There is substantial potential for graywater and stormwater to contribute to local water supply needs while providing other benefits such as stormwater pollution reduction, water supply diversification, and increased local control of water supplies. Graywater and stormwater use could be an important part of a broader effort to reimagine urban water infrastructure to efficiently use water, energy, and financial resources while enhancing water supply reliability, resiliency, and the livability of cities. However, as discussed in Chapter 9, major gaps exist in our understanding that make decision making more difficult, particularly with regard to neighborhood- and regional-scale stormwater and graywater capture initiatives.
This chapter highlights the major research needs identified by the committee that should be addressed to better support decision making. Additionally, this chapter presents research needs that look forward to ways to improve the water and energy efficiency of our nation’s water infrastructure and maximize financial, environmental, and social benefits. These research needs, if addressed, have the potential to advance the use of graywater and stormwater to expand local water supplies and ensure its safe and reliable use. Twelve research needs are categorized according to five themes outlined in Box 10-1:
- Risk and water quality
- Treatment technology
- Social science and decision analysis
- Policy and regulatory
Although no documented reports of adverse human health effects from the use of stormwater or graywater have been identified, additional examination of risk from microbial and chemical contaminants is necessary to support safe and appropriate design and implementation of stormwater and graywater use systems—particularly for large-scale systems. Such efforts can also facilitate adoption of water-saving practices in areas lacking a regulatory framework.
1. Assess the occurrence and fate of human pathogens in graywater and stormwater
Pathogens represent the most significant risks in nonpotable graywater and stormwater applications, considering the potential for adverse health effects from a single exposure to a small volume of water. Currently, most of the information on microorganisms in graywater and stormwater is limited to the occurrence of indicator microorganisms (e.g., total and fecal coliform bacteria, enterococci) rather than human pathogenic organisms. However, it is well documented that no consistent quantitative relationship exists between the occurrence or concentrations of indicator organisms and pathogenic organisms. This is particularly true for roof runoff, in which the sources of indicator bacteria may be primarily derived from animal waste that may or may not contain human pathogens. Depending on the project scale and contributing source areas, human waste from leaking sewers or faulty septic systems may contribute to the pathogen loads of stormwater. Therefore, additional work is needed to characterize the occurrence of pathogens in stormwater from various source areas (e.g., rooftops [tree covered and unshaded], open space, mixed use) and scales to inform guidance on appropriate treatment or exposure control to protect human health.
In graywater, for which the source of pathogens is typically human waste from laundry or showers, the usefulness of indicator data to predict risk will vary based on the scale of the project and other vehicles of disease spread (graywater would represent one of many potential vehicles of infectious disease within a single household). Pathogen data from graywater are extremely limited and insufficient for comprehensive risk analysis. Research is needed to assess the variabil-
ity in pathogen concentrations in graywater under different scales, storage conditions, and with and without source control practices, with the goal of bracketing likely and possible pathogen concentrations for the basis of broad risk calculations. These data are needed to identify appropriate practices to limit exposure when untreated graywater is used and to devise effective treatment systems where appropriate. Additionally, the implications of scale to the calculation of risk need to be examined.
Because hundreds of potential pathogens could be present in graywater or stormwater, it is necessary to choose representative pathogens for which the water is to be analyzed. The choice of pathogens could be based on a “worst-case” approach, or other approach based on the specific local situation.
2. Assess the occurrence and fate of chemical contaminants in stormwater
Stormwater quality is highly variable over space and time and is a direct function of land use, catchment size, and climatic and seasonal factors. Thus, stormwater water quality is difficult to predict. Organic contaminants, pathogens, and salts pose a particular concern for groundwater recharge. Some organic contaminants in urban stormwater are not common to municipal wastewater, and therefore their occurrence and fate remain poorly understood. Future research should investigate the occurrence and persistence of hazardous organic contaminants in urban stormwater. This research should identify those contaminants, such as urban pesticides and additives used in automotive and commercial applications, that are likely to be present in urban stormwater and may pose human health risks when stormwater is used for groundwater recharge. Information on the occurrence and fate of these compounds in urban settings can inform the development of appropriate source control and/or treatment strategies. In the case of large-scale projects, field studies and demonstration projects should proceed in tandem with planning for greater beneficial use of stormwater for water supply.
3. Understand the implications of enhanced water conservation on graywater quality and use
Indoor water conservation campaigns have had the most impact on use of water for flushing toilets and laundry
(Chapter 3). As discussed in Chapter 3, decreased water use, particularly in laundry machines, has the potential to impact the quality of graywater, making it more concentrated. This has the most impact on laundry-to-landscape systems, which are the most practical graywater systems for implementation in existing development. Research to date on impacts of graywater irrigation on soil quality has focused on graywater systems that include water from multiple sources (bathroom and laundry water; Sharvelle et al., 2012). New improvements in laundry machines continue to be made, reducing their water use. Such advances may render laundry-to-landscape programs obsolete. If homeowners are willing to adopt ultra-low water use washing machines, then this may be a more efficient way to reduce water demand than using laundry water for irrigation. The impact of high-efficiency laundry machines use for graywater irrigation needs to be better understood and evaluated in terms of water availability, water quality, and subsequent impact to soil quality.
4. Develop risk-based water quality guidance for various uses that could serve as a basis to develop standards of practice
Risk-based water quality guidance should be developed for various stormwater and graywater uses based on an understanding of anticipated human exposures from nonpotable uses of graywater and stormwater and possible contaminant concentrations (to be informed by research needs 1 and 2). Additional research is needed to better understand the impacts on risk and reliability associated with organic matter and turbidity levels recommended by the National Sanitation Foundation (NSF) International NSF 350 standard for graywater and stormwater for toilet flushing at a range of scales, because these factors significantly affect treatment costs. This information is needed to help localities that are struggling with the lack of existing standards or guidance and project developers (e.g., multi-residential buildings using graywater or stormwater for toilet flushing) who want to know what level of treatment is necessary and appropriate to protect public health. Such guidance, if rigorously developed based on comprehensive water quality and exposure data, would also reassure stakeholders that stormwater and graywater nonpotable use projects meet common acceptable-risk standards. Once risk-based water quality guidance is developed, standards of practice can be developed to meet these objectives, which can allow for more focused and cost-effective technology development and production, ultimately reducing costs for end users. Risk-based guidance also applies to large-scale groundwater recharge projects, although such guidance could vary with hydrogeologic conditions.
5. Develop monitoring technologies and strategies to assure compliance with regulatory criteria
The beneficial use of stormwater and graywater is achieved through distributed systems of varying scales. Because stormwater capture and recharge systems may be distributed over many locations, advanced monitoring and online control technologies could help reduce the costs associated with assessing system function and use. Research is needed to develop approaches for real-time monitoring of graywater and stormwater treatment systems for projects that have substantial human exposure. Technology advancement on rapid pathogen detection would be beneficial for large-scale graywater reuse as well as other sectors of water management and reuse, although surrogate detection systems may be feasible.
Stormwater capture and use systems are often installed as part of a broader effort to control stormwater discharges and combined sewer overflows. Advanced distributed monitoring systems could also assess the timing of water use relative to the demand for stormwater capture to assess the benefits provided and to identify strategies to optimize multiple benefits.
As graywater and stormwater use expands in scale and for uses other than subsurface or restricted access irrigation, treatment will likely be necessary to protect public health (Chapters 5 and 6). Additionally, managed recharge of stormwater may require treatment to prevent groundwater contamination (Chapter 5). Although treatment technology is relatively advanced, some focused research and development could improve the cost-effectiveness and reliability of graywater and stormwater use.
6. Develop treatment systems to meet tailored (fit-for-purpose) water quality across a range of scales
There is a need to assess and design treatment systems for various beneficial uses of graywater and stormwater. In the absence of fit-for-purpose water quality guidance, common standards of practice for graywater and stormwater treatment for various applications have yet to be developed. A wide array of technologies is applied, with varying levels of treatment. For managed recharge of stormwater, existing unit processes and the sequence in which they are applied must be tailored to achieve efficient treatment in terms of cost, energy, and maintenance. Until a standard of practice for stormwater treatment for water supply augmentation is developed, systems will continue to be developed as one-off
designs that remain costly and may lack sufficient proof-of-concept for wide-scale adoption.
The current lack of a standard of practice results in a time-consuming design phase (particularly for neighborhood- or regional-scale systems), which is often followed by iterations of design modifications once the system is in operation. Achieving a standard of practice that includes treatment process trains that are appropriate for stormwater or graywater use projects requires extensive application of technologies through demonstration projects. Rigorous monitoring of pilot and demonstration projects with extensive data collection is needed to adequately synthesize findings and to develop a standard of practice. Treatment system development, where feasible, should examine the capacity of natural, passive, or low-energy processes (e.g., wetlands, soils, engineered media, aeration) to reliably meet contaminant removal objectives while reducing energy use.
7. Understand the long-term performance and reliability of graywater and stormwater systems (from small to large scales)
Data on long-term performance of graywater and stormwater systems are lacking. Many systems do not include water quality monitoring, and therefore performance of systems remains unreported and unknown. Concerns regarding long-term performance and reliability of graywater systems continue to be a limitation for acceptance of such systems. Public health departments remain wary of treatment system performance and the potential health risks that may result from lack of maintenance or system failures. An extensive effort to characterize long-term performance and reliability is needed, which can include monitoring and data collection from systems that are currently operational as well as installation and monitoring of new pilot or demonstration projects, especially at neighborhood or regional scales. Development of an online database such as the International Stormwater BMP Database1 is recommended to serve as a portal for collecting and sharing information on the costs and performance of graywater and stormwater capture and beneficial use systems (see research need 11). Research on long-term performance should also assess the human behavioral dimensions of operation and maintenance of graywater and stormwater systems at various scales to assess long-term risks and develop strategies for better training or oversight, if needed.
With respect to the groundwater recharge of stormwater, there is a need to better understand water quality improvements during storage and infiltration, including long-term performance of natural treatment processes. Research is needed to assess the level of water quality improvement that can be expected in urban stormwater during infiltration and storage (including pathogen and organic contaminant removal) to determine additional treatment or system maintenance needs. Regulatory flexibility may be needed to allow demonstration tests at reasonable scales in the field. Demonstration tests will inform risk management and operational procedures for improved resilience and reliability.
Today’s urban water infrastructure is not designed for large-scale graywater or stormwater use. In many places infrastructure does not exist for urban stormwater or graywater capture, requiring new visions of water infrastructure to reach potential water and energy saving efficiencies. These issues suggest the following research needs with respect to infrastructure:
8. Envision new opportunities for water- and energy-conserving infrastructure and demonstrate their performance
Conventional design of stormwater and wastewater systems was developed many years ago when the beneficial uses were not envisioned, and retrofitting existing urban water infrastructure to accommodate beneficial uses of on-site water sources is expensive. However, new construction at building or neighborhood scales could include these features at a relatively small incremental cost. New urban water management models that incorporate graywater and stormwater use offer the potential to be much more water- and resource-efficient than traditional models. Research is needed to continue to develop new visions for urban water infrastructure that conserve water, reduce energy use (and generate energy where feasible), and reduce waste by recycling nutrients and other valuable resources. Important questions exist relative to key system components, including resource recovery and treatment technologies. Yet, this highly significant transformation can be achieved only if this potential is demonstrated. These systems could be incorporated into new urban development, and practical learning from the demonstration of these models could subsequently be incorporated into urban redevelopment.
9. Identify strategies to retrofit existing infrastructure for enhanced beneficial use of on-site water sources
Research is needed on cost-effective strategies for retrofitting these drainage and delivery components in existing developed areas to meet additional objectives, including im-
proving urban hydrology, reducing pollutant discharges, and enhancing water availability. Methods should be evaluated for retrofitting existing stormwater systems for enhanced stormwater capture, such as exploring options for conversion of stormwater detention ponds to enable stormwater capture. Given investments in existing systems and the future build-out of water reuse facilities in some areas, the benefits and feasibility of joint reclaimed wastewater and stormwater infiltration systems should be assessed. Additionally, the implications of extensive graywater use on existing wastewater conveyance infrastructure need to be better understood and the costs of managing such impacts evaluated.
Understanding the human dimensions of graywater and stormwater use will inform better project design and implementation. In addition, compiling existing performance data in a centralized database could improve support for decision making. Key priorities for research include
10. Understand behavioral impacts on overall water use in the context of graywater and stormwater projects
A major unresolved issue is the extent to which behavioral factors affect the benefits provided by graywater and stormwater projects. As noted in Chapter 3, two pilot projects on laundry-to-landscape graywater systems showed no reduction in potable water use, on average, and one of these studies showed increased potable water use. Similar studies on Australian household-scale stormwater capture projects have shown that actual water savings are only 0.3 to 0.7 times the theoretical value (see Box 3-2). It is well known that many rain barrels and cisterns are installed but not used sufficiently, compromising both water savings and pollution prevention benefits. The availability of a new low- or zero-cost water supply could cause households to plant more water-intensive landscaping or simply maintain existing landscaping more intensively. Similarly, knowledge that laundry water is being reused for irrigation may lead residents to do more laundry because the net cost of each load has been reduced. It is also possible that the positive emotional feeling (or “warm glow”) from making investments in green infrastructure may impact potable water consumption elsewhere, either positively or negatively. Therefore, research is needed to understand how such systems affect water use behavior.
Research to assess user knowledge and experience with on-site graywater and stormwater use systems is also important to assess the extent to which best management practices are understood, systems are properly installed and maintained, and appropriate source control practices are used. Additionally, research to understand homeowner-scale applications of stormwater and graywater would be useful to assess probable contaminant exposures as opposed to those anticipated if only best practices are followed. Such research would provide an improved understanding of household-scale graywater and stormwater risks and benefits and could be used to identify opportunities for public outreach and education to maximize potential benefits.
11. Collect performance data (including cost, energy, water savings, and water quality) in support of integrated water supply management, decision making, and refinement of decision tools
Because of the absence of ample documentation of costs, performance, risks, and co-benefits (see Chapter 7), many utilities are hesitant to integrate graywater or stormwater capture and use into their long-term water resource plans. Alternatively, some individuals and organizations may be over optimistic about the comparative benefits and costs of such projects for their communities. The U.S. Environmental Protection Agency (EPA) has a National Menu of Stormwater Best Management Practices, but this menu contains limited case studies and performance data on stormwater capture projects. Additional data are needed to quantify the multiple benefits and costs of small- to large-scale stormwater and graywater projects.
Improved financial cost data will reduce uncertainty for cost and benefits analyses and facilitate comparisons among alternatives. Well-documented case studies can better clarify capital costs and maintenance requirements, including documented energy costs and savings. Costs are easier to quantify than benefits, but benefits are equally important because graywater and stormwater use can provide multiple benefits beyond water savings or supply, including many that are not easily monetized. Systematic approaches to accounting for the full range of benefits (including multi-sectorial benefits and costs) should be developed to facilitate synthesis of project-specific information. In addition, water quality performance data, potential water supply capture benefits, and the broad array of potential co-benefits should be collected in a database of graywater and stormwater projects. In many cases, the benefits information may be missing or quite limited, in which case a systematic effort to develop such information will be of considerable value. Many innovative types of small-scale stormwater capture projects have minimal documentation of performance metrics.
To advance the state of the art, this information should be synthesized, including both smaller- and larger-scale graywater and stormwater projects, so that utilities, cities, building developers, and residents can understand the state of the practice and the relative costs and benefits (e.g., wa-
ter supply, energy savings, water quality, aesthetics). Such information could also be incorporated into decision tools to improve the statistical basis of the many variables considered for on-site water systems.
Beneficial use of graywater and stormwater is a relatively new and growing practice in many regions of the United States, and, as a result, legal and regulatory policies have not evolved as quickly as the practices. Consequently, communities would benefit from additional research on effective practices in regulatory programs and policies.
12. Examine how incentives and various regulatory strategies have proven effective in the implementation of stormwater or graywater systems to conserve water supplies
Implementation of stormwater capture and graywater strategies may be constrained by institutional “silos” that include multi-jurisdictional inefficiencies that may hinder the maximal use of these two potential supply sources. In addition, regulatory challenges to optimizing the capture and storage of stormwater for water supply purposes can be a significant hurdle for implementing an integrated stormwater capture and recharge strategy. Local governmental entities that have responsibility for stormwater or wastewater management vary significantly throughout the United States, so the institutional structure and constraints will likely vary from state to state and also within states.
These issues point to the need to examine various regulatory strategies for effective practices that can enhance broader implementation of stormwater and graywater systems to conserve or enhance potable water supplies. Research should assess regulatory innovations to increase onsite water use including market incentives, pollution credit markets, and integrated watershed management planning that optimizes local sustainable water supplies. Research should include an assessment of regulatory barriers, including those that may arise if conveyances and delivery systems cross property lines (or water management boundaries). This research should also assess the legal challenges and water rights issues for coastal and inland states and identify specific low barriers that can be overcome.