Much information has been provided in this report about the quality and quantity of stormwater and graywater available in different locations, possible end uses, known risks, reported costs and benefits, and legal and regulatory constraints. Decision makers should understand these factors to determine the potential risks, costs, and benefits of investments in graywater and stormwater capture and use systems at a range of scales at the local level. For small-scale systems, home and business owners may want to determine whether their investments are better served by graywater or stormwater capture for their given geographic and building circumstances, water supply needs, and personal objectives. At a regional scale, graywater and stormwater use affects many aspects of water, wastewater, and stormwater management, and decision making is best served by a holistic view of costs and benefits. This chapter attempts to synthesize this information within a water supply planning framework to help local decision makers, at the household, neighborhood, or regional scale, consider key information in assessing the potential role of stormwater and/or graywater as alternative local supplies to meet water needs.
Over the long term, with increasing urban population growth and the potential for more climate variability, there will be an increasing trend to maximize water conservation. Efforts will also continue to address stormwater pollution through retrofits and new construction designs. The potential role for graywater and stormwater within this future will play out with different priorities and urgencies, depending on which drivers described in Chapter 1 are most relevant to local decision makers. Opportunities for graywater use will increase with (re)development and growth of urban populations residing in multiple dwelling units. As a practical matter, beneficial use of stormwater will be driven primarily by water scarcity and pollution regulations. The next 20 to 30 years is likely to see continued evolution in the nation’s approach to water management, but from today’s viewpoint, no clear pathway or single technology is evident. Political and geographical realities will affect decision making region by region.
Figure 9-1 presents broad decision steps for those considering stormwater and/or graywater capture and use. The major steps include defining objectives, identifying opportunities and constraints, characterizing sites, identifying candidate strategies, selecting the system design, implementing the system, and engaging stakeholder involvement throughout the process. Each of these will be discussed below in the context of the major findings of this report.
Successful implementation of alternative water systems requires the effective engagement of a broad range of groups and individuals, typically referred to as stakeholders. A common definition of a stakeholder is any individual or group who can affect implementation of the subject project or program. Stakeholders are defined based on their legitimate interests in the matter at hand. Stakeholders may oppose or support the subject project. They may be internal or external to the responsible organization. They are not only those who will benefit from or be impacted by a project but also those who are involved in implementing and operating the required infrastructure and/or are concerned about this practice. Therefore, stakeholders are relevant for projects across a range of scales and may include individual residents, frequent visitors, business owners, employees, and organizations. An effective stakeholder engagement process will identify the relevant stakeholders and involve them proactively in the decision process so that opponents’ concerns are fully considered and negative impacts are mitigated, if feasible, and supporters are fully informed.
For larger stormwater or graywater beneficial use projects, stakeholders can be identified by determining the issues relevant to the set of decisions to be made. These issues may relate to, for example, public health, environmental impacts, implementation, and costs in the context of other available
water supply options. A stakeholder analysis (Table 9-1) is used to identify the groups and individuals that could be affected by these issues and their credible representatives (e.g., governmental and nongovernmental organizations). The significance of the impact of each identified issue on the decisions to be made is also characterized. The next step is to reach out and engage the identified stakeholders in the decision process.
Stakeholder engagement should begin relatively early in the decision process so that the relevant issues are appropriately considered throughout the process. For large projects, it is certainly possible that different issues become more relevant at different times in the decision process. If this is the case, then the extent of involvement of specific stakeholders may vary during the process. However, engagement of all relevant stakeholders from the beginning will reduce the tendency for newly involved stakeholders to want to bring the process “back to square one.” Consistent and early involvement helps to build a base of understanding and commitment to reach the necessary decisions by all relevant stakeholders.
A rich literature is available on effective decision processes (Lockie and Rockloff, 2005; NRC, 2005, 2008b, 2009c, 2012b; World Bank, 2012), and it is beyond the committee’s charge to explore this in detail. In general, the process should be structured to involve three groups of participants—stakeholders, subject matter experts, and facilitators—whose relevant roles and responsibilities are clearly defined. Stakeholders define the issues and considerations that should be addressed in the decision process, and they bring a set of values that in an effective process are used to prioritize the relative importance of the identified issues and considerations. The subject matter experts bring technical knowledge that provides a factual basis for decision making. Facilitators structure the overall process to enable the various participants to function efficiently and effectively in the context of their legitimate roles, responsibilities, and interests. As a practical matter, effective stakeholder engagement works best among groups that fundamentally trust one another and see the benefits of working together, even though strongly held opinions may vary. An example
TABLE 9-1 Example Framework for Stakeholder Analysis
|Issue||Stakeholder Group||Key People and Representative Groups||Type of Impact||Significance of Impact||Significance of Group Impact on Decision|
is the Watershed Management Groups in greater Los Angeles, which consists of agencies, cities, and nongovernmental organizations that focus on water resource management and future funding. Agencies and environmental groups (including TreePeople and Green LA Coalition) are responsible for developing stormwater capture projects among stakeholders in Los Angeles (Luthy and Sedlak, 2015). However, stakeholder groups comprising disparate interests and intractable adversaries are unlikely to resolve conflicts or achieve implementable projects. The CALFED Bay-Delta Program has been criticized for such failures (LAO, 2006).
A critical early step of any alternative water supply project is to determine the objectives of the project. This step is sometimes overlooked, to the detriment of project effectiveness and stakeholder satisfaction with the project, once completed. Chapter 1 discusses some major drivers for stormwater or graywater capture and use projects, including water supply, water reliability, pollution prevention, energy savings, and environmental stewardship and education. Brief summaries of the potential for graywater or stormwater to address these drivers are provided in Tables 9-2 and 9-3. Related project objectives could include reduced use of potable water supplies, enhanced local control of water supplies, and delayed need for infrastructure investments. Environmental objectives could include reduced stormwater discharges during rain events, reduced discharges to combined sewer systems to reduce the number and magnitude of overflows, reduced energy use, and enhanced groundwater recharge to reduce damage to urban streams from high-volume flows.
Stormwater or graywater projects can often be designed to optimize particular objectives if they are clearly identified in advance. For example, if reduction of potable water use is the primary objective, stormwater capture and use systems can be designed with tanks that are sized to meet as much of the water demand as is feasible under local climate conditions. Using stormwater and/or graywater for indoor nonpotable applications and limiting irrigation to that needed to preserve native landscaping would maximize water conservation. However, in many regions—particularly those working to minimize combined sewer overflows—pollution prevention is the primary objective of stormwater projects, and water supply provides a secondary benefit. In fact, to reduce stormwater pollution most cost-effectively, projects might instead use distributed shallow groundwater infiltration rather than stormwater capture. However, systems can, in many cases, be designed to balance multiple, sometimes competing objectives. For example, in areas working to manage combined sewer overflows, real-time weather forecasting can be used to automatically drain a tank to the sewer system in advance of a storm so that sufficient tank storage is available to capture runoff from the predicted storm (see Chapter 6). Such a system sacrifices some water supply to enhance the use of the tank to minimize stormwater pollution effects. The level of desired benefits also needs to be considered. For example, a project to advance public education may not necessitate large benefits to be effective, but a project to meet regulated pollution reduction measures, such as those contained in combined sewer overflow consent decrees, would necessitate large quantifiable outcomes.
Opportunities and Constraints
Next, local opportunities and constraints should be identified. Understanding the legal and regulatory controls on stormwater and graywater use is a key first step. As discussed in Chapter 8, water rights may limit large- or small-scale stormwater or graywater use in the western United States. This issue is discussed in more detail later in the chapter in several project examples. If downstream water rights could be impacted by the project, a water right permit may need to be secured. Graywater and stormwater use is regulated only at the state or local level, and relevant regulations may be found in state plumbing codes, wastewater disposal regulations, environmental regulations, and state or local public health laws (see Tables B-1 and B-2 for major state regulations, although this list is not exhaustive). A few states provide water quality guidance to determine when treatment is necessary for nonpotable uses (see Chapter 8, Box 8-2). Most of the existing regulations govern household- or small-building-scale projects, and neighborhood/multi-residential projects or regional capture projects necessitate consultation with appropriate state and local agencies to determine project design requirements.
TABLE 9-2 Capacity to Address Drivers Through Graywater Reuse
|Household Scale||Neighborhood to Regional Scales|
|Water Supply (Quantity drivers)|
|Water scarcity||Graywater can reduce indoor and outdoor household use (see Chapter 3), although indoor graywater use requires substantial treatment, dual plumbing, and rigorous maintenance.||Multi-residential buildings can achieve significant reductions in indoor water use through graywater reuse (approximately 24% when used for toilet flushing, and more if other nonpotable uses are included), with no impacts to the water available to downstream users.|
If water savings are the primary objective, then homeowners should first address outdoor irrigation demand by converting to native landscaping. Such efforts could reduce the size and complexity of the graywater irrigation systems needed while making more water available to downstream users.
If water savings are the primary objective, then opportunities to reduce outdoor irrigation demand should be considered by converting to native landscaping.
|Water supply reliability||During drought restrictions, graywater provides a modest but re native landscaping.||liable water source for irrigation that can help maintain|
|Water supply diversification||Not a major driver.||GW provides an additional drought-resistant supply to diversify a community’s water portfolio.|
|Water Quality (Pollution drivers)|
|Pollution prevention||Not a major driver.||Not a major driver.|
|Energy savings and greenhouse gas reductions||Laundry-to-landscape systems should provide low-energy on-site reuse for irrigation.||Large graywater systems with pumps and treatment likely require more energy than conventional drinking water sources, but graywater treatment, even at small scales, requires less energy than municipal wastewater treatment. Overall, the life-cycle energy requirements of various systems remain unknown.|
|Larger systems with pumps and treatment may require more energy than conventional water sources, although life-cycle energy requirements of various systems remain unknown.|
|Environmental Stewardship||May be a major driver at the household scale. However, ways to minimize outdoor water use should also be considered when optimizing environmental stewardship.||May be an important driver for developers, who sometimes benefit from higher rental or resale values for green buildings.|
|Hydromodification||Not a driver.|
|Extend life of existing infrastructure||Graywater systems have been cited as ways to extend the life of septic systems, although the committee was unable to find data to support this claim.||In dense urban areas, graywater reuse can extend the life of existing wastewater infrastructure by allowing additional development without expanding conveyance capacity.|
|Financial benefits||Rebates may be offered in some locations.||Incentives may be available for large projects that extend the life of existing urban wastewater infrastructure.|
|Cost savings may be feasible for simple laundry-to-landscape systems based on potable water savings.||Cost savings may be feasible based on potable water savings, although capital and maintenance costs of large-scale systems are not well defined.|
Once the legal and regulatory framework is understood, the opportunities for on-site use can be considered within existing constraints. Chapter 2 presents an array of applications for nonpotable water at the household, neighborhood, and regional scales. Opportunities should be considered that address multiple objectives, where possible, and that deliver a wide range of benefits, including some that are not easily monetized, such as aesthetic enhancements, public education, and aspirational value (see Chapter 7).
Site Characterization: Water Availability and Quality
Understanding water availability and quality to meet the intended uses is an essential next step in the planning process.
The total annual quantity of graywater and/or stormwater from various sources should be assessed, along with its inter-annual variability. Key questions include the following:
- Is sufficient stormwater and/or graywater available on an average annual basis to meet water supply objectives considering the target end uses? If not, then is supplemental water use acceptable?
- What is the timing of the water availability relative to the water demands? What storage capacity is needed to provide consistent water availability?
TABLE 9-3 Capacity to Address Drivers Through the Beneficial Use of Stormwater
|Household Scale||Neighborhood to Regional Scales|
|Water Supply (Quantity drivers)|
|Water scarcity||Stormwater can reduce indoor and outdoor household use, although indoor stormwater use requires treatment, dual plumbing, and rigorous maintenance.||Substantial potential exists to enhance regional water supplies by capturing and recharging stormwater, if suitable aquifers and recharge conditions exist, although water rights may need to be acquired.|
|If water savings are the primary objective, then opportunities to reduce outdoor irrigation demand should be considered by converting to native landscaping||Under suitable climatic conditions, multi-residential buildings can achieve significant reductions in indoor water use (up to 24% when used for toilet flushing, and more if other nonpotable uses are included), with no impacts to the water available to downstream users.|
|If water savings are the primary objective, opportunities to reduce outdoor irrigation demand should be considered by converting to native landscaping.|
|Water supply reliability||Not a major driver; during drought, roof runoff could provide some irrigation supply, but household tanks are rarely large enough to provide reliability during an extended dry spell, and during drought conditions, the roof runoff amounts available will be less than normal because of the lack of rain.||Neighborhood or regional stormwater recharge during wet periods can significantly enhance groundwater availability during times of drought.|
|Neighborhood-scale stormwater capture using large tanks can also enhance water reliability.|
|Water supply diversification||Not a major driver.||Large-scale stormwater recharge provides a means to diversify a community’s water portfolio.|
|Water Quality (Pollution drivers)|
|Pollution prevention||Stormwater capture at the household scales can reduce runoff from the site, particularly with larger tanks, but pollution prevention is not usually a major driver at this scale.||Reduction of stormwater pollution is often a major driver behind large stormwater capture and use projects, which aim to provide multiple benefits from large required investments to reduce stormwater runoff.|
|Energy savings and greenhouse gas reductions||Household-scale irrigation systems without pumps or treatment require minimal energy.||Large graywater systems with pumps and treatment likely require more energy than conventional water sources, although life-cycle energy requirements of various systems remain unknown.|
|Larger systems with pumps and treatment may require more energy than conventional water sources, although life-cycle energy requirements of various systems remain unknown.|
|Environmental stewardship||May be a major driver at the household scale. However, ways to minimize irrigation use should also be considered.||May be an important driver for developers, who sometimes benefit from higher rental or resale values for green buildings.|
|Hydromodification||Not typically a major driver at the household-scale because the benefits are small.||Large-scale stormwater capture or recharge systems can reduce stormwater runoff, improve the timing of surface water flows, and reduce erosion, which may be an important driver in urban areas with degraded streams.|
|Extend life of existing infrastructure||Not typically a major driver.||Neighborhood and regional stormwater infiltration or capture systems can be part of distributed strategies to address combined sewer overflows, in place of an expensive new separate storm sewer system.|
|Financial incentives||Rebates for rain barrels and tanks may be offered locally. Long-term cost savings may be feasible based on potable water savings.||Incentives may be available for large projects that contribute to regional water quality goals.|
|Long-term cost savings may be feasible based on potable water savings.|
Chapter 3 describes the quantities and timing of stormwater and graywater availability in six locations in the United States based on 1995-1999 precipitation data for a medium-density residential scenario. The chapter also broadly discusses the potential for graywater and stormwater to address water supply needs by examining graywater and stormwater use scenarios for irrigation and/or toilet flushing. The information in Chapter 3, although not intended for site-specific planning, illuminates how local climate, storage capacity, and on-site applications all affect how onsite water resources can reduce potable water use. For cities located in the central and eastern United States, where the timing of rainfall is better matched to irrigation demand, both graywater reuse and roof runoff capture with moderate
tank sizes can lead to substantial potential reductions in total water demand (up to 26 percent for whole-house graywater and 28 percent for stormwater for the medium-density residential scenarios analyzed; see Tables 5-1 and 5-5). In contrast, in the arid Southwest, where precipitation is limited and concentrated in winter months when irrigation demand is low, very large storage capacity is needed to significantly reduce potable water demand through stormwater capture. In these areas, large-scale groundwater recharge is an attractive water supply management alternative, if appropriate conditions for infiltration are available. In the arid Southwest, whole-house graywater can provide a substantial and consistent water source, although small relative to average outdoor irrigation demand. If reducing potable water demand is the primary objective, then conservation efforts to convert nonnative vegetation to xeriscaping should be encouraged, because reductions in outdoor water use provide the greatest opportunities for overall water savings, particularly in the arid Southwest.
An advantage of on-site graywater or stormwater use is the capacity to match treatment needs to the end use, with the potential for minimal or no treatment for some uses with little or no human exposures. Planning, therefore, requires an understanding of the quality of the local graywater or stormwater (Chapter 4) and the potential human exposures (Chapter 5) to calculate the potential risks associated with those exposures. These risks can then be used to assess the need for additional treatment. For graywater, multi-residential-scale systems have an averaging effect on graywater quality, and representative data are available on physical and chemical properties, although pathogen data are more limited. At the household scale, quality can vary widely based on whether best management practices for source control are implemented, although the additional risk of pathogenic illness from untreated household-scale graywater is lower considering the other potential pathways for disease spread within a household. For stormwater, a wide array of factors affect water quality (e.g., climate, rainfall intensity, land use, properties of surface materials), and there remains a significant shortage of information on human pathogens in stormwater (Chapter 4). For most nonpotable uses, human pathogens are the primary concern, although groundwater infiltration projects, particularly at a large scale, also necessitate a thorough characterization of organic and inorganic chemical constituents, including salts, to determine an appropriate system design (Chapter 4). When alternate water sources are considered for irrigation use, the salt content of the source water should be considered along with local soil conditions. Some applications may not be appropriate when salt content is high and local soil has a high clay content and/or elevated background salt content. In addition, opportunities for source control of pollutants can be considered.
Identify Candidate Strategies and Components
With a firm understanding of the project objectives (Chapter 1), opportunities for on-site use (Chapter 2), available water quantity (Chapter 3) and water quality (Chapter 4), potential human exposures (Chapter 5), and legal and regulatory constraints (Chapter 8), planners can appropriately narrow the suite of design and treatment options (see Chapter 6). For cities and water utilities, a key question to consider is project scale and whether to emphasize larger-scale projects (neighborhood or regional) or incentivize household- and building-scale projects. Household-scale projects are relatively easy to implement, could be partially subsidized by utilities, and would not require land purchases. However, household-scale, on-site, graywater or stormwater capture and use projects must be maintained by individual homeowners, and the use of these systems (and therefore the associated benefits) could be challenging to assess. For example, if stormwater tanks are not routinely used for irrigation or other nonpotable uses, then they provide neither the water supply nor pollution prevention benefits that are intended. Neighborhood-scale projects are typically paid for and managed by a water utility or a facility owner that can provide periodic maintenance and oversight, allowing the benefits to be documented if so desired. Neighborhood-scale systems also typically include more extensive treatment so that the water can be used safely for a wider range of beneficial uses. Neighborhood- and regional-scale stormwater and graywater projects may provide efficiencies of scale. Recent estimates from LADWP (2014) for stormwater capture suggest that neighborhood and regional stormwater infiltration systems can be comparable to other new water supply alternatives, while offering an array of additional benefits, such as pollution control and expanded greenspace in the urban environment (see Chapter 7). The availability of land and appropriate geology to support such projects should be determined in areas considering this option.
System Design Selection
Final design selection involves weighing how well the project objectives are achieved, overall costs (including capital and operations and maintenance costs minus any subsidies or incentives), and an assessment of financial, societal, and economic benefits. Projects at all scales should consider the acceptability to stakeholders.
For larger projects, where many stakeholders are involved, structured decision tools, such as multi-criteria decision analysis, can be used. These tools provide a structured and transparent mechanism by which various alternatives are evaluated to support final project design selection. The stakeholders and subject matter experts collaborate to create a decision hierarchy, as illustrated in Figure 9-2, that summarizes the key factors (or criteria) that affect the attractiveness of various options. This hierarchy can then be used to rate each alternative by establishing quantitative or semi-quantitative measures of each factor, and each factor is weighted by stakeholders based on its relative importance. The resulting computation blends the technical qualities of each option, as determined by the subject matter experts, and the relative importance of each factor, reflecting the values of the stakeholders (see Box 9-1).
The process is not complete when the relative value of each option is computed, for several reasons. First, not all stakeholders will have the same value set. This can be addressed by assigning different relative weights, thereby allowing the value of each option to be calculated and reflect the preference of individual stakeholders. Second, the results can lead to insights into which criteria are most important in distinguishing the relevant options. The most highly weighted criteria that are also the most different from a technical perspective will create the greatest difference in the assessment of alternatives. Both of these outcomes can be used to develop further options which, for example, can incorporate the most desirable elements of some of the original options as well as the key concerns of multiple stakeholders. Therefore, one can understand that it is not only the computation of scores but also the discussion that the process elicits that is important when allowing a diverse group of stakeholders to reach a decision that all can support.
Implementation issues vary in complexity, depending on the project’s scale. Project implementation issues include securing financing, working with regulators for necessary permits, developing mechanisms for system maintenance, and establishing appropriate system monitoring. Routine monitoring to assess treatment performance is a critical component of system implementation to minimize health risks. Depending on the extent of exposures, real-time monitoring may be appropriate to provide quality assurance, with an automatic shut-off when the water treatment system malfunctions (see Box 2-2). Monitoring on-site stormwater use may also be valuable, to ensure that the system is providing the intended benefits. Long-term maintenance and operations plans are also essential for effective operation.
San Francisco Public Utilities Commission has developed a guidebook for implementing alternative on-site water supplies that identifies relevant requirements for permitting, design and construction, inspection, maintenance, and monitoring (SFPUC, 2015). A few other localities have developed specific, consolidated, on-site, water reuse programs (see SFPUC, 2014, for a blueprint on developing such programs); in other locations, project permitting may require approval from numerous local agencies.
The decision framework outlined in Figure 9-1 and described in the preceding section can be used at a range of scales to determine whether on-site graywater and/or stormwater capture and use is a sound alternative considering the risks, costs, and benefits, and if so, what designs are most appropriate to implement. In the following section, the committee uses this framework to examine decisions in a household scale example and two neighborhood-scale examples. This information is presented to help synthesize information described elsewhere in the report (Chapters 1-8) for specific decision-making contexts.
Defining objectives is the first step of any on-site water supply decision process. Typical drivers behind graywater and stormwater use projects and the extent to which household-scale projects address these drivers are summarized in Tables 9-2 and 9-3. At a household scale, common objectives of on-site alternative water supply systems include reducing potable water demand, environmental stewardship, cost savings from reduced potable water and wastewater fees, pollution prevention (stormwater), and reliability of water supply during drought (graywater) (see Chapter 1). Some conflicts may exist among objectives, such as the objective to reduce stormwater pollution versus the objective to maximize water conservation and preserve supplies for downstream users. The relative importance of any one of these objectives over the others may determine the most applicable strategies. For example, if reliability of irrigation water during periods of extended droughts is a key objective, then only graywater systems offer a constant supply of water regardless of climatic conditions at a household scale.
Identify Opportunities and Constraints
The legal and regulatory framework should be understood because some states have specific permitting requirements or limit the potential uses of stormwater and graywater. Water rights permitting can be a key constraint to stormwater capture and use in western states (Chapter 7), but several states offer permitting exemptions for household-scale projects. For example, California, Utah, and Washington currently allow capture of rooftop runoff without a water rights permit for small projects (see Table B-1). In Arizona and Texas, onsite capture from rooftops, paved surfaces, and landscaped areas is exempt from water rights permitting. Although no specific laws address stormwater use in Idaho and New Mexico, state publications encourage the capture and use of rooftop runoff. The capture and use of roof runoff is generally not permitted in Colorado unless a water right permit is secured (see Table B-1 for exemptions).
State and local regulations may impact the potential end uses of graywater and stormwater and require specific best management practices or design standards (see Table B-3). Additionally, wastewater disposal regulations, environmental regulations, and state or local public health laws could impact implementation. Although Chapter 7 and Appendix B attempt to summarize the major legal and regulatory frameworks affecting the on-site beneficial use of graywater and stormwater, state and local laws are likely to continue to evolve as more people express interest in these practices and as courts continue to assess the legal implications. Therefore, interested homeowners should seek clarity from state or local government agencies on the latest legal and regulatory context for on-site graywater and stormwater use.
Along with constraints, opportunities for on-site use should be identified. At the household scale, the most common use of graywater and stormwater is landscape irrigation. Use for washing or toilet flushing may be feasible but will require more extensive treatment because of the potential human exposures.
Site Characterization—Water Availability and Quality
When considering stormwater or graywater capture and beneficial use at a household or building scale, it is critical to understand the amount of water available relative to the intended uses. At the household level, a water availability assessment for graywater is fairly straight-forward, given available water use data and the number of people living in the home. On average, 9.6 gpcd of graywater is provided from laundry water, with as low as 4 gpcd provided from high-efficiency washers with 14 gpcd or more from older, low-efficiency washers (Figure 3-4). On average, 26 gpcd graywater is available from all household water use, including bathroom faucets, showers, bathtubs, and laundry (see DeOreo et al., 2016). The use of water saving appliances and fixtures may reduce the amount of graywater available (see Figure 3-4). Use of graywater for toilet flushing requires dual plumbing and diligent maintenance. Therefore, most homeowners use graywater for irrigation and not toilet flushing. Chapter 3 outlines potential water savings for a medium-density, residential development in six cities considering conservation irrigation of turfgrass, but an individual’s outdoor water use will vary with the local climate, type of vegetation, irrigation rates and frequency, and other behavioral factors. Homeowners should consider available graywater supply versus irrigation needs, recognizing that in arid climates, available graywater may only provide a small fraction of outdoor water demands for typical, non-native vegetation. However, graywater may be sufficient to provide a reliable supply of irrigation water for water-efficient landscaping.
Stormwater availability for beneficial use at the household level will vary widely based on local climate conditions, the source area available for stormwater capture (i.e., square footage of roof area), the storage volume, and the timing of rainfall relative to water demands. In the arid Southwest, the volume and timing of rainfall is poorly matched to the irrigation demand (see Figure 3-2), and extremely large storage tanks are needed to substantially reduce potable water use. In California, for example, beneficial use of large volumes of stormwater can be achieved by neighborhood- or regional-scale capture facilities and storage by groundwater recharge. In the committee’s scenario analysis of potable water savings potential in Los Angeles (see Chapter 3), two rain barrels used for irrigation only reduced potable water use by 1 percent, and a moderate 2,200-gallon storage tank reduced potable water use by 4 percent. In contrast, in Lincoln, Nebraska, moderate-size storage tanks used only for outdoor irrigation resulted in a potential 21 percent reduction in overall water use, while two rain barrels resulted in 5 percent potential savings (see Tables 3-5 and 3-6).
Graywater and stormwater use present several water quality concerns at the household level. To minimize the risks of untreated graywater reuse at the household scale, residents should comply with best management practices and use subsurface (including landscape-covered drip) irrigation and only irrigate non-food crops. For stormwater use, roof runoff is primarily used because of its preferred water quality, but use of water from roofs with copper or galvanized steel materials should be avoided because of elevated metal content (see Chapter 4). Additionally, tree cover over roofs (as habitat for squirrels and birds) may result in high levels of indicator bacteria in the runoff, although the occurrence of human pathogens in roof runoff and other stormwater
Identify Candidate Strategies and System Components
With the objectives, constraints, and opportunities identified and the site characterized, candidate strategies can be considered in more detail. Graywater systems at the household scale include simple, low-cost, laundry-to-landscape systems to more complex, whole-house systems that require a storage tank and pump. If the system is used for applications with potential human exposures (e.g., spray irrigation, toilet flushing), then disinfection is also required. Whole-house systems that include treatment and disinfection require substantial maintenance that is usually beyond the skills of the typical homeowner (see Chapter 6).
For stormwater capture, household options include capturing roof runoff in cisterns or rain barrels or constructing rain gardens for shallow groundwater infiltration. Pitt et al. (2011) describes how to calculate benefits provided by various tank sizes for various roof areas and precipitation rates. Additionally, local or state regulations may influence system design requirements. In most cases, treatment is not necessary for irrigation, although human exposures should be minimized to reduce health risks. Disinfection may be desirable for spray irrigation at commercial buildings or other areas with substantial potential human contact (Chapter 5). Household roof runoff capture systems without treatment are relatively easy to implement and require minimal maintenance. If installed, then treatment systems would need to be relatively simple and not require significant expertise or attention (see Chapter 6).
Select System Design
Once identified, alternative designs and treatment options can be assessed for their capacity to deliver water supply benefits, water reliability, pollution control, and other project objectives relative to the cost. A full range of benefits, including social and environmental benefits (see Box 7-1), should be considered, although the data to assess these benefits may not always be available. For example, energy savings may also be possible for on-site graywater and stormwater systems, but the data are lacking to quantify life-cycle energy benefits (or costs) at the household scale.
Costs for household-scale, on-site, graywater and stormwater use can range widely depending on conveyance systems, tank size, whether treatment is included, and whether the system is self-installed or professionally installed (see Chapter 7). The committee calculated payback periods based on costs reported in the available literature and modeled potential water savings from the scenario analyses in Chapter 3. The payback periods vary depending on the uses and climate factors. For conservation irrigation use1 only, calculated payback periods based on the scenario analysis and the many associated assumptions (see Chapter 3) range from 5 to 26 years for rain barrels, 14 years to more than 50 years for a self-installed 2,200-gallon (8,300 liter) tank, and 2.5 to 6.0 years for a laundry-to-landscape system, assuming water use is actually reduced by the amount of graywater or stormwater utilized (see Chapter 7). These payback periods address only equipment and do not include the value of homeowner labor or the costs of maintenance. Local costs and benefits are needed to inform decision making for on-site reuse, because cost and benefits can vary substantially by location.
Homeowners must weigh the various benefits against their own objectives and budgets. Consider, for example, a homeowner who lives in the arid Southwest and whose primary objectives are sustainability and water conservation. The largest water savings would be provided by approaches to reduce or eliminate potable water demand for irrigation, such as the use of xeriscaping and other types of climate-appropriate, low-water-use landscapes (Mayer et al., 2015). Graywater irrigation through a simple laundry-to-landscape system could help maintain those landscapes, particularly during extended droughts, and reduce the costs associated with irrigation. In arid climates, simple laundry-to-landscape graywater systems have much shorter payback periods than do rain barrels or cisterns. If that homeowner lives in a city that already reuses wastewater through a centralized water reclamation facility, then graywater reuse would not change regional water savings, although simple laundry-to-landscape systems could reduce total energy use (definitive data are not available). Based on the scenario analyses, in Lincoln, Nebraska, rain barrels and laundry-to-landscape graywater systems are fairly comparable in terms of potential payback periods, although the volume of water conserved is smaller than other system designs. If sustainability and water pollution control are important objectives, then moderate-sized cisterns can provide substantial water savings (although with longer payback periods) and can reduce the adverse environmental effects of stormwater runoff. There is no single “best” configuration to maximize on-site water supply at the household scale, because of site-specific factors and individual differences in overall project objectives.
1 The committee recognizes that many residents irrigate at rates far above that required to meet the evapotranspiration deficit; thus, potential potable water savings could be higher than reported here. However, behavioral factors that would lower actual water savings were not considered.
At the household scale, installation can be performed by skilled do-it-yourselfers or professional installers. Maintenance needs should be well understood, because even the simplest rain barrel systems require periodic maintenance to remove sediment and ensure proper functioning. Owners should be aware of how water use impacts the desired project benefits. For example, stormwater capture systems provide minimal pollution prevention benefits if the tanks are not regularly emptied so that they are available to capture runoff from the next storm. Likewise, homeowners who increase the extent of landscaping to take advantage of newly available graywater supplies can ultimately increase potable water use even with the installation of graywater systems.
For the neighborhood scale, two examples are considered in the context of the decision framework presented in Figure 9-1—a multi-residential building development and an office-park development. These two examples provide different opportunities and considerations.
The objectives of a neighborhood-scale project (either a multi-residential development or a business park with many distributed buildings) might include cost savings through reduced potable water and wastewater fees, financial incentives related to stormwater management or extending the capacity of existing water and wastewater infrastructure, enhanced water reliability, environmental stewardship, public education, pollution prevention, and projecting a “green” image that could attract residents or businesses (see Chapter 1 and Tables 9-2 and 9-3). The relative priority of these objectives may influence the on-site water supply strategy selected.
Identify Opportunities and Constraints
Stormwater capture for beneficial use at the neighborhood scale can be constrained by water rights laws and local and state regulations. As discussed in the household-scale example, several states (e.g., California, Utah, Washington) exempt small rooftop capture systems, but a large multi-residential development or business park would likely exceed the capacity limits of these exemptions. Arizona and Texas water law appears to allow for stormwater capture before it has entered a natural water course (see Chapter 8 for more details). Coastal cities with no downstream users (e.g., San Francisco, Los Angeles) may be exempt from water rights permitting requirements. In prior appropriation states without exemptions for stormwater capture, a water rights permit must be acquired. Local and state regulations may also constrain potential applications for captured stormwater. Some states and localities do not permit stormwater use for toilet flushing (or for any use other than irrigation) (see Chapter 8).
Graywater regulations vary significantly from state to state. Five states (Arizona, California, New Mexico, Oregon, and Washington) have tiered regulatory frameworks that prescribe increased requirements for large facilities (see Table 8-3). States without a tiered framework may require additional consultation so that state and local agencies are comfortable that the project is adequately protective of public health. Several states (e.g., Idaho, Nevada, Ohio, Utah) only allow graywater to be used for irrigation, and because multi-residential units (particularly high-rise buildings) are likely to generate much more graywater than needed for landscape irrigation, such restrictions would limit the usefulness of these projects. However, state and local laws on graywater and stormwater use are evolving quickly, so interested developers should consult with local and state government agencies to understand the implications of the current legal and regulatory framework.
In a multi-residential building or a business park development, possible applications include landscape irrigation, shallow groundwater recharge, toilet flushing, ornamental water features, heating, ventilation, and air conditions (HVAC) cooling water, and washing (see Chapter 2).
Site Characterization—Water Availability and Quality
Stormwater can be captured from rooftops, driveways, and parking areas, although on-site stormwater capture is typically limited to rooftop runoff, because it tends to have the highest quality (see Chapter 4). Potential on-site water supply benefits can be calculated based on the stormwater capture area, local climate conditions, tank size or infiltration basin design, and the timing of water demands (see Chapter 3 for regional specifics or Pitt et al., 2011 for water availability calculations). High-rise buildings, which have a small area of capture, provide limited water supply relative to the overall on-site water use, but office parks may have substantial roof area. Stormwater availability relative to tank size will be greatest where rainfall timing is well-matched to water demand for the intended applications. Among the cities analyzed, Lincoln, Madison, and Newark showed the best match between stormwater availability and irrigation demand. When considering the capacity of stormwater to address a continuous demand, such as toilet flushing, Madi-
son, Newark, Lincoln, and Birmingham provided the largest reductions in potable water use because of the substantial, near-year-round precipitation (see Table 3-6 and Figure 3-2). In the arid Southwest the highly seasonal rainfall that occurs when irrigation is typically not needed makes stormwater capture for beneficial use more challenging. Careful consideration of roofing materials is advised to minimize metal contamination (Box 4-1).
If a new multi-residential building is constructed with dual plumbing to capture all graywater from bathroom faucets, showers, bathtubs, and laundry, then approximately 26 gpcd of graywater (or 45 percent of indoor water used) could be available for reuse for indoor or outdoor nonpotable uses. In cases of multi-residential buildings, irrigation demand is often small relative to the amount of graywater generated. In such cases, graywater use for toilet flushing may be a more viable option to achieve reduced demand for potable water. For a large multi-residential building, because source control becomes more difficult to manage and potential human exposures increase, graywater treatment is needed for most applications. For a business park, a separate analysis of on-site sources of graywater is advised prior to further planning because graywater represents a fairly small percentage of total wastewater generated in most businesses and institutional buildings (see Chapter 3) unless laundry or showers represent a significant part of average water use.
Identify Candidate Strategies and System Components
In a business park or institutional setting with large rooftop collection areas, stormwater could be captured, stored in large tanks, and treated for various on-site uses, or used to recharge groundwater (see Chapter 6). The design alternatives would need to be developed considering overall objectives, water availability, potential nonpotable uses, and water demand. For example, a system designed to capture all runoff from a 1-inch storm may be quite different from a system designed to optimize potable water savings. In the arid Southwest, where rainfall is concentrated during periods of low irrigation demand, very large stormwater storage tanks are typically needed to significantly reduce potable water use on an annual basis. Under appropriate hydrogeologic conditions, on-site or neighborhood-scale groundwater infiltration can instead be used to enhance regional water supply while reducing stormwater pollution. Infiltration projects tend to have significantly lower costs compared to large-scale stormwater capture and use projects, although the benefits of such projects are distributed regionally rather than to the building developer. Source areas for infiltration projects should be selected to minimize contamination (see Chapter 4), although infiltration basins can be designed to provide additional water quality treatment during infiltration (Chapter 6). If stormwater is captured and used on-site, then the level of service provided by the on-site, non-potable water system should be considered in the system design to optimize its use (relative to potable water). For example, at a fire station where stormwater is used for washing and tank filling, the use of nonpotable water could be encouraged by adjusting the flow rate and pressure to be greater than the potable water system.
New multi-residential buildings may be good candidates for dual-plumbed facilities that use treated graywater and/or stormwater for toilet flushing and other possible uses, such as laundry or HVAC. A building-wide treatment system can be managed and routinely maintained by trained operators, minimizing overall risk. Chapter 6 discusses the treatment necessary for specific applications of graywater (Figure 6-5) and stormwater (Figure 6-8), and an array of technologies are available at the multi-residential building scale to address the treatment objectives (Table 6-3) for potential end uses and exposures (see Chapter 5). However, few localities have specified treatment guidelines or requirements objectives, so developers may have to work closely with local public health agencies to develop treatment strategies that are protective of public health until such guidance is developed. Both graywater and stormwater can be used to meet nonpotable water demands, but combined systems typically involve separate treatment of the two sources before they are combined into a single collection (see Chapter 6). In some new developments, wastewater from toilets is also captured on site to maximize energy recovery (see Box 2-2). Different system designs can be developed to maximize various project objectives.
Select System Design
The potential benefits of on-site graywater and/or stormwater capture and use (including potable water savings, averted wastewater fees, other incentives, pollution prevention, energy recovery, environmental stewardship, public education, and improved public image) can be compared to the costs of various design alternatives and to a conventional water and wastewater system in the context of the overall project objectives. A full range of benefits, including social and environmental benefits (see Box 7-1), should be considered, although not all benefits have been thoroughly quantified. The financial costs and benefits of projects at this scale are site-specific and can be calculated by design engineers for the purpose of comparison among alternatives. General cost and benefit information for comparable projects would be helpful to inform decision making, but such information is currently not readily available.
Project implementation will necessitate close coordination with several local agencies (e.g., water, public health, building) for appropriate permitting of an on-site water capture and use project. SFPUC (2014, 2015) outlined a streamlined permitting process for on-site use of alternative water supplies in the San Francisco region and produced a “blueprint” for other cities that want to develop an on-site water program to encourage large-scale implementation. System maintenance and monitoring to assess treatment performance is essential to minimize human health risks. Additionally, residents should be informed about source control strategies to help maintain good system operation and minimize public health risk (see Chapter 2).
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, site conditions, source water availability, and project budgets, all vary widely. This chapter lays out a decision framework that can be used when considering the use of graywater or stormwater to meet various objectives and summarizes information from the report relevant to key decision steps at both the household and neighborhood scales.
Information is generally available to support water management decision making for on-site, nonpotable applications for simple, household-scale, graywater and/ stormwater systems with minimal human exposures. However, additional research would enhance decision making for larger systems or those with treatment requirements. Adequate information is available (or could be obtained) on graywater and stormwater availability for small-scale systems such as basic water quality parameters, system design and treatment technology effectiveness, and the existing regulatory framework. This information can be used to assess the capacity for on-site or local alternative water supplies to meet water demands while providing other benefits. However, as projects grow in size and scope, detailed analysis is
required to explore options, assess siting issues, and address concerns about water quality and availability. Therefore, key uncertainties affect the capacity to make fully informed decisions on appropriate and cost-effective designs for larger or more complex graywater or stormwater beneficial use systems. These uncertainties, which could be reduced by additional research, include:
- Water quality objectives for various uses 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.
Lack of clarity on water rights and legal and regulatory inconsistencies are also impediments to water management decision making in some states. More discussion on research needs is provided in Chapter 10.
Stakeholder engagement is crucial to the evaluation, selection, and implementation of any urban water management system and is particularly important when new options for distribution throughout the urban area, such as stormwater and graywater reuse, are being considered. The first step is understanding that stakeholders are those groups and individuals that can affect selection and implementation of the relevant system. Fortunately, effective and proven approaches exist to identify and engage appropriate stakeholders in the process, leading to the selection of implementable solutions. Experience shows that co-benefits, such as expanded greenspace or environmental stewardship, are often important to gaining stakeholder support. As noted in the discussion, experience shows that stakeholder engagement founded on trust and shared responsibility can be effective in planning and implementing projects, while stakeholder groups comprised of intractable adversaries are likely to have just the opposite effect.