormwater in the United States to Enhance Water Supplies and Relevant Scales
Category of Use | Specific Types of Use | Gray water Application | Stormwater Application | Limitations | ||||
Household/Small building | Neighborhood/Multi-residential | Regional | Household/Small building | Neighborhood/Institutions | Regional | |||
Urban Applications | Toilet and urinal flushing |
X |
X |
X |
X |
X |
|
|
Heating and air conditioning makeup water or evaporative cooling |
X |
X |
||||||
Fire fighting |
X |
|||||||
Laundries |
X |
|||||||
Vehicle washing |
X |
X |
X |
X |
||||
Street cleaning |
X |
X |
X |
|||||
Decorative fountains/other ornamental water features |
X |
X |
||||||
Landscape irrigation | Lawns, flowerbeds |
X |
X |
X |
X |
X |
|
|
Parks, golf courses |
X |
|||||||
Playgrounds/schools |
X |
|||||||
Agriculture |
X |
X |
||||||
Greenways |
X |
|||||||
Wildlife habitats and recreational uses | Wetlands |
X |
X |
X |
X |
|
||
Ornamental or recreational water body |
X |
X |
||||||
Large-scale water supply augmentation | Surface impoundments |
X |
X |
|
||||
Groundwater recharge |
X |
X |
NOTES: Other uses are possible but not commonly applied. Certain uses require a high level of treatment. Scales are defined in Table 2-1.
COMMON APPLICATIONS TO CONSERVE CONVENTIONAL WATER SUPPLIES
Graywater and stormwater are being used in many places in the United States and worldwide in ways that conserve conventional water supplies. Common graywater and stormwater uses are illustrated in Table 2-2, but other uses could be considered in the future for each water source. As noted in Chapter 1, there are many drivers behind graywater and stormwater use, and water supply may be a primary objective or only a peripheral benefit supplementing the primary drivers for the project. These applications are discussed in the following section, along with water quality concerns for particular end uses.
Urban Applications
An array of on-site uses for stormwater and graywater exist for urban applications, including toilet and urinal flushing, wash water, air conditioner chiller water, firefighting, commercial laundries, vehicle washing, street cleaning, decorative fountains, and other water features. As an alternative supply source for these nonpotable uses, stormwater requires treatment mainly to address particulate matter and pathogens for applications with significant potential human exposures (see also Chapter 5). Graywater requires treatment prior to most uses other than subsurface irrigation to minimize human health risks from microbial contaminants and to provide a stable water quality because microbial growth occurs when untreated graywater is stored (see Chapter 5). Treatment systems (discussed in Chapter 6) can vary substantially in complexity and the degree of treatment achieved.
Toilet Flushing
A popular beneficial application that reduces consumption of conventional water supplies is the use of captured stormwater or graywater for toilet flushing. Toilet flushing makes up approximately 24 percent of domestic water use (equivalent to about 14 gallons [53 liters] per person per day); thus, the savings potential for potable water is significant (Reichel et al., 2011; DeOreo et al., 2016). Water use for toilet flushing varies with population, toilet design (i.e., low-flow versus standard toilets), and demand fluctuations. For example, weekend versus weekday water use patterns are different, and seasonal variations occur because of varying workforce numbers or student populations at schools or in cities.
Using a nonpotable water supply for toilet flushing requires a plumbing system separate from the potable water system (known as dual plumbing). Dual plumbing is most cost-effective to install in new construction, although some buildings have been retrofitted to accommodate stormwater
and graywater for toilet flushing. Treatment systems are typically used, with the primary objective of controlling microbial growth and human health risks from pathogens (discussed further in Chapters 5 and 6). Given the treatment system maintenance requirements, the use of on-site water sources for toilet flushing is more common in multi-residential or office buildings (see Box 2-2) rather than individual households. Treatment systems for single residences are commercially available and have been successfully installed and operated.
Outdoor Water Features
Stormwater can be used to provide water for outdoor fountains or other ornamental water features (Box 1-3). In Germany, there are several examples of stormwater beneficial uses where aesthetic values were important in the stormwater management system. Dreiseitl (1998) states, “[S]tormwater is a valuable resource and opportunity to provide an aesthetic experience for the city dweller while furthering environmental awareness and citizen interest and involvement.” For example, in Berlin at the Potsdamer Plaza, roof runoff is captured in large underground tanks. Some of the runoff is treated for toilet flushing and irrigation use, and the rest flows into a 3.8-acre (1.4 ha) artificial lake in the center of the developed area (Figure 2-2), reducing stormwater discharges into local rivers. The Potsdamer Plaza project also has numerous fountains (Dreiseitl, 1998). Pitt et al. (2011) describe several other cases where stormwater has been used for aesthetic purposes. At the Cincinnati Zoo, harvested stormwater is used as makeup water for moats surrounding animal enclosures and for flowing water features (artificial streams) through the zoo grounds (Box 2-3). Water treatment typically focuses on removal of human pathogens and possibly particulate matter to minimize clogging and other maintenance issues depending on the storage and conveyance systems used.
Washing
Stormwater and graywater can be used for washing vehicles, equipment, and paved surfaces. Graywater reuse is commonly practiced by commercial car washes, which often reuse their wash water to substantially reduce overall water use. In Washington, DC, a number of fire stations are equipped with stormwater capture systems, and the harvested stormwater is available for daily fire engine spray washing and refill of fire truck day tanks. Although the DC stormwater capture systems are primarily designed to reduce runoff flowing to the combined sewer during wet weather, the captured stormwater is also available onsite to reduce potable water use. Typical vehicle wash water is filtered and disinfected at the time of use to reduce risk associated with any human pathogens that might be present.
Firefighting
Water for firefighting can be supplemented with stormwater runoff generated on site or within the immediate neighborhood, provided there is sufficient storage. Wet detention ponds can provide much needed reliability of access to firefighting water during times of natural disasters. As an example, during re-building of Veteran’s Administration hospitals in Los Angeles after the Northridge earthquake, stormwater detention ponds were planned that would hold sufficient additional water for firefighting needs. If stormwater flow is insufficient to maintain the water level in the ponds to the level needed to meet firefighting needs, then the detention pond would be supplemented with makeup water. Firefighting water volume needs for specific building types and sizes can be calculated based on the International Fire Code (ICC, 2012). As an example, a 50,000 ft2 building made from heavy timber with noncombustible external materials would require a fire flow of 4,000 gpm (15,000 L/min) for a duration of 4 hours. Therefore, a total volume of about 960,000 gallons (3.6 million liters) must be available for on-site storage of firefighting water. In most cases, sufficient treatment is provided in the wet detention pond, although additional disinfection may be required if fecal indicator bacteria levels exceed use requirements.
Heating, Ventilation, and Air Conditioning (HVAC)
Increasingly HVAC systems are being integrated into on-site water use. Captured and treated stormwater can be used as makeup water in evaporative cooling systems. Cooling water has been estimated as 15 percent of total water use for commercial and industrial land uses (Gleick et al., 2003). Barclays Center in Brooklyn, New York, utilizes retained stormwater as a water supply source for the cooling towers.
Air conditioning condensate is also a reliable source of onsite water supply, although the water can be high in heavy metals (e.g., copper, lead). Some on-site stormwater capture systems include collection of condensate for landscape irrigation or other nonpotable uses. This approach has been used at a range of scales from small household air conditioning systems to major commercial properties. The rate of condensate generation in HVAC systems typically correlates well with seasonal demands for irrigation water.
Landscape Irrigation
Stormwater and graywater are commonly used for landscape irrigation at both household and neighborhood scales. Three basic irrigation methods are as follows:
- Subsurface irrigation, in which graywater or stormwater is supplied through drip systems either through buried distribution pipes below the ground surface or beneath a thick layer of mulch or through direct drainage into mulch basins;
- Surface irrigation, in which water is supplied through a drip system without a thick mulch cover; and
- Spray irrigation, in which graywater or stormwater is supplied through spray nozzles (i.e., sprinklers).
Irrigation systems that use graywater and stormwater can be very simple (see Chapter 6) and have been widely accepted within the regulatory community, although state laws may restrict surface or spray irrigation and the irrigation of food-crops (see Chapter 8). Important chemicals of concern in graywater for irrigation uses are sodium, chloride, and boron. In stormwater, deicing salts (particularly, sodium chloride) also pose a concern because they can negatively impact soil quality and plant health. These contaminants are typically managed through source control.
Graywater, at the household scale, is most commonly used for landscape irrigation via simple laundry-to-landscape systems, which provide subsurface irrigation of untreated graywater (Box 2-4). Larger graywater systems in offices and other commercial buildings may also be used for landscape irrigation, although large-scale graywater supplies are commonly disinfected prior to use to reduce possible human health risks, unless exposures are carefully controlled (see Chapter 5 for a more detailed discussion of risk and exposure).
Many cities and water utilities have implemented rain barrel rebate programs to encourage household stormwater capture and outdoor irrigation use with the hope of reducing adverse environmental impacts of stormwater. Small household irrigation systems do not typically provide treatment, but treatment systems can be added to facilitate longer-term storage. Similarly, neighborhood-scale stormwater systems have captured area runoff for irrigation of parks, playgrounds, school lawns, and greenways. For example, the National Park Service recently completed a major project designed to capture local stormwater for irrigation of the National Mall in Washington, DC, for the purpose of reducing potable water use (Box 2-5). Large storage facilities can be designed to extend the use of stormwater for landscape irrigation, in particular through periods with low and no precipitation, at greater project cost.
Wildlife Habitats and Recreational Uses
The capture and use of stormwater can enhance and in some cases create aquatic habitat. Possible uses include neighborhood-scale wetlands to absorb nutrients and filter sediment from stormwater or stormwater retention ponds. The use of stormwater for habitat creation is a potentially important application of stormwater, especially in rapidly growing regions with limited availability of surface water. Wetlands, such as the Rory M. Shaw Wetlands Park (Box 2-6), provide recreational features, community amenities, educational opportunities, and urban habitat. As such, stormwater-created wetlands are gaining support of environmental groups and stakeholder communities. However, inherently variable stormwater flows complicate the design of systems intended to enhance or create aquatic habitats and may necessitate access to other water sources during extended dry periods. Wet ponds are generally better at providing wildlife habitat and require more space than do dry ponds (Barnes and Adams, 1998). The Rio Salado Environmental Restoration Project established through a partnership between the City of Phoenix and the U.S. Army Corps of Engineers serves as an example to restore native wetlands and riparian habitats with recreational and educational uses at a regional scale using a 5-mile section of the Salt River (DeSemple, 2006).
Stormwater wetlands provide habitat services when designed properly (NCSU, 2011; Duffield, 1986). Because urban stormwater wetlands may accumulate contaminants from the urban landscape, these effects should be monitored and mitigated if possible. Sparling et al. (2004) investigated
13 stormwater wetlands in the Maryland suburbs near Washington, DC. The accumulation of zinc in hatchlings of red-winged blackbirds inhabiting wetlands in industrial areas suggested nestling stress and impairment. Overall nestling success compared favorably with national averages. Although the zinc concentrations were elevated, the authors concluded that the benefit of stormwater habitat provided by stormwater wetlands might outweigh the negative impacts of contaminant accumulation provided by the stormwater habitat because of the scarcity of such habitats in urban areas. Controlling sedimentation rates and periodic dredging of sediments would prevent urban stormwater wetlands from accumulating toxic chemicals (Sparling et al., 2004).
Surface Impoundments and Groundwater Recharge
On-site capture and infiltration of urban stormwater is often used to mitigate stormwater runoff and contaminant loading to surface waters and reduce flows to combined sewer systems. Under appropriate hydrogeological conditions and project design, stormwater infiltration can be used to recharge local aquifers and thereby expand groundwater supplies. Several projects exist in southern California to recharge stormwater (see Box 1-4 and Figure 2-3), and more are in the planning stage (Box 1-1). In some areas underlain by low permeability rock or sediment (e.g., clay), infiltration does not reach the deeper aquifers that provide water supply (Figure 2-4). However, in areas with appropriate hydrogeology, stormwater recharge and storage basins can be designed at neighborhood or regional scales in engineered infiltration facilities to recharge local aquifers.
Groundwater infiltration projects capture surface runoff from relatively large and diverse source areas, and the stormwater is likely to contain a broad array of contaminants, such as petrochemicals, urban pesticides, and flame retardants (discussed in more detail in Chapter 4) (Eriksson et al., 2007). If these chemicals are not removed by soil-aquifer treatment upon infiltration, then additional water quality treatment may be necessary to prevent groundwater contamination. Suspended sediment also needs to be removed prior to infiltration to prevent clogging of the infiltration basins.
Surface water impoundments may also capture stormwater runoff for water supply and can sometimes be operated to optimize groundwater recharge (Box 1-4). Singapore harvests and treats stormwater on a large-scale for its public drinking water supply (PUB, 2015). Referred to as one of the “four taps,” stormwater is harvested from a network of drains and canals and stored in reservoirs—including river estuaries dammed to hold runoff—as part of a diversified water supply portfolio for water self-sufficiency.
The major benefits from use of regional solutions for captured stormwater are that larger infiltration or storage sites can
be designed, operated, and maintained by water supply agencies to maximize stormwater capture and provide treatment as needed to remove hazardous contaminants. Regional facilities can also bring an efficiency of scale (see Chapter 6).
SUMMARY
A large number of potential beneficial use options exist for graywater and stormwater depending on scale and treatment provided. Stormwater capture projects can serve a range of uses from irrigation to toilet flushing and HVAC cooling water to large-scale groundwater recharge. Stormwater treatment needs will vary with land use and catchment size, potential human exposures, and whether captured stormwater will contribute to urban water supplies. Traditional applications of graywater have focused on household-level nonpotable uses, including landscape irrigation and toilet flushing. However, neighborhood- and regional-scale projects are now emerging worldwide that integrate graywater reuse into the design of fast-growing urban centers and facilitate decentralized resource recovery in large new developments.