Water Reuse: Potential for Expanding the Nation's Water Supply Through Reuse of Municipal Wastewater (2012)

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A New Era of Water Management

As the world enters the 21st century, the human community finds itself searching for new paradigms for water supply and management in light of expanding populations, sprawling development, climate change, and the limits of existing conventional supplies. This introductory chapter explores the context for this new era of water management, within which water reuse is attracting increasing attention.

POPULATION GROWTH AND WATER SUPPLY

In the year 1900, the population of the world was between 1.6 and 1.8 billion persons (U.S. Census, 2010e). By the end of the 20th century, it was just short of 6.1 billion persons (U.S. Census, 2010d), an increase of approximately 270 percent. The United States finds itself in the same situation. Between 1900 and 2000, the population of the United States grew from 76 million persons to 282 million persons, an increase of 240 percent (U.S. Census 2010c). Along with this increase in population has come an increase in the demand for water.

To address the water supply needs of this expanding population in the United States, the 20th century was a time for building major water infrastructure, particularly dams (Figure 1-1) and aqueducts (Morgan, 2004). In the southwestern United States, ambitious projects built on the Colorado River, the Central Valley of California, and in central Arizona provided water and power that supported rapid population growth and increases in irrigated agriculture. Smaller projects in Texas, Florida, Colorado, and Georgia also expanded the nation’s water supply capacity as population growth accelerated. Although a limited number of water supply and storage projects are still being built, the rate of construction of water supply infrastructure has dropped off significantly in recent decades (Graf, 1999; Gleick, 2003).

FIGURE 1-1 Reservoir capacity in the continental United States from 1900 to 1996.
SOURCE: Data from Graf (1999).

This decline in construction of new capacity has occurred in spite of continuing projections for increased demand, suggesting that the strategy of fulfilling increased water demand by building large dams and aqueducts to capture water from freshwater streams is reaching its limit. This change is attributable to a number of causes, among them: (1) a diminishing number of rivers whose flow is not already claimed by other users, (2) increased concern about adverse impacts of

impoundments on stream ecology, and (3) a better understanding of water quality problems caused by irrigated agriculture (NRC, 1989).

Regional development and migration have placed further stress on our water sources. Large populations have migrated to warmer climates in California, Nevada, Arizona, Texas, and Florida, causing growth rates of 85 percent to more than 400 percent between 1970 and 2009 in those states while the national population has increased by less than 50 percent (Figure 1-2). In some places, these changes have necessitated infrastructure to collect and move water on a grand scale (e.g., the infrastructure on the Colorado River, the California State Water Project, and the Central Arizona Project).

An even broader perspective on this migration is provided in the U.S. county-level population projections through 2030 prepared by the U.S. Global Change Research Program (Figure 1-3). Continued development of these population centers in the southwest and arid west and continued migration from population centers in the eastern and midwestern United States will require substantial transformation in the way water is procured and used by the people who live and work in these geographies.

The shift in population and associated water demand is further complicated by potential impacts of climate change on the water cycle. Increases in evapotranspiration due to higher temperatures will increase water use for irrigated agriculture and landscaping while changes in precipitation patterns (see Figure 1-4) may diminish the ability of existing water infrastructure to capture water. This is particularly important in the western United States where shifts in the timing and location of precipitation and decreases in snowfall are expected (NRC, 2007).

FIGURE 1-2 Population growth in selected states between 1970 and 2009.
SOURCE: Data from U.S. Census (2010b).

Considerable uncertainty remains about the impacts of climate change on water supplies. Improvements in models and the collection of additional data are likely to reduce the uncertainties associated with these estimates in coming decades. However, the pressures placed on water supplies by the combination of population growth and the likely impacts of climate change necessitate a reexamination of the ways in which water is acquired and used, before all of the questions about climate change impacts on the hydrological cycle are resolved (NRC, 2011a).

NEW APPROACHES TO WATER MANAGEMENT

The increase in population coupled with the decreased rate of construction of reservoirs, dams, and other types of conventional water supply infrastructure is leading to a new era in water management in the United States. The pressures on water supplies are changing virtually every aspect of municipal, industrial, and agricultural water practice. These changes in water management strategies take two principal forms: reducing water consumption through water conservation and technological change and seeking new sources of water.

Reducing Water Consumption

Improvements in water efficiency and programs for water conservation have begun to change our national water use habits, reducing per capita water consumption. More changes of this kind are likely in the future across many sectors. In Table 1-1, selected data on water use collected by the U.S. Geological Survey (Kenny et al., 2009) are summarized, where changes in water use by both agriculture and industry are clearly evident.

While the U.S. population grew from roughly 150 million to 300 million persons during the 60-year period, industrial water use—an application that was once the third highest use of water in the United States—grew only modestly between 1950 and 1970 and has been on the decline for 45 years now. These decreases are due to increased efficiency, higher prices for water and energy, and a shift away from water-

FIGURE 1-3 County-level population growth trends in the United States between 1970 and 2030. Each block on the map illustrates one county in the United States. The height of each block is proportional to that county’s population density in the year 2000, and so the volume of the block is proportional to the county’s total population. The color of each block shows the county’s projected change in population between 1970 and 2030, with shades of orange denoting increases and blue denoting decreases.
SOURCE: USGCRP (2000).

intensive manufacturing. More recently transfer of manufacturing outside the United States may also have been important.

Water use for irrigation peaked in 1980 and has now declined below 1970 levels. New technologies have been developed in irrigation practice (Gleick, 2003) and indications are that these technologies, if more widely adopted, could result in significant additional improvement (Postel and Richter, 2003). Water exchanges between municipal and agricultural entities are also taking place with increasing frequency. Agreements with agricultural interests by both the Metropolitan Water District of Southern California and the San Diego Water Authority are examples. This practice puts further pressure on agriculture to get value for the water it uses.

FIGURE 1-4 Downscaled climate projections showing the change in 30-year mean annual precipitation between 1971– 2000 and 2041–2070, in centimeters per year. The median difference is based on 112 projections.
SOURCE: Brekke et al. (2009).

TABLE 1-1 Summary of Water Use (billion gallons per day) in the United States, 1950–2005

NOTE: Includes both freshwater and saline water sources.
SOURCE: Data from Kenny et al. (2009).

Thermoelectric power use also peaked in 1980, but this use is misleading because a large fraction consists of “once-through” cooling water, which is primarily a nonconsumptive use (Kenny et al., 2009). Thus, reduction of use of this water would not necessarily provide new water resources, although it may have other environmental benefits. Furthermore, plants employing freshwater once-through cooling are often located in areas with ample water resources where water demands are not growing rapidly.

Whereas the total consumption for industry and irrigation have both decreased in recent decades, water use for primarily public supply continues to rise. During the period between 1950 and 2005, water used for public supply more than tripled as the nation’s population doubled. Much of the increase in per capita consumption of water during this period (most notably between 1950 and 1985) can be tied to increased water use for landscaping, especially in arid climates. Consequently, there is significant potential for water conservation in the public supply sector.

Overall, U.S. water use (excluding thermoelectric power uses) has been stable at approximately 210 billion gallons per day (BGD; 795 million cubic meters per day [m³/d]) since 1985. This flat water-use trend corresponds with the slowdown in construction of new impoundments in the United States (Figure 1-1).

When these water use data are combined with population data from the U.S. Census Bureau and examined on a per capita basis, it becomes clear that irrigation and nonpower industrial use are now on the

FIGURE 1-5 Past trends in water use in the United States, expressed on a per capita basis.
SOURCE: Data from Kenny et al. (2009).

decline (Figure 1-5). Per capita industrial water use has been on the decline since 1965; per capita agricultural use was flat between 1955 and 1980 and has been declining since then. Municipal use (referred to as public water supply in Kenny et al., 2009) continued to grow until 1990, but even this sector has begun to see the effects of water conservation in recent years. It is reasonable to expect that conservation will continue to play an increasingly important role in the nation’s water management in the decades ahead, thereby reducing the demand for new water supplies. Including all sectors (except thermoelectric power), per capita water

FIGURE 1-6 Changes in U.S. water use and implications for the future. Population and total U.S. water use shown on left axis; per capita water use on right axis. Per capita water use includes all water uses except thermoelectric power, which is dominated by once-through cooling.
SOURCE: Data from Kenny et al. (2009) and U.S. Census Bureau (2008).

use was relatively stable between 1950 and 1980 but has dropped precipitously since that time (Figure 1-5).

The U.S. Census Bureau predicts that the nation’s population will increase by over 50 percent between 2010 and 2060. This population growth is displayed in Figure 1-6 along with the history of total water use and the history of per capita water use as well. If the U.S. Census estimates are correct, then, barring the development of major new water sources, per capita use must decline further. Both more efficient water use and the development of new sources of water beyond those the nation has traditionally used may be necessary in areas with limited existing water supplies.

Searching for New Water Sources

In addition to conservation efforts, the other major emphasis in the new era of water management involves a search for untapped water sources. These sources include the desalination of seawater and brackish groundwater, the recovery of groundwater impaired by previous anthropogenic activity, off-stream or underground storage of seasonal surpluses from existing impoundments, the recovery of rainwater and stormwater runoff, on-site greywater¹ reuse, and the reuse of

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¹ Greywater is water from bathing or washing that does not contain concentrated food or human waste.

municipal wastewater effluent. The role of each of these approaches in the nation’s future water supply portfolio is likely to be dictated by considerations related to public health, economics, impacts on the environment, and institutional considerations. The NRC recently published studies on desalination (NRC, 2008b), stormwater management (NRC, 2009c) and underground storage (NRC, 2008c). In this new water era, the reuse of municipal effluent for beneficial purposes may also be important. This topic—herein termed water reuse—is the focus of this report. See Box 1-1 for additional reuse terminology.

Water Reuse

During the past several decades, treated wastewater (also called reclaimed water) has been reused to accomplish two primary purposes: (1) to create a new water supply and thereby reduce demands on limited traditional water supplies and (2) to prevent ecological impacts that can occur when nutrient-rich effluent is discharged into sensitive environments.² Increasingly, the basic need for additional water supply is becoming the central motivator for water reuse. In addition to growing water demands, the further adoption of water reuse will be affected by a variety of issues, including water rights, environmental concerns, cost, and public acceptance.

The context for water reuse and common reuse applications for nonpotable reuse (e.g., water reuse for irrigation or industrial purposes) and potable water reuse (e.g., returning reclaimed water to a public water supply) are described in detail in Chapter 2. Potable reuse is commonly broken into two categories: indirect potable reuse and direct potable reuse. This classification considered potable reuse to be “indirect” when the reclaimed water spent time in the environment after treatment but before it reached the consumer. Inherent in this distinction was the idea that the natural environment (or environmental buffer, discussed in Chapter 2) provided a type of treatment that did not occur in engineered treatment systems. An example of these definitions can be found in the NRC (1998) report, Issues in Potable Reuse. The committee has chosen not to use these terms but rather to speak about the project elements required to protect public health when potable reuse is contemplated and to try to understand the attributes of the protection provided by an environmental buffer (see Chapters 2, 4, and 5).

In NRC (1998) a distinction was also made between “planned” and “unplanned” potable water reuse. For this report, the committee has chosen not to use these terms, because they presume that water managers are unaware of the integrated nature of the nation’s

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² For example, the water reuse program in St. Petersburg, Florida, was started in response to state legislation in 1972 (the Wilson-Grizzle Act) requiring all wastewater treatment plants discharging to Tampa Bay to either upgrade to include advanced wastewater treatment (including nutrient removal) or to cease discharging to Tampa Bay (Crook, 2004).

FIGURE 1-7 Reduction in per capita flow to the Los Angeles County Joint Outfall during the beginning of the 21st century (2000–2007).
SOURCE: Data from S. Highter, Los Angeles County Sanitation District, personal communication, 2010.

water system (e.g., when downstream drinking water systems use surface waters that receive upstream wastewater discharges). In the committee’s view, the use of effluent-impacted water supplies is reuse in fact, if not reuse in name. Therefore, the committee will refer to the less carefully scrutinized practice of using effluent-impacted water supplies for potable water sources as “de facto” reuse, rather than the term unplanned reuse (see Chapter 2 for more discussion of de facto reuse).

Municipal wastewater effluent is produced from households, offices, hospitals, and commercial and industrial facilities and conveyed through a collection system to a wastewater treatment plant. In 2004, over 16,000 publicly owned wastewater treatment plants were in operation in the United States, receiving over 33 BGD (120 million m³/d) of influent flow (EPA, 2008b). These publicly owned wastewater plants serve approximately 222 million Americans, or 75 percent of the population. Thus, the total discharge averages approximately 150 gallons (0.56 m³) per day per person.³ Recently, however, per capita wastewater flows have been decreasing, largely because of conservation practices (see Figure 1-7 for one example). Thus, water conservation and water reuse are linked, and projections of water available for reuse based on today’s wastewater flows need to take some allowance for reductions in wastewater production due to conservation and reduced sewer flows during future periods of water restriction.

Although a map depicting the location of all of the effluent discharges in the country is not available, the distribution of wastewater discharges should roughly track the population distribution, assuming similar per capita domestic and industrial wastewater generation rates occur across the country (Figure 1-8). Figure 1-8 illustrates that much of the nation’s wastewater is discharged to inland waterways. As a result, de facto reuse of wastewater is already an important part of the current water supply portfolio. The ongoing practice of de facto reuse and the likelihood that all of the reclaimed water will not be returned to the water supply also means that increased water reuse will not necessarily increase the nation’s net water resource by an equal amount. In fact in many western U.S. jurisdictions, downstream users possess a water right that could prevent or inhibit municipal reuse (see Chapter 10).

Based on data provided by the U.S. Environmental Protection Agency (EPA, 2008c), the committee calculated that approximately 12 BGD (45 million m³/d) of U.S. municipal wastewater was discharged directly into or just upstream of an ocean or estuary in 2008 out of 32 BGD (120 million m³/d) discharged nationwide (38 percent).⁴ Because there are no downstream cities that rely on these discharges to augment their water supplies, reuse of coastal discharges could directly augment the nation’s overall water resource. If all of these coastal discharges were reused, the additional water available would represent approximately 6 percent of estimated U.S. total water use or about 27 percent of municipal use in 2005 (Kenney et al., 2009). However, not all of the water available for reuse is located in areas where it is needed. Additionally, the health of some coastal estuaries may be dependent on the freshwater inflows provided by coastal wastewater discharges, particularly in water-scarce regions. Thus, the extent of availability

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³ Calculated from 33 BGD divided by 222 million people. Thus, this per capita discharge includes all discharges to wastewater treatment plants, not just residential discharges.

⁴ The raw data of the wastewater treatment plants along the continental U.S. coastline is from EPA’s Clean Watersheds Needs Survey: 2008 Data and Reports. The cited numbers are the sum of the outflow from wastewater treatment plants that discharge into watersheds having a fourth-level hydrologic unit code–defined area that directly borders or is immediately upstream of a major estuary or ocean, such that the wastewater discharge is unlikely to be part of the water supply of any downstream users.

FIGURE 1-8 Distribution of the U.S. population in 2009, which can be used to approximate discharge volumes of municipal wastewater effluent.
SOURCE: U.S. Census Bureau (http://www.census.gov/popest/gallery/maps/PopDensity_09.pdf).

of these coastal discharges for reuse would be dependent on site-specific analysis.

If reclaimed water was used largely for nonconsumptive uses, the water supply benefit of water reuse could be even greater because, in many cases, the wastewater can be again captured and reused. It is also evident that many inland discharges could be productively used as well, suggesting the potential for an even larger impact from water reuse on the nation’s water supplies.

CURRENT CHALLENGES

Important challenges remain that must be addressed before the potential of municipal water reuse can be fully harnessed. These challenges are discussed in this section and explored in more depth in the remainder of the report.

It is important to recognize that many communities currently practicing water reuse have already “picked the low-hanging fruit,” through practices such as irrigating golf courses, landscapes, municipally owned parks, and medians near wastewater treatment plants or by converting industrial applications that are less sensitive to water quality (e.g., cooling) to reclaimed water. Where these projects have been implemented, communities have become familiar with the advantages of reuse, particularly improved reliability and drought resistance of the water supply and reduced nutrient loading to sensitive downstream ecosystems. On the other hand, while many of these initial types of water reuse projects were inexpensive and relatively simple to implement, many future water reclamation projects are likely to pose greater challenges.

In addition, utilities will have to consider public skepticism about the health risks associated with reuse projects, and the public decision-making process can be a difficult one, particularly for projects with a potable reuse component. People have been trained

for generations to provide separation in both time and space between their wastes and their water supplies, and therefore the public is concerned about the safety of using wastewater effluent for domestic purposes. At the same time, several high-profile reports detailing the presence of pharmaceuticals and personal care products in water supplies (e.g., Kolpin et al., 2002; Benotti et al., 2009) have increased awareness of the common practice of de facto water reuse, which has increased with population growth. Today, many U.S. communities rely on drinking water sources that are exposed to wastewater discharges. Nevertheless, the quality of U.S. drinking water continues to improve, largely because of improvements in treatment technology. Perhaps the question is not whether reuse should be considered; rather the question should be how reuse can be planned so that it better incorporates appropriate engineered barriers. In many cases the alternative to building new, engineered water reuse systems is increased reliance on de facto water reuse, with fewer engineered controls and monitoring.

A century ago, circumstances as well as best professional judgment supported policies in which water was considered to be potable after it spent a certain period of time in the natural environment. This is illustrated by an official policy of the state of Massachusetts allowing sewage (untreated wastewater) discharges to rivers serving as a drinking water supply provided the outfall was located more than 20 miles (32 km) upstream of the drinking water intake (Hazen, 1909; Sedgwick, 1914; Tarr, 1979). Today, we increasingly rely on the application of treatment technologies and sophisticated monitoring to ensure that safe drinking water conditions are achieved. In recent decades, advances in the capability of water treatment systems have been substantial, and these systems are now able to routinely achieve a level of protection that exceeds anything imaginable in the middle of the 20th century. Despite this progress, how do we determine when treated wastewater has reached the point where it has become suitable for potable supply? How can this decision be made in a way that engenders public confidence? What monitoring tools are needed to provide assurance that promised performance is being delivered on a continuous basis?

Every treatment technique takes advantage of the specific properties of each contaminant in order to remove it, and no one treatment technique or combination of treatment techniques can be relied upon to reduce all possible contaminants to levels below the limits of detection. Robust analytical methods will continue to be developed that will detect organic compounds and pathogens at increasingly lower levels. Thus, water managers are faced with the challenge of knowing a contaminant is present at low levels without knowing if its presence at those levels is significant.

In the decades since the NRC published its groundbreaking report Risk Assessment in the Federal Government: Managing the Process (NRC, 1983), the nation has developed a sophisticated infrastructure for assessing the risk of anthropogenic chemicals in the environment and a significant cadre of experts trained in its application. Significant progress also has been made in the assessment of risks from waterborne pathogens. Whereas this infrastructure is well suited for the support of national regulations designed to manage risk and also for application to the assessment of important regional decisions, it is not as well suited to facilitate the decisions of individual communities comparing the costs, risks, and benefits of planned reuse with other water supply alternatives. Thus, communities face challenges in finding adequate technical support for complex water management decisions.

STATEMENT OF COMMITTEE TASK AND REPORT OVERVIEW

The challenges discussed in the previous section have limited the application of water reuse in the United States. In 2008, the 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. Effluent reuse has long been a topic of discussion and the NRC has issued several reports on the subject in the past (see Box 1-2).

This broad study considers a wide range of uses, including drinking water, nonpotable urban uses, irrigation, industrial process water, groundwater recharge, and water for environmental purposes. The study also considers technical, economic, institutional, and social challenges to increased adoption of water reuse to pro-

vide practical guidance to decision makers evaluating their water supply alternatives. The study is sponsored by the EPA, 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 District, and the Monterey Regional Water Pollution Control Agency.

The committee was specifically tasked to address the following 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?

The committee’s report and its conclusions and recommendations are based on a review of relevant technical literature, briefings, and discussions at its eight meetings, field trips to water reuse facilities, and the experience and knowledge of the committee members in their fields of expertise.

Following this brief introduction, the statement of task is addressed in nine subsequent chapters of this report:

• Chapter 2 provides context for this report by describing the history of reuse, common reuse applications, and the use of reuse technologies in the United States and globally.

• Chapter 3 discusses water quality and contaminants of concern in wastewater effluent.

• Chapter 4 provides an overview of the state of the science in water reuse with respect to treatment technology.

• Chapter 5 examines design and operational strategies to ensure reclaimed water quality.

• Chapter 6 discusses the risk assessment framework as it applies to water reuse.

• Chapter 7 explores the risks of reuse in context by evaluating the relative risks of various reuse practices to human health compared with de facto reuse practices that are generally perceived as safe.

• Chapter 8 discusses applications of water reuse for ecological enhancement.

• Chapter 9 examines the financial and economic circumstances surrounding reuse and examines the benefits of reuse.

• Chapter 10 describes the social and institutional factors, including regulatory concerns, legal considerations, and public perception.

• Chapter 11 discusses actions needed to advance the capacity to use reuse to address water demands, including research needs and the roles of federal and nonfederal agencies.

Note that this report covers all types of reuse, but not all chapters include equal coverage of all reuse applications. The committee has chosen to focus more intensely on applications for which there are specific unresolved issues that may be limiting the ability of communities and local decision makers to make wise choices about their future water supply options; thus, the reader will find greater discussion on potable reuse relative to nonpotable reuse. Additionally, on the basis of the statement of task, the committee focused its efforts on the reuse of municipal wastewater effluent. The issues discussed in the report have applicability to both large and small municipal wastewater treatment plants.

However, the committee does not discuss building-scale reuse or greywater reuse in depth in this report.

CONCLUSION

As populations are increasing, particularly in water-limited regions, water managers are looking toward sustainable water management solutions to address shortfalls in supply from conventional water sources. Efforts to increase the efficiency of water use through enhanced conservation and improved technologies and the development of new sources of water may both be necessary to address future water demand in areas facing extreme water shortfalls. Potable and nonpotable reuse are attracting increasing attention in the search for untapped water supply sources. Out of the 32 BGD (121 million m³/d) of municipal wastewater effluent discharged nationwide, approximately 12 BGD (45 million m³/d) is discharged to an ocean or estuary (equivalent to 6 percent of the estimated total U.S. water use or 27 percent of public supply). Reuse of these coastal discharges, where feasible, in water-limited regions could directly augment available water resources. When reclaimed water is used for nonconsumptive 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 (e.g., in-stream habitats, coastal estuaries) in water-limited settings. Municipal wastewater reuse, therefore, offers the potential to significantly increase the nation’s total available water resources. However, reuse alone cannot address all of the nation’s water supply challenges, and the potential contributions of water reuse will vary by region.