As the world enters the 21st century, the human community finds itself searching for new paradigms for water supply and management. As communities face water supply challenges amidst continued population growth and climate change, water reuse, or the use of highly treated wastewater effluent (also called reclaimed water) for either potable or nonpotable purposes, is attracting increasing attention. Many communities have implemented inexpensive water reuse projects, such as irrigating golf courses and parks or providing industrial cooling water in locations near the wastewater reclamation plant. In the process, these communities have become familiar with the advantages of water reuse, such as improved reliability and drought resistance of the water supply. However, increased use of reclaimed water typically poses greater financial, technical, and institutional challenges than traditional sources and some citizens are concerned about the safety of using reclaimed water for domestic purposes. These challenges have limited the application of water reuse in the United States.
The National Research Council’s (NRC’s) Committee on Assessment of Water Reuse as an Approach for Meeting Future Water Supply Needs was formed to conduct a comprehensive study of the potential for water reclamation and reuse of municipal wastewater to expand and enhance the nation’s available water supply alternatives (see Box S-1 for the statement of task). The study is sponsored by the Environmental Protection Agency, the Bureau of Reclamation, the National Science Foundation, the National Water Research Institute, the Centers for Disease Control and Prevention, the Water Research Foundation, the Orange County Water District, the Orange County Sanitation District, the Los Angeles Department of Water and Power, the Irvine Ranch Water District, the West Basin Water District, the Inland Empire Utilities Agency, the Metropolitan Water District of Southern California, the Los Angeles County Sanitation Districts, and the Monterey Regional Water Pollution Control Agency.
In this report, the committee analyzes technical, economic, institutional, and social issues associated with increased adoption of water reuse and provides an updated perspective since the NRC’s last report, Issues in Potable Reuse (NRC, 1998). This report considers a wide range of reuse applications, including drinking water, nonpotable urban uses, irrigation, industrial process water, groundwater recharge, and ecological enhancement.
CONTEXT AND POTENTIAL FOR WATER REUSE
Municipal wastewater reuse offers the potential to significantly increase the nation’s total available water resources. Approximately 12 billion gallons of municipal wastewater effluent is discharged each day to an ocean or estuary out of the 32 billion gallons per day discharged nationwide. Reusing these coastal discharges would directly augment available water resources (equivalent to 6 percent of the estimated total U.S. water use or 27 percent of public supply).1 When reclaimed water is used for nonconsumptive
1 See Chapter 1 for details on how the committee calculated this discharge total and the percentages.
Statement of Task
A National Research Council committee, convened by the Water Science and Technology Board, conducted a comprehensive study of the potential for water reclamation and reuse of municipal wastewater to expand and enhance the nation’s available water supply alternatives. The committee was tasked to address the following issues and questions:
1. Contributing to the nation’s water supplies. What are the potential benefits of expanded water reuse and reclamation? How much municipal wastewater effluent is produced in the United States, what is its quality, and where is it currently discharged? What is the suitability—in terms of water quality and quantity—of processed wastewaters for various purposes, including drinking water, nonpotable urban uses, irrigation, industrial processes, groundwater recharge, and environmental restoration?
2. Assessing the state of technology. What is the current state of the technology in wastewater treatment and production of reclaimed water? How do available treatment technologies compare in terms of treatment performance (e.g., nutrient control, contaminant control, pathogen removal), cost, energy use, and environmental impacts? What are the current technology challenges and limitations? What are the infrastructure requirements of water reuse for various purposes?
3. Assessing risks. What are the human health risks of using reclaimed water for various purposes, including indirect potable reuse? What are the risks of using reclaimed water for environmental purposes? How effective are monitoring, control systems, and the existing regulatory framework in assuring the safety and reliability of wastewater reclamation practices?
4. Costs. How do the costs (including environmental costs, such as energy use and greenhouse gas emissions) and benefits of water reclamation and reuse generally compare with other supply alternatives, such as seawater desalination and nontechnical options such as water conservation or market transfers of water?
5. Barriers to implementation. What implementation issues (e.g., public acceptance, regulatory, financial, institutional, water rights) limit the applicability of water reuse to help meet the nation’s water needs and what, if appropriate, are means to overcome these challenges? Based on a consideration of case studies, what are the key social and technical factors associated with successful water reuse projects and favorable public attitudes toward water reuse? Conversely, what are the key factors that have led to the rejection of some water reuse projects?
6. Research needs. What research is needed to advance the nation’s safe, reliable, and cost-effective reuse of municipal wastewater where traditional sources of water are inadequate? What are appropriate roles for governmental and nongovernmental entities?
uses, the water supply benefit of water reuse could be even greater if the water can again be captured and reused. Inland effluent discharges may also be available for water reuse, although extensive reuse has the potential to affect the water supply of downstream users and ecosystems in water-limited settings. Water reuse alone cannot address all of the nation’s water supply challenges, and the potential contributions of water reuse will vary by region. However, water reuse could offer significant untapped water supplies, particularly in coastal areas facing water shortages.
Water reuse is a common practice in the United States. Numerous approaches are available for reusing wastewater effluent to provide water for industry, irrigation, and potable supply, among other applications, although limited estimates of water reuse suggest that it accounts for a small part (<1 percent) of U.S. water use. Water reclamation for nonpotable applications is well established, with system designs and treatment technologies that are generally accepted by communities, practitioners, and regulatory authorities. The use of reclaimed water to augment potable water supplies has significant potential for helping to meet future needs, but planned potable water reuse only accounts for a small fraction of the volume of water currently being reused. However, potable reuse becomes more significant to the nation’s current water supply portfolio if de facto (or unplanned) water reuse2 is included. The de facto reuse of wastewater effluent as a water supply is common in many of the nation’s water systems, with
2 De facto reuse is defined by the committee as a drinking water supply that contains a significant fraction of wastewater effluent, typically from upstream wastewater discharges, although the water supply has not been permitted as a water reuse project. There is no specific cutoff for how much effluent in a water source is considered de facto reuse, because water quality is affected by the extent of instream contaminant attenuation processes and travel time. However, water supplies where effluent accounts for more than a few percent of the overall flow are usually considered to be undergoing de facto reuse. For a detailed discussion of the extent of effluent contributions to water supplies, see Chapter 2.
some drinking water treatment plants using waters from which a large fraction originated as wastewater effluent from upstream communities, especially under low-flow conditions.
An analysis of the extent of de facto potable water reuse should be conducted to quantify the number of people currently exposed to wastewater contaminants and their likely concentrations. A systematic analysis of the extent of effluent contributions to potable water supplies has not been made in the United States for over 30 years. Such an analysis would help water resource planners and public health agencies understand the extent and importance of de facto water reuse.
WATER QUALITY AND WASTEWATER RECLAMATION TECHNOLOGY
The very nature of water reuse suggests that nearly any substance used or excreted by humans has the potential to be present at some concentration in the treated product. Modern analytical technology allows detection of chemical and biological contaminants at levels that may be far below human and environmental health relevance. Therefore, if wastewater becomes part of a reuse scheme (including de facto reuse), the impacts of wastewater constituents on intended applications should be considered in the design of the treatment systems. Some constituents, such as salinity, sodium, and boron, have the potential to affect agricultural and landscape irrigation practices if they are present at concentrations or ratios that exceed specific thresholds. Some constituents, such as microbial pathogens and trace organic chemicals, have the potential to affect human health, depending on their concentration and the routes and duration of exposure (see Chapter 6). Additionally, not only are the constituents themselves important to consider but also the substances into which they may transform during treatment. Pathogenic microorganisms are a particular focus of water reuse treatment processes because of their acute human health effects, and viruses necessitate special attention based on their low infectious dose, small size, and resistance to disinfection.
A portfolio of treatment options, including engineered and managed natural treatment processes, exists to mitigate microbial and chemical contaminants in reclaimed water, facilitating a multitude of process combinations that can be tailored to meet specific water quality objectives. Advanced treatment processes are also capable of addressing contemporary water quality issues related to potable reuse involving emerging pathogens or trace organic chemicals. Advances in membrane filtration have made membrane-based processes particularly attractive for water reuse applications. However, limited cost-effective concentrate disposal alternatives hinder the application of membrane technologies for water reuse in inland communities.
Natural systems are employed in most potable water reuse systems to provide an environmental buffer. However, it cannot be demonstrated that such “natural” barriers provide any public health protection that is not also available by other engineered processes (e.g., advanced treatment processes, reservoir storage). Environmental buffers in potable reuse projects may fulfill some or all of three design elements: (1) provision of retention time, (2) attenuation of contaminants, and (3) blending (or dilution). However, the extent of these three factors varies widely across different environmental buffers under differing hydrogeological and climatic conditions. In some cases engineered natural systems, which are generally perceived as beneficial to public acceptance, can be substituted for engineered unit processes, although the science required to design for uniform protection from one environmental buffer to the next is not available. The lack of clear and standardized guidance for design and operation of engineered natural systems is the biggest deterrent to their expanded use, in particular for potable reuse applications.
Reuse systems should be designed with treatment trains that include reliability and robustness. Redundancy strengthens the reliability of contaminant removal, particularly important for contaminants with acute affects, while robustness employs combinations of technologies that address a broad variety of contaminants. Reuse systems designed for applications with possible human contact should include redundant barriers for pathogens that cause waterborne diseases. Potable reuse systems should employ diverse processes that can function as barriers for many types of chemi-
cals, considering the wide range of physiochemical properties of chemical contaminants.
Reclamation facilities should develop monitoring and operational plans to respond to variability, equipment malfunctions, and operator error to ensure that reclaimed water released meets the appropriate quality standards for its use. Redundancy and quality reliability assessments, including process control, water quality monitoring, and the capacity to divert water that does not meet predetermined quality targets, are essential components of all reuse systems. A key aspect involves the identification of easily measureable performance criteria (e.g., surrogates), which are used for operational control and as a trigger for corrective action.
Monitoring, contaminant attenuation processes, post-treatment retention time, and blending can be effective tools for achieving quality assurance in both nonpotable and potable reuse schemes. Today most projects find it necessary to employ all these elements, and different configurations of unit processes can achieve similar levels of water quality and reliability. In the future, as new technologies improve capabilities for both monitoring and attenuation, it is expected that retention and blending requirements currently imposed on many potable reuse projects will become less significant in quality assurance.
The potable reuse of highly treated reclaimed water without an environmental buffer is worthy of consideration, if adequate protection is engineered within the system. Historically, the practice of adding reclaimed water directly to the water supply without an environmental buffer—a practice referred to as direct potable reuse—has been rejected by water utilities, by regulatory agencies in the United States, and by previous NRC committees. However, research during the past decade on the performance of several full-scale advanced water treatment operations indicates that some engineered systems can perform equally well or better than some existing environmental buffers in diluting and attenuating contaminants, and the proper use of indicators and surrogates in the design of reuse systems offers the potential to address many concerns regarding quality assurance. Environmental buffers can be useful elements of design that should be considered along with other processes and management actions in formulating the composition of potable water reuse projects. However, environmental buffers are not essential elements to achieve quality assurance in potable reuse projects. Additionally, the classification of potable reuse projects as indirect (i.e., includes an environmental buffer) and direct (i.e., does not include an environmental buffer) is not productive from a technical perspective because the terms are not linked to product water quality.
UNDERSTANDING THE RISKS
Health risks remain difficult to fully characterize and quantify through epidemiological or toxicological studies, but well-established principles and processes exist for estimating the risks of various water reuse applications. Absolute safety is a laudable goal of society; however, in the evaluation of safety, some degree of risk must be considered acceptable (NAS, 1975; NRC, 1977). To evaluate these risks, the principles of hazard identification, exposure assessment, dose-response assessment, and risk characterization can be used, as outlined in Chapter 6. Risk assessment screening methods enable estimates of potential human health effects for circumstances where dose-response data are lacking. Although risk assessment will be an important input in decision making, it only forms one of several such inputs, and risk management decisions incorporate a variety of other factors, such as cost, equitability, social, legal and regulatory factors, and qualitative public preferences.
The occurrence of a contaminant at a detectable level does not necessarily pose a significant risk. Instead, only by using dose-response assessments can a determination be made of the significance of a detectable and quantifiable concentration.
A better understanding and a database of the performance of treatment processes and distribution systems are needed to quantify the uncertainty in risk assessments of potable and nonpotable water reuse projects. Failures in reliability of a water reuse treatment and distribution system may cause a short-term risk to those exposed, particularly for acute contaminants (e.g., pathogens) where a single exposure is needed to produce an effect. To assess the overall risks of a system, the performance (variability and uncertainty) of each of the steps needs to be understood. Although a good understanding of the typical
performance of different treatment processes exists, an improved understanding of the duration and extent of any variations in performance at removing contaminants is needed.
When assessing risks associated with reclaimed water, the potential for unintended or inappropriate uses should be assessed and mitigated. If the risk is then deemed unacceptable, some combination of more stringent treatment barriers or more stringent controls against inappropriate uses would be necessary if the project is to proceed. Inadvertent cross connection of potable and nonpotable water lines represent one type of unintended outcome that poses significant human health risks from exposure to pathogens. To significantly reduce the risks associated with cross connections, particularly from exposure to pathogens, nonpotable reclaimed water distributed to communities via dual distribution systems should be disinfected to reduce microbial pathogens to low or undetectable levels. Enhanced surveillance during installation of reclaimed water pipelines may be necessary for nonpotable reuse projects that distribute reclaimed water that has not received a high degree of treatment and disinfection.
EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT
It is appropriate to compare the risk of water produced by potable reuse projects with the risk associated with the water supplies that are presently in use. In Chapter 7, the committee presents the results of an original comparative analysis of potential health risks of potable reuse in the context of the risks of a conventional drinking water supply derived from a surface water that receives a small percentage of treated wastewater. By means of this analysis, termed a risk exemplar, the committee compares the estimated risks of a common drinking water source generally perceived as safe (i.e., de facto potable reuse) against the estimated risks of two other potable reuse scenarios.
The committee’s analysis suggests that the risk from 24 selected chemical contaminants in the two potable reuse scenarios does not exceed the risk in common existing water supplies. The results are helpful in providing perspective on the relative importance of different groups of chemicals in drinking water. For example, disinfection byproducts, in particular nitrosodimethylamine (NDMA), and perfluorinated chemicals deserve special attention in water reuse projects because they represent a more serious human health risk than do pharmaceuticals and personal care products. Despite uncertainties inherent in the analysis, these results demonstrate that following proper diligence and employing tailored advanced treatment trains and/or natural engineered treatment, potable reuse systems can provide protection from trace organic contaminants comparable to what the public experiences in many drinking water supplies today.
With respect to pathogens, although there is a great degree of uncertainty, the committee’s analysis suggests the risk from potable reuse does not appear to be any higher, and may be orders of magnitude lower, than currently experienced in at least some current (and approved) drinking water treatment systems (i.e., de facto reuse). State-of-the-art water treatment trains for potable reuse should be adequate to address the concerns of microbial contamination if finished water is protected from recontamination during storage and transport and if multiple barriers and quality assurance strategies are in place to ensure reliability of the treatment processes. The committee’s analysis is presented as an exemplar (see Appendix A for details and assumptions made) and should not be used to endorse certain treatment schemes or determine the risk at any particular site without site-specific analyses.
ECOLOGICAL APPLICATIONS OF WATER REUSE
Currently, few studies have documented the environmental risks associated with the purposeful use of reclaimed water for ecological enhancement. Water reuse for the purpose of ecological enhancement is a relatively new and promising area of investigation, but few projects have been completed and the committee was unable to find any published research in the peer-reviewed literature investigating potential ecological effects at these sites. As environmental enhancement projects with reclaimed water increase in number and scope, the amount of research conducted with respect to ecological risk should also increase, so that the potential benefits and any issues associated with the reuse application can be identified.
The ecological risk issues and stressors in ecological enhancement projects are not expected to exceed those encountered with the normal surface water discharge of municipal wastewater. Further, the presence of contaminants and potential ecological impacts may be lower if additional levels of treatment are applied. The most probable ecological stressors include nutrients and trace organic chemicals, although stressors could also include temperature and salinity under some circumstances. For some of these potential stressors (e.g., nutrients), there is quite a bit known about potential ecological impacts associated with exposure. Less is known about the ecological effects of trace organic chemicals, including pharmaceuticals and personal care products, even though aquatic organisms can be more sensitive to these chemicals than humans. Sensitive ecosystems may necessitate more rigorous analysis of ecological risks before proceeding with ecological enhancement projects with reclaimed water.
Financial costs of water reuse are widely variable because they are dependent on site-specific factors. Financial costs are influenced by size, location, incoming water quality, expectations and/or regulatory requirements for product water quality, treatment train, method of concentrate disposal, extent of transmission lines and pumping requirements, timing and storage requirements, costs of energy, interest rates, subsidies, and the complexity of the permitting and approval process. Capital costs in particular are site specific and can vary markedly from one community to another. Data on reuse costs are limited in the published literature, although Chapter 9 provides reported capital and operations and maintenance costs for nine utilities (representing 13 facilities) that responded to a committee questionnaire.
Distribution system costs can be the most significant component of costs for nonpotable reuse systems. Projects that minimize those costs and use effluent from existing wastewater treatment plants are frequently cost-effective because of the minimal additional treatment needed for most nonpotable applications beyond typical wastewater disposal requirements. When large nonpotable reuse customers are located far from the water reclamation plant, the total costs of nonpotable projects can be significantly greater than potable reuse projects, which do not require separate distribution lines.
Although each project’s costs are site specific, comparative cost analyses suggest that reuse projects tend to be more expensive than most water conservation options and less expensive than seawater desalination. The costs of reuse can be higher or lower than brackish water desalination, depending on concentrate disposal and distribution costs. Water reuse costs are typically much higher than those for existing water sources. The comparative costs of new water storage alternatives, including groundwater storage, are widely variable but can be less than those for reuse.
To determine the most socially, environmentally, and economically feasible alternative, water managers and planners should consider nonmonetized costs and benefits of reuse projects in their comparative cost analyses of water supply alternatives. Water reuse projects offer numerous benefits that are frequently not monetized in the assessment of project costs. For example, water reuse systems used in conjunction with a water conservation program can be effective in reducing seasonal peak demands on the potable system, which reduces capital and operating costs and prolongs existing drinking water resources. Water reuse projects can also offer improved reliability, especially in drought, and can reduce dependence on imported water supplies. Depending on the specific designs and pumping requirements, reuse projects may have a larger or smaller carbon footprint than existing supply alternatives. They can also reduce water flows to downstream users and ecosystems.
Current reclaimed water rates do not typically return the full cost of treating and delivering reclaimed water to customers. Nonpotable water reuse customers are often required to pay for the connection to the reclaimed water lines; therefore, some cost incentive is needed to attract customers for a product that is perceived to be of lower quality based on its origin. Frequently, other revenue streams, including fees, drinking water programs, and subsidies, are used to offset the low rates. As the need for new water supplies in water-limited regions becomes the driving motivation for water reuse, reclaimed water rates are
likely to climb so that reclaimed water resources are used as efficiently as the potable water supplies they are designed to augment.
SOCIAL, LEGAL, AND REGULATORY FACTORS
Water rights laws, which vary by state, affect the ability of water authorities to reuse wastewater. States are continuing to refine the relationship between wastewater reuse and the interests of downstream entities. Regardless of how rights are defined or assigned, projects can proceed through the acquisition of water rights after water rights have been clarified. The right to use aquifers for storage can be clarified by states through legislation or court decision. The clarification of these legal issues can provide a clearer path for project proponents.
Scientifically supportable risk-based federal regulations for nonpotable water reuse would provide uniform nationwide minimum acceptable standards of health protection and could facilitate broader implementation of nonpotable water reuse projects. Existing state regulations for nonpotable reuse are developed at the state level and are not uniform across the country. Further, no state water reuse regulations or guidelines for nonpotable reuse are based on rigorous risk assessment methodology that can be used to determine and manage risks. The U.S. Environmental Protection Agency (EPA) has published suggested guidelines for nonpotable reuse that are based, in part, on a review and evaluation of existing state regulations and guidelines and are not based on rigorous risk assessment methodology. Federal regulations would not only provide a uniform minimum standard of protection, but would also increase public confidence that a water reuse project does not compromise public health. If nonpotable reuse regulations were developed at the federal level through new enabling legislation, this process should be informed by extensive scientific research to address the wide range of potential nonpotable reuse applications and practices, which would require resources beyond the reach of most states. A more detailed discussion of the advantages and disadvantages of federal reuse regulations is provided in Chapter 10. EPA should fully consider the advantages and disadvantages of federal reuse regulations on the future application of water reuse to address the nation’s water needs while appropriately protecting public health.
Modifications to the structure or implementation of the Safe Drinking Water Act (SDWA) would increase public confidence in the potable water supply and ensure the presence of appropriate controls in potable reuse projects. Although there is no evidence that the current regulatory framework fails to protect public health when planned or de facto reuse occurs, federal efforts to address potential exposure to wastewater-derived contaminants will become increasingly important as planned and de facto potable reuse account for a larger share of potable supplies. The SDWA was designed to protect the health of consumers who obtain potable water from supplies subject to many different sources of contaminants but does not include specific requirements for treatment or monitoring when source water consists mainly of municipal wastewater effluent. Presently, many potable reuse projects include additional controls (e.g., advanced treatment and increased monitoring) in response to concerns raised by state or local regulators or the recommendations of expert advisory panels. Adjustment of the SDWA to consider such requirements when planned or de facto potable reuse is practiced could serve as a mechanism for achieving a high level of reliability and public health protection and nationwide consistency in the regulation of potable reuse. In the process, public confidence in the federal regulatory process and the safety of potable reuse would be enhanced.
Application of the legislative tools afforded by the Clean Water Act (CWA) and SDWA to effluent-impacted water supplies could improve the protection of public health. Increasingly, we live in a world where municipal effluents make up a significant part of the water drawn for many water supplies, but this is not always openly and transparently recognized. Recognition of this reality necessitates increased consideration of ways to apply both the CWA and SDWA toward improved drinking water quality and public health. For example, the CWA allows states to list public water supply as a designated use of surface waters. Through this mechanism, some states have set up requirements on discharge of contaminants that could adversely affect downstream water supplies.
Updates to the National Pretreatment Program’s list of priority pollutants would help ensure that water reuse facilities and de facto reuse operations are protected from potentially hazardous contaminants. The National Pretreatment Program has led to significant reductions in the concentrations of toxic chemicals in wastewater and the environment. However, the list of 129 priority pollutants presently regulated by the National Pretreatment Program has not been updated since its development more than three decades ago, even though the nation’s inventory of manufactured chemicals has expanded considerably since that time, as has our understanding of their significance. Updates to the National Pretreatment Program’s priority pollutant list can be accomplished through existing rulemaking processes. Until this can be accomplished, EPA guidance on priority chemicals to include in local pretreatment programs would assist utilities implementing potable reuse.
Enhanced public knowledge of water supply and treatment are important to informed decision making. The public, decision makers, and decision influencers (e.g., members of the media) need access to credible scientific and technical materials on water reuse to help them evaluate proposals and frame the issues. A general investment in water knowledge, including improved public understanding of a region’s available water supplies and the full costs and benefits associated with water supply alternatives, could lead to more efficient processes that evaluate specific projects. Public debate on water reuse is evolving and maturing as more projects are implemented and records of implementation are becoming available.
The committee identified 14 water reuse research priorities that are not currently being addressed in a major way. These research priorities in the areas of health, social, and environmental issues and performance and quality assurance (detailed in Chapter 11, Box 11-1) hold significant potential to advance the safe, reliable, and cost-effective reuse of municipal wastewater where traditional sources are inadequate.
Improved coordination among federal and nonfederal entities is important for addressing the long-term research needs related to water reuse. Addressing the research needs identified by the committee will require the involvement of several federal agencies as well as support from nongovernmental research organizations. If the federal government decides to develop national regulations for water reuse, a more robust research effort will be needed to support that initiative with enhanced coordination among federal and nonfederal entities. Such an effort would benefit from the leadership of a single federal agency, which could serve as the primary entity for coordination of research and for information dissemination.
* * *
Solutions to the nation’s water challenges will require an array of approaches, involving conservation, supplemented as needed by alternative water supply technologies, such as reuse. Both potable and nonpotable reuse can increase the nation’s water supply, although nonpotable reuse can be more expensive in existing communities that are not already equipped with dual water distribution systems. With recent advances in technology and treatment design, potable reuse can reduce the concentrations of chemical and microbial contaminants to levels comparable to or lower than those present in many drinking water supplies. Adjustments to the federal regulatory framework, including scientifically supportable risk-based regulations for nonpotable reuse and modifications to the structure or implementation of the SDWA for potable reuse projects, would ensure a high level of public health protection for both planned and de facto reuse and increase public confidence in water reuse. Additionally, improved coordination among federal and nonfederal entities could more effectively address key research needs.