Triple bottom line costs for multiple potable reuse treatment schemes

Abstract

As populations around the world continue to grow and communities appreciate the difficulty in securing new water supplies, water reuse is expected to expand in the coming years. Other factors, such as localized drought severity and increased community and regulatory pressure may also increase the application of water reuse. The level of treatment provided in water reuse projects varies significantly throughout the world depending on numerous factors, such as regulations, water quality, end uses of the treated water, and public influence. Selecting the appropriate treatment technology and level of treatment can be a complex decision. Recent experiences within the water reuse industry have demonstrated that governmental and nongovernmental organizations and advocacy groups can influence selection of a higher or more costly level of treatment than is fit for the water purpose. This is partially because of a failure to consider the full financial, environmental, and social elements of the triple bottom line (TBL). The focus of this report was to develop and apply a TBL framework to help guide sound selection of the treatment process. The objective is to match the treatment to the intended use without expending unnecessary funds or energy or emitting excess greenhouse gas (GHG) and other air emissions, while minimizing other environmental and social costs. Although the present research addresses water reuse only, the TBL approach is equally applicable toward evaluating the full suite of water supply and demand alternatives.

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Water impact

More communities than ever are investigating the feasibility of implementing potable reuse projects to increase their water supply and protect against periods of drought. The complexity of this task is compounded by the variety of reuse treatment technologies, which can differ in terms of benefit to the end user as well as in the true cost of implementation. This paper examines the benefits and costs of various levels of treatment for potable reuse applications. A triple bottom line (TBL) analysis was performed that includes financial, environmental, and social elements to help ensure that the right treatment process is applied for the intended use without expending unnecessary funds, energy, greenhouse gases, and other social and environmental costs.

Introduction

Project background

The beneficial use of municipal wastewater effluent for nonpotable and potable use (water reuse) is currently practiced in various regions of the world. The level of treatment provided in water reuse projects varies significantly throughout the world depending on factors, such as regulations, water quality of the wastewater effluent, water quality goals, end uses of the treated water, and public influence. Recent experiences within the water reuse industry have demonstrated that governmental and nongovernmental organizations and other advocacy groups are influencing selection of a higher level of treatment to minimize a perceived risk to members of the public or the environment. However, selection and implementation of higher-level treatment is often done without full consideration of triple bottom line (TBL) components that include financial, environmental, and social elements that can be equally important to stakeholders. The focus of this report is to develop and apply a TBL framework to help ensure that the right treatment process for the intended use is selected without expending unnecessary funds, energy, and greenhouse gas (GHG) emissions or generating other social costs that waste society's resources because they fail to generate a corresponding benefit to society. Included in this report is the application of the TBL framework to pairs of water reuse treatment train alternatives that serve the same end use to provide transparent evidence for regulatory and policy deliberation purposes of how much added TBL cost is incurred by society to meet certain water reuse requirements. This provides sound evidence to enlighten broader policy and regulatory debates about treatment requirements and goals to avoid codifying requirements or practices that are not aligned with intended uses and associated risks. Note that in some instances treatment technologies with higher TBL costs are required for an intended application. However, it is important to clearly understand the TBL costs of each process so that informed decision making can be made and that “overtreatment” is not provided.

With the expectation that the need for developing new sources of affordable water supply will grow significantly in the near future in both arid and less arid climates, the proper examination of TBL costs of water reuse is especially important to assist utilities in the proper selection of treatment to help meet that need. In addition, a better understanding of TBL costs will help those communities developing new water reuse regulations and policy to properly address the financial, environmental, and social components included in a TBL analysis. Finally, although the present research addresses water reuse only, the TBL approach is equally applicable toward evaluating the full suite of water supply and demand alternatives.

Potable reuse regulations and treatment implications

The two categories of potable reuse are indirect and direct. Indirect potable reuse involves the discharge of treated water into an environmental receiving body (e.g., reservoir, groundwater aquifer) where it is subsequently withdrawn and treated for distribution in a drinking water system. Direct potable reuse follows the same principle except there is no intermediate receiving water body and treated reclaimed water is piped directly to the drinking water plant or into the potable water distribution system. Therefore, the main difference between indirect potable reuse and direct potable reuse is that indirect potable reuse includes an environmental barrier that provides natural treatment and increased retention time to allow mitigation in the event of water quality degradation. The level of treatment and online automation needed to implement direct potable reuse is currently being studied by the water reuse industry. Table 1 lists some well-known examples of indirect and direct potable reuse schemes currently in operation worldwide.¹ Historically, the majority of potable reuse schemes have been in the indirect category—with the one exception of the direct potable reuse scheme practiced in Windhoek, Namibia, since 1968. However, recently much more attention has been given to direct potable reuse as evidenced by projects in Texas and New Mexico, and California's recent legislation requiring the state to study the feasibility of direct potable reuse by 2016.² Note that a significant amount of unplanned, or “de facto,” indirect potable reuse occurs throughout the world. As reported in the 2012 Water Reuse report by the National Research Council,³ “The de facto reuse of wastewater effluent as a water supply is common in many of the nation's water systems, with some drinking water plants using waters from which a large fraction originated as wastewater effluent from upstream communities, especially under low-flow conditions.” Therefore, although not widely understood, indirect potable reuse is fairly common throughout the world.

Table 1 Examples of indirect and direct potable reuse schemes

Indirect potable reuse	Direct potable reuse
NEWater, Singapore	Windhoek, Namibia
Montebello Forebay Groundwater Recharge Project, Los Angeles County, California	Big Spring, Texas, United States
Orange County Water District Groundwater Replenishment System, Orange County, California	Cloudcroft, New Mexico, United States
Western Corridor Recycled Water Scheme, South East Queensland, Australia
Upper Occoquan Service Authority, Centreville, Virginia

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Although potable reuse has been practiced since the 1960s, its application is not as prevalent as nonpotable reuse and, therefore, regulations have not been developed within certain states in many locations. Although potable reuse guidelines have been developed in the United States and Australia, no federal regulations currently exist in the United States. Australia's water is regulated through the National Water Quality Management Strategy Guidelines which states and territories can incorporate into regulation when necessary. However, in the United States, a few states (e.g., California and Florida) have developed comprehensive potable reuse regulations because of the significant amount of potable reuse practiced in those locations. Some other states (e.g., Georgia, Texas) have not developed potable reuse regulations but allow the practice on a case-by-case basis with project specific permits established accordingly. A review of potable reuse regulations from the United States and Australia reveals the following:

• Most regulations and guidelines are focused on pathogen removal, organic removal, nitrogen removal, and compliance with drinking water regulations.

• Multiple barrier advanced treatment is required in most U.S. and Australian locations.

• California enforces a total organic carbon (TOC) limit of 0.5 mg L⁻¹ for 100% injection of recycled water (no diluent water), which is much more stringent than that required by Florida and recommended by EPA, 3 mg L⁻¹ and 2 mg L⁻¹, respectively.⁴ This has led to significantly different treatment approaches between the western and eastern United States for potable reuse projects, which is further explained in the Potential for Overtreatment in Water Reuse section.

• Use of soil aquifer treatment via spreading basins for potable reuse treatment is allowed in California and can reduce treatment costs significantly, because it can avoid the use of mechanically intensive equipment (e.g., microfiltration [MF], reverse osmosis [RO], and ultra-violet advanced oxidation process [UVAOP]) and the power and chemical consumption associated with these treatment processes.

• Advanced treatment typically is not needed to meet the 10 mg L⁻¹ total nitrogen (TN) limit stipulated by California and Florida provided the wastewater treatment plant practices include nitrification and denitrification treatment processes.

Various treatment technologies have been employed to meet these regulatory requirements and project specific water quality goals. California has traditionally used soil aquifer treatment where effluent is applied via surface spreading basins or dual membrane (microfiltration/ultrafiltration [MF/UF] plus RO) with direct injection. Projects in the eastern United States have been implemented using granular activated carbon (GAC) and natural treatment processes. The international community has primarily used a dual membrane approach, with the exception of Windhoek, Namibia.

Treatment provided in potable reuse projects is typically a combination of multiple barriers for the removal of pathogens and organics. Multiple barriers for pathogens typically are provided through a combination of filtration (granular or membrane), coagulation, softening, and disinfection (chlorine or ultraviolet [UV]). Multiple barriers for organic removal typically are provided through a combination of advanced treatment processes (RO, GAC, soil aquifer treatment [SAT], UVAOP, ozone), although conventional treatment processes (coagulation, softening) also provide removal at some locations. Most full-scale potable reuse plants include a robust organics removal process of GAC, RO, or SAT, which act as an effective barrier to bulk and trace organics and are the backbone of the potable treatment process:

• SAT based. Where SAT is used, advanced treatment beyond GMF and disinfection is not always employed. This is especially relevant in California where recharge of a major potable water aquifer has occurred via spreading basins since 1962.

• GAC based. GAC is used at a number of locations for the removal of bulk and trace organic compounds. GAC has a long history of use in potable reuse projects with operational installations in Virginia (1978), Texas (1985), Georgia (2000), and Colorado (2010). RO is not used where GAC is used.

• RO based. RO has become the gold standard for potable reuse projects in California and internationally (e.g., Singapore and Australia) because of its excellent performance in the removal of dissolved solids and trace organics. California regulations require the use of RO for direct injection potable reuse projects or a comparable alternative with regulatory approval. RO creates a concentrate stream that can be difficult and costly to dispose of, especially at inland locations. Most locations where RO has been implemented are located near the ocean where disposal of RO concentrate is convenient and much less costly than inland locations.

The use of SAT can only be implemented in areas with favorable geological conditions and, therefore, cannot be implemented at all locations. Conversely, RO and GAC can be implemented at any location because they are engineered processes. Consequently, the use of RO and GAC is more prevalent than SAT for potable reuse projects and this trend will likely continue as more projects are implemented. Note that both GAC and RO provide excellent removal of organic matter and neither can remove all constituents below detection limits. RO does provide for a lower overall dissolved organic carbon concentration, but note that the GAC effluent dissolved organic carbon concentration is lower than many raw waters provided to drinking water treatment plants. However, for water supplies with high dissolved solids content, partial or full RO treatment may be necessary to avoid cycling up of salts in both the potable and reclaimed water.

UVAOP has been implemented for most recent potable reuse projects to remove contaminants of emerging concern (CECs) and other compounds not well removed by RO (e.g., nitrosamines). The addition of ozone, or ozone with hydrogen peroxide, also has gained recent attention as a potential replacement for UVAOP and currently is being used at plants in Gwinnett County, Georgia and Windhoek, Namibia. Ozone has shown excellent removal of CECs.⁵ However, unlike UVAOP, ozone is not effective in removing nitrosamines (unless coupled with a biological process, such as SAT or biologically activated carbon [BAC]) and therefore an alternative mitigation technique would be required if nitrosamines are of concern.

Selection of a potable reuse treatment plant's backbone organics removal approach (GAC, RO and/or SAT) is dependent on many factors including raw water quality, finished water quality goals, cost, geographic considerations, type of potable reuse, public perception, and other site-specific factors. Although the RO-based approach appears to be gaining popularity and has been implemented in most of the recent potable reuse projects, all three types of organic removal processes have been successfully implemented at full scale, and careful consideration of all TBL factors should be given to each approach prior to treatment selection to truly understand all cost, environmental, and social effects.

Potential for overtreatment in potable reuse

Fifteen potable reuse schemes are operational in the United States as of 2010, with projects in California, Virginia, Georgia, Texas, Arizona, Colorado, and New Mexico.⁶ Eight of these schemes use RO as the primary mechanism for organics removal, four use GAC, and three use SAT. The use of SAT is not always feasible because of site constraints and geological conditions. RO and GAC are often easier to implement from an engineering and construction perspective but are usually more costly. RO has predominantly been used in the western United States, whereas GAC is predominantly used in the East. For example, in areas where SAT is not utilized, 80% of the projects implemented in the West have used RO (8 out of 10 projects), compared to 0% in the East (0 out of 2 projects—both projects use GAC). Regulations, geographic location, and source water quality have driven this dichotomy in potable reuse treatment. In California, state reuse regulations require utilities to provide RO and advanced oxidation treatment for potable groundwater recharge applications where SAT is not used.⁷ A total organic carbon (TOC) concentration of less than 0.5 mg L⁻¹ must be achieved to allow complete reuse of treated water without additional blending water. RO also removes dissolved solids, which can prevent increased salinity levels in recharged aquifers. In contrast, the approach taken in Virginia and Georgia for potable reuse has been significantly different. Implementation of the Upper Occoquan Service Authority's (UOSA) indirect potable reuse project in northern Virginia began in 1978 to consolidate 11 small WWTPs that were causing significant eutrophication in a downstream drinking water reservoir into one regional advanced treatment plant. The primary purpose of the regional plant (current capacity is 54 MGD [204 MLD]) was to protect the downstream drinking water reservoir, and the water quality parameters included in the discharge permit were established for this purpose: chemical oxygen demand (COD) < 10 mg L⁻¹; total kjeldahl nitrogen (TKN) < 1 mg L⁻¹, total phosphorus (TP) < 0.1 mg L⁻¹, and turbidity < 0.5 NTU. The 60 MGD (227 MLD) potable reuse project in Georgia's Gwinnett County is similar to UOSA in that protection of the downstream drinking water reservoir (Lake Lanier) was the primary purpose of the locally developed discharge permit for the advanced treatment plant, which included the following limits: COD < 18 mg L⁻¹, NH₃ < 0.4 mg L⁻¹, TP < 0.08 mg L⁻¹, and turbidity < 0.5 NTU. UOSA and Gwinnett County both successfully use a GAC-based treatment train to meet their discharge limits, whereas California uses RO-based and SAT-based treatment trains. Table 2 summarizes the main factors affecting selection of advanced treatment processes at these locations.

Table 2 Significant factors affecting selection of advanced treatment processes in California, Virginia, and Georgia

Parameter	California (for direct groundwater recharge; no SAT)	UOSA (Northern Virginia)	Gwinnett County, Georgia
a The RWC is the quantity of recycled water applied at a recharge site divided by the sum of recycled water applied at a recharge site and diluent water used for blending. b Alternative treatment technologies can be used with regulatory approval.
Potable reuse regulations	Yes	Yes (Occoquan Policy)	No
Organics limit	TOC ≤ 0.5 mg L^−1/recycled water contribution (RWC)^a	Chemical Oxygen Demand (COD) ≤ 10 mg L^−1	COD < 18 mg L^−1
Regulatory treatment required	RO and advanced oxidation^b	UOSA plant treatment train	None specified
Total Dissolved Solids (TDS) concern	Yes; TDS is high in some locations	No; reclaimed water TDS is <500 mg L^−1
Total nitrogen	Several coastal wastewater treatment plants (WWTPs) do not practice nitrification/denitrification; RO provides a nitrogen barrier in these cases to meet the 10 mg L^−1 TN limit	Both wastewater treatment plants practice nitrification/denitrification and therefore additional nitrogen removal is not required
Ease of RO concentrate disposal	Historically less expensive through ocean disposal	Expensive because of inland location and difficulty in accessing ocean for disposal

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The primary difference between California and the eastern United States is California's requirement for RO treatment driven by the very low TOC limit, TDS concerns, and statewide regulatory mandate for RO. In contrast, RO treatment is not required at the Virginia and Georgia potable reuse plants because of the higher discharge limit for organics (COD based). Consequently, GAC is used for organics removal at these plants because of its significantly lower total costs and ability to meet the required COD limits easily. Naturally, the question arises as to which treatment approach is more appropriate because both RO and GAC have been used successfully for many years at full-scale facilities. The answer is often location-specific and dependent on numerous issues that will be examined in detail in this report. The intent of this research is to determine TBL costs for different treatment approaches that will provide regulators, water utilities, and practitioners with an in-depth understanding of the consequences of regulatory requirements and treatment selection decisions.

Because of the high-energy requirements for RO and costly disposal requirements of its concentrate waste for inland locations, it may not be the preferred alternative in all cases after careful consideration of all TBL factors. This is supported in a recently published NRC report titled, Water Reuse: Potential for Expanding the Nation's Water Supply through Reuse of Municipal Wastewater:³

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 applications for water reuse in inland communities.

Overtreatment scenarios for analysis

Cases of overtreatment in the water reuse industry do not appear widespread; however, because water reuse has grown significantly in recent years and is expected to grow more as population densities increase and water scarcity amplifies, a clear understanding of the TBL costs for different treatment approaches is beneficial to current and future water reuse practitioners.

Based on the case studies presented earlier, TBL costs for a potable reuse scenario comparing California's “RO-Advanced Oxidation” approach to the East Coast's “GAC-based” approach. Three concentrate management approaches will be analyzed for the RO-based approach: ocean disposal, mechanical evaporation, and evaporation ponds.

These scenarios are not exhaustive, because many treatment process selections are available during the implementation of potable reuse projects. For example, use of soil aquifer treatment can be an effective and efficient potable reuse treatment process and should be considered in geographic locations that support its use. However, because the practicality of its use is site-specific, cost estimates using this technology are generally not transferrable, and more of the recently implemented potable reuse projects are using mechanically based technologies, SAT treatment has not been included in the potable reuse treatment scenario analyzed. The scenarios selected for analysis represent approaches frequently considered and therefore will be directly applicable to many utilities during implementation of their reuse projects. Analysis of these scenarios also provides a framework that can be applied to the TBL evaluation of other treatment train comparisons. These scenarios are fully described in the Triple Bottom Line Methodology section. Note that the intent of this research is not to criticize those technologies that have higher TBL costs, because in some cases those technologies are necessary for the intended application. Instead, the intent is to clearly understand the TBL costs of each process so that informed decision making can be made and that “overtreatment” is not provided when it is unnecessary.

What is triple bottom line accounting?

The three components that comprise the TBL are financial, social, and environmental. As such, TBL accounting offers an alternative to evaluating organizational performance purely on the basis of the direct financial return to the organization to include the environmental, social, and financial elements that matter to stakeholders both internal and external to the organization.⁸ By balancing environmental and social effects with financial ones, organizations avoid achieving financial gains at the expense of the environment and societal aims.

Utilities involved in water reuse and other organizations that understand and strive to improve their performance along each of these dimensions are sending the signal that they are well managed and that they take a long-term perspective on their operations.⁵ For the purposes of this report, these three elements are defined as follows:

• Environmental elements include effects on natural resources (e.g., land, air, and water) and the flow of ecosystem services that directly and/or indirectly support human wants and needs for current and future generations. This includes resources (e.g., water, energy, chemicals, land, and materials) that reuse water utilities rely on as “inputs,” as well as resources that are affected by discharges, air emissions, or solid waste disposal in the course of “producing” or using reuse water. It is important to note that a disconnection can exist between perceived risks on the part of the public and actual risks based on sound science. In such circumstances it can be important to expend resources to bring perceived risks and actual risks into closer alignment to avoid making faulty decisions or the appearance of flawed decisions.

• Social elements relate to quality of life that are deemed important from a societal perspective and are not otherwise covered by the financial or environmental dimensions. Examples of social elements include human health, worker safety, education, and crime. Of these social factors, it is likely that human health is the only one that would be affected by different water reuse treatment trains. For example, different reuse treatment trains have different energy requirements and thus vary in terms of their emissions of air pollutants damaging to human health.

• Financial elements include the direct costs and returns to the organization, as well as the financial effects on stakeholders outside of the organization.

As Kenway and colleagues note, TBL reporting on beneficial and adverse effects makes the full social costs and social benefits of water alternatives transparent to decision makers.⁵ This is important to facilitating selection of the least costly reuse treatment alternative for society as a whole.

This research uses a cost-benefit analysis approach toward TBL accounting. Each of the water reuse treatment trains are compared in terms of their full social costs and benefits. Environmental and social effects are quantified in their natural units (e.g., kWh of energy utilization, tons of carbon dioxide [CO₂] equivalents) as stakeholders are interested in tracking how alternatives directly contribute to certain societal goals, including energy conservation and reducing GHG emissions. Where reasonable, effects are then quantified in dollars to facilitate comparing alternatives on the basis of a single measure of net social cost. Environmental and social effects that were not expressed in monetary terms are quantified or qualitatively characterized in the summary comparison of alternative treatment trains to ensure their consideration in identifying the TBL preferred alternative.

Finally, this TBL approach also relies on life-cycle assessment (LCA), a second well-established method for evaluating alternatives. By incorporating LCA into the approach toward evaluating treatment train alternatives, this analysis considers effects that are upstream of the water reuse treatment facility (e.g., at the power plant that produces the energy to run the water reuse treatment plant), as well as downstream effects (e.g., brine waste disposal from the reuse water treatment plant). The application of cost-benefit analysis and LCA approaches into the TBL framework addresses the research objective which is to create a framework document to help ensure that the right process and technology is applied to match water quality with its intended use, without expending unnecessary funds, energy, and GHG emissions to treat water beyond what is suitable or necessary for the intended application. Then, by applying the TBL framework to pairs of treatment train alternatives, the project provides documented and transparent evidence—for regulatory and policy deliberation purposes—of how much added cost (including external, nonmarket costs) is incurred by society to meet some water reuse regulatory requirements. This provides sound evidence to enlighten broader policy and regulatory debates about treatment requirements that are out of synch with intended uses and associated risks.

Triple bottom line methodology applied to a potable reuse scenario for reservoir augmentation

This scenario is a potable reuse scenario comparing the RO-based approach (MF-RO-UVAOP) used extensively in California and internationally to the GAC-based approach used in the eastern United States for reservoir augmentation. This scenario addresses the situation where a utility implements the more recently recognized RO-based approach for potable reuse without understanding the potential TBL effects, especially for inland locations where RO concentrate disposal can be particularly challenging. Multiple concentrate handling approaches are analyzed for this scenario including ocean disposal, mechanical evaporation, and evaporation ponds. Process flow diagrams for this scenario are divided into scenario A, which represents the GAC-based approach and scenario B, which represents the RO-based approach. These scenarios are shown in Fig. 1 and 2 respectively.

Fig. 1 Scenario A: reuse treatment for potable reuse using a GAC-based treatment approach.

Fig. 2 Scenario B: reuse treatment for potable reuse using a RO-based treatment approach.

Assumptions critical to the development of this scenario are included in Table 3. Both treatment trains (scenarios A and B) provide multiple barriers to organics and pathogens, which is important to all potable reuse projects.

Table 3 Critical assumptions for development of scenarios A and B

Item	Discussion
Water reuse plant is located at an inland location where ocean disposal is not readily available.	Disposal of RO concentrate has historically been much easier and less costly for facilities located along the coast because of the availability of ocean disposal. However, because of the increased difficulty in permitting new ocean disposals, the increased interest in potable reuse at inland locations where ocean disposal is not available, and the perception by many that RO technology must be used for potable reuse, it was assumed that ocean disposal would not be available in development of this alternative. Therefore, RO concentrate handling and disposal costs were included in this alternative.
Climate at plant location is semiarid or arid.	Semiarid and arid locations have evaporation rates that are high enough to allow consideration of using evaporation ponds for RO concentrate handling. This allows for alternative comparison to mechanically intensive RO concentrate handling technologies, such as brine concentrators and crystallizers.
WWTP practices nitrification and denitrification, which results in a total nitrogen concentration of less than 10 mg L^−1	More stringent WWTP nutrient discharge regulations being discussed and implemented in many states will likely result in significantly lower total nitrogen values in secondary effluent. For example, Florida's proposed Numeric Nutrient Criteria will likely require total nitrogen concentrations of less than 5 mg L^−1 in the effluent from many WWTPs. These lower total nitrogen concentrations will reduce treatment requirements at potable reuse plants because nitrogen removal will not be required to meet the nitrate MCL of 10 mg L^−1 that is often required at potable reuse plants. Many WWTPs located away from the coast include biological nitrogen removal and produce total nitrogen effluent below 10 mg L^−1.
WWTP secondary effluent TDS is less than 500 mg L^−1 or blending with other waters is provided	The GAC-based train (scenario A) does not remove TDS. Thus, to meet the U.S. Environmental Protection Agency's (EPA's) secondary maximum contaminant level (MCL) of 500 mg L^−1 for TDS, the WWTP secondary effluent TDS concentration must be less than 500 mg L^−1 or blending with other waters is required. For locations that have higher TDS levels and no blending is available, the GAC-based train could still be implemented based on public acceptance of a higher TDS concentration or partial RO treatment for TDS removal.
Regulatory limits for TOC that follow the East Coast's regulatory approach and EPA's 2012 Water Reuse Guidelines	TOC regulations can dictate the type of treatment process required. For example, in California a TOC concentration of less than 0.5 mg L^−1 must be achieved for groundwater recharge via direct injection (not surface spreading) unless the reuse water is blended with other supplies. From a practical standpoint, RO is the only treatment process that can meet this requirement. In contrast, regulations and permitted projects in other states are not this strict with respect to TOC, which allow other advanced organic removal processes such as GAC to be considered. For example, potable reuse projects in Virginia and Georgia are permitted with a COD limit of 10 mg L^−1 and 18 mg L^−1, respectively, which is approximately equivalent to a TOC concentration of 3–6 mg L^−1. Potable reuse regulations in Florida require a TOC of less than 3 mg L^−1. The 2012 EPA Water Reuse Guidelines suggest a TOC of less than 2 mg L^−1. The target finished water TOC for this scenario is nominally 3 mg L^−1.
Reuse plant influent is withdrawn from WWTP prior to chlorination to avoid formation of N-nitrosodimethylamine (NDMA)	NDMA has been shown to form during the chlorination process at WWTPs, especially when chloramines are used for disinfection. Withdrawal of secondary effluent prior to chlorination for reuse treatment allows for alternative treatment processes to be considered that don't include NDMA removal. For example, UVAOP provides excellent NDMA removal, but consumes large amounts of power. Alternative oxidation technologies that don't remove NDMA well, such as ozone, can provide a similar removal of other contaminants with potentially lower TBL costs. Note that ozone also has the potential to form some NDMA, but this will be well removed in the downstream BAC process. Ozone can also form bromate in some waters; in these cases ammonia addition may be needed to inhibit bromate formation.

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Triple bottom line approach: identifying the TBL factors

As described in the ‘What is Triple Bottom Line Accounting?’ section, this research uses a cost-benefit approach toward TBL accounting. Previous applications of cost-benefit analysis and TBL reporting involving the water reuse sector include numerous guidelines and studies.^8–15 The present paper builds upon previous work by demonstrating how to use the framework to support better decisions involving trade-offs among different treatment technologies. Each of the water reuse treatment trains is compared in terms of its economic costs and benefits to society and the environment, and not simply to those internal to the utility. The objective is to quantify the most significant factors in monetary terms to facilitate comparisons among alternatives on the basis of societal welfare. In addition to quantifying effects in dollars, it can be important to some stakeholders to readily compare alternatives on the basis of their environmental or social metrics, such as energy utilization, GHG emissions, and human health effects. Thus, these factors also are tracked in their “natural” units. However, to the extent that they also are quantified in dollars, these benefits and costs are only to be “counted” once.

This TBL approach incorporates principles from LCA to identify the questions enabling a more complete accounting of effects over the life cycle of the water reuse treatment process. Specifically, this analysis requires asking the following questions:

1. What are the social costs and benefits incurred at the treatment plant itself? Addressing this question includes capturing the direct costs of treatment, as well as external costs owing to the treatment process (e.g., ecological footprint of the treatment facility, capital and operation and maintenance [O&M] costs of treatment, utilization of energy, and chemicals to “produce” reclaimed water). These effects originating at the plant are called “direct effects” as they emanate directly from the treatment phase of the process.

2. What are the net social costs and benefits caused by producing and transporting inputs to the water reuse treatment process? This question indicates that one must look “upstream” of the water reuse treatment facility to capture external environmental impacts created prior to any utilization of the inputs in the water reuse treatment process (e.g., energy must first be produced at a power plant thereby creating GHG emissions and other emissions of air pollutants harmful to human health). For our purposes, these are called “upstream effects.”

3. What are the net social costs and benefits to the end users “downstream” of the treatment processwhere the “end users” can be households, businesses, industry or the environment (e.g., disposal or utilization of brine waste from the reuse water treatment plant, effects of the nutrients in reuse water on agricultural or landscape irrigation end users; discharge of reuse water to surface waters or percolation into groundwater)? Effects that occur post-treatment are appropriately called “downstream effects.”

In identifying the social costs and benefits of each reuse treatment train, the alternatives are compared with each other and not to other water supply alternatives, such as desalination or reservoir expansions. That is, the decision to employ a reuse alternative is taken as a given for the purposes of this analysis. Thus, any social benefits and costs that are common to all reuse alternatives are not considered here, because they would not be of value in differentiating among reuse treatment trains. However, TBL assessment of reuse water in relation to other water supply alternatives, such as reservoir creation, water conveyance, and desalination is an important topic in its own right and has been addressed by Stratus Consulting and others.^6,10

These TBL questions are illustrated in Fig. 3 TBL Factors to Consider in Selecting a Water Reuse Treatment Process. Note that the “direct TBL effects” and the “upstream TBL effects” occur on almost all water reuse projects in differing degrees, but the “downstream effects” are project specific and the applicability of each must be determined for each project analyzed.

Fig. 3 TBL factors to consider in selecting a water reuse treatment process.

The TBL endpoints associated with each of the reuse treatment trains included in the present analysis were identified by answering these questions with the aid of a literature review of other TBL applications.

Utility survey

A significant part of the TBL analysis is estimating the financial costs, both capital and operating, of a reuse water treatment process. Capital costs and annual operation and maintenance (O&M) costs are dependent on many site-specific factors, such as the type of treatment processes provided, design and operational criteria for those treatment processes, raw water quality, treated water quality requirements, and local market conditions related to power, chemical and labor costs. The cost estimating tool used in this research, CH2M HILL Parametric Cost Estimating System (CPES), is designed to accommodate variations in these factors and has an extensive unit cost database to provide accurate cost estimates, but a utility survey was conducted with project participants to allow comparison of cost estimates to cost data from full-scale potable reuse plants. Cost estimates also will be compared to cost information collected from the literature, which was presented earlier in this report.

The utility survey included 188 questions related to the utility's operational water reuse plant. The primary focus of the survey was to collect enough data from each water reuse plant to allow analysis and fair comparison of reported costs to those generated from the CPES cost estimating tool and costs collected from other water reuse plants. Much of the data collected provided specific information on plant design and operation that can significantly affect costs. The CPES cost model was calibrated to the cost information collected.

Triple bottom line costs

This analysis includes comparison of the GAC-based potable reuse approach (scenario A) to the RO-based potable reuse approach (scenario B). Scenario B includes three concentrate handling approaches: ocean disposal, mechanical evaporation, and evaporation ponds. A hybrid approach combining the use of brine concentration and evaporation ponds is discussed at the end of the section.

Capital and annual operating costs for all treatment trains analyzed are shown in Fig. 4 and 5 as a function of flow rate. Annual operating costs are based on an average flow factor of 0.6; thus, for the 70 MGD (265 MLD) plant, operating costs are based on an annual average flow of 42 MGD (159 MLD). Inspection of Fig. 4 and 5 reveals the following about the capital and operating costs:

Fig. 4 Capital costs in US dollars.

Fig. 5 Annual operating costs in US dollars.

• Lowest cost: the capital and annual operating costs for scenario A (GAC-based) are the lowest for all flows analyzed and the savings increase with increasing flow rate.

• GAC-based versus RO-based costs: scenarios A and B have similar capital and annual operating costs at low flow, but costs are significantly different at higher flows:

∘ At a flow rate of 5 MGD (19 MLD) the capital costs for scenarios A and B are $50 million and $52 million, respectively. However, the difference grows significantly at higher flows. Capital costs for scenario B are about 30% higher at a flow of 20 MGD (78 MLD) and 70% higher at a flow of 70 MGD (265 MLD). This increasing difference at higher flows is because of the better economies of scale for scenario A at higher flows because of its larger percentage of concrete construction (e.g., ozone contactor, BAC filters, GAC adsorbers). Scenario B includes more mechanically intensive equipment (MF, RO, UVAOP) that does not realize as significant economies of scale.

∘ At a flow rate of 5 MGD (19 MLD) the annual operating costs for scenarios A and B are $1.9 million and $2.4 million, respectively. However, the difference grows significantly at higher flows. Annual operating costs for scenario B are about 40% higher for the 20 MGD (78 MLD) case and 50% higher for the 70 MGD (265 MLD) case when compared to the 2 year GAC replacement frequency case for scenario A. The cost difference between these scenarios is even greater if GAC is replaced on an 8 year frequency. The increasing difference at higher flows is because of higher power consumption and larger replacement costs associated with major process equipment associated with scenario B (i.e., MF, RO, UVAOP).

• Concentrate handling costs for scenario B: where sewer or ocean disposal of concentrate is not available the need for concentrate management increases scenario B capital and annual operating costs significantly:

∘ At 5 MGD (19 MLD), the total plant capital cost using mechanical evaporation is approximately 30% more than ocean disposal ($67 million versus $52 million). Evaporation ponds are 75% higher than ocean disposal. If land has to be purchased for construction of the evaporation ponds, capital costs would increase further. At 20 MGD (78 MLD), the total plant capital costs for mechanical evaporation and evaporation ponds are 40% and 150% higher respectively, than for ocean disposal.

∘ At 5 MGD (19 MLD), the total annual operating cost using the mechanical evaporation approach is approximately 60% more than the ocean disposal approach ($3.9 million versus $2.4 million). Evaporation ponds are approximately 60% higher than ocean disposal. At 20 MGD (78 MLD), the total operating costs for mechanical evaporation and evaporation ponds are 85% and 50% higher, respectively, than ocean disposal.

∘ Where sewer or ocean disposal is not available, the capital and annual operating costs for concentrate handling is extremely high, which may limit the use of RO technology at inland locations. Reducing the volume of RO concentrate through new volume reduction technologies should be considered to reduce costs; however, although numerous technologies are currently being researched, none at present have been proven at full-scale to substantially reduce concentrate handling costs to the extent that would allow implementation of RO-based plants at inland locations without significant additional costs.

The environmental costs associated with GHG emissions and other air emissions are shown in Fig. 6 and 7, respectively. Inspection of these figures reveals the following about the monetized environmental costs:

Fig. 6 Annual greenhouse gas (GHG) costs in US dollars.

Fig. 7 Other air emissions annual costs in US dollars.

• Lowest cost: the monetized costs for scenario A (GAC-based) are the lowest for all flows analyzed and the savings increase with increasing flow rate.

∘ At a flow rate of 5 MGD (19 MLD) the annual GHG costs for scenarios A and B-ocean disposal are $40 000 and $130 000, respectively. The annual costs for other air emissions for these scenarios are $150 000 and $500 000, respectively.

∘ Scenario B-ocean disposal environmental costs increase significantly with flow because of the large amount of power consumption associated with this scenario. For example, at 70 MGD (265 MLD), the annual GHG and other air emissions costs for scenario B are $1.2 million and $4.2 million, respectively.

• Concentrate handling costs: where ocean disposal of concentrate is not available for scenario B, implementation of concentrate management increases scenario B environmental costs significantly:

∘ The environmental costs for the mechanical evaporation approach are much more significant because of the increased power use in this treatment scenario. At 5 MGD (19 MLD), the annual GHG and other air emissions costs are $370 000 and $1.6 million, respectively.

• Comparison between GHGs and other air emissions costs:

∘ Other air emissions costs are more significant than CO₂ emissions costs because the monetized health effects of PM_2.5 and PM_2.5 precursors (SO₂ and NO_x) are much higher than the monetized health effects of CO₂. For example, the monetized health effects of PM_2.5 and SO₂ from electricity generation is $136 877 per ton and $36 852 per ton, respectively, versus $26.91 per ton for CO₂ equivalents. Higher quantities of CO₂ equivalents are released during electricity generation, but not enough to offset the higher monetized health effects of PM_2.5 and its precursors.

∘ GHG costs are dominated by electricity production, which accounts for 70 to 90% of all environmental costs associated with each treatment train. GHG costs associated with chemical production are significant, ranging from 6 to 27%. GHG costs associated with trucking are low because of small quantity of CO₂ emissions from this source.

∘ Other air emissions costs are dominated by the release of SO₂ and NO_x from energy production, which represent approximately 60% and 25%, respectively, of all other air emission costs. Other air emissions from trucking of chemicals and residuals range from 3 to 17% of the total.

Net present value comparisons

This section provides the results from applying the TBL accounting methodology to each of the treatment scenarios. For all scenarios, the NPV calculations assume that the facility is designed in 2012 and constructed between 2013 and 2015, with operation, maintenance and replacement costs distributed evenly over the life of the facility. Each facility is assumed to have a 30 year life (2016 to 2045) with annual operation and maintenance costs and replacement of worn equipment. Following Office of Management and Budget (OMB) Guidance, the NPV of the treatment trains is calculated at a 3% and 7% discount rate, except where otherwise noted.¹⁶ Factors that could not be quantified in monetary terms are described qualitatively.

The NPV results for the potable reuse scenarios are reported in Table 4. Scenario A, the GAC-based treatment train has the lowest NPV costs in each category no matter what size facility. TBL costs for scenario A range from $86 million for a 5 MGD (19 MLD) facility to $466 million for a 70 MGD (265 MLD) facility. The environmental costs range from $4.9 million to $37 million and account for about 6 to 8% of the TBL costs, respectively. The next least cost option is scenario B1, the RO with ocean disposal treatment train. Looking only at the NPV capital and O&M costs, ignoring environmental costs for the moment, suggests that this scenario is somewhat comparable to scenario A at the 5 MGD (19 MLD) scale. Choosing the RO treatment train when ocean disposal is an option adds about $11.2 million (14%) to the capital and O&M cost of a GAC-based treatment train. This percentage increases rapidly with facility scale. For a 20 MGD (78 MLD) facility, the cost increases by $59 million (36%) and for a 70 MGD (265 MLD) facility, the incremental cost is $261 million, reflecting a 60% increase in capital and O&M costs over the GAC-based facility. However, once the environmental costs are taken into consideration, the gap between scenario A and scenario B with ocean disposal is much more significant, even for the 5 MGD (19 MLD) facility. The incremental TBL cost differential between treatment trains increases to $24 million (28%). This TBL differential increases in significance with the scale of the facility. At 20 MGD (78 MLD), the TBL cost differential is $93 million (54%), and at 70 MGD (265 MLD) the RO based facility with ocean disposal costs $381 million (82%) more than the GAC-based treatment train.

Table 4 NPV results ($2012 US dollars; 3% discount rate)

Flow	Treatment trains
	A (GAC-based)	B1 (RO-based w/ocean disposal)	B2 (RO-based with mechanical evaporation)	B3 (RO-based with evaporation ponds)
Financial NPV (capital and O&M costs)
5 MGD (19 MLD)	$80 960 000	$92 170 000	$133 430 000	$145 240 000
20 MGD (78 MLD)	$161 140 000	$219 925 000	$358 190 000	$449 570 000
70 MGD (265 MLD)	$428 970 000	$690 610 000	$1 128 780 000	$1 556 730 000
Environmental NPV (monetized GHGs and other air emissions)
5 MGD (19 MLD)	$4 900 000	$17 790 000	$54 080 000	$24 000 000
20 MGD (78 MLD)	$11 990 000	$46 780 000	$175 000 000	$62 300 000
70 MGD (265 MLD)	$36 830 000	$156 100 000	$566 480 000	$175 480 000
Total NPV
5 MGD (19 MLD)	$85 860 000	$109 960 000	$187 510 000	$169 240 000
20 MGD (78 MLD)	$173 130 000	$266 705 000	$533 190 000	$511 870 000
70 MGD (265 MLD)	$465 800 000	$846 710 000	$1 695 260 000	$1 732 210 000

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The NPV differences between the GAC-based treatment train and the RO-based treatment trains requiring alternative methods of brine disposal, such as would be necessary at inland locations, are quite dramatic at every scale. Choosing scenario B RO-based with mechanical evaporation over the GAC-based scenario A would increase the cost by a factor of 2.2 to 3.6 times, for a 5 MGD (19 MLD) to 70 MGD (265 MLD) facility, respectively. Scenario B RO-based with evaporative ponds is similar; TBL costs for a 5 MGD (19 MLD) and 70 MGD (265 MLD) facility are 2.0 times and 3.7 times higher, respectively, than the scenario A GAC-based approach.

In most locations throughout the country, the quality of the reuse water is such that none of the treatment trains require additional management measures on the part of the end user. However, there are circumstances where the level of TDS in the source water can be problematic for scenario A, the GAC-based treatment train, which unlike the RO-based treatment trains does not remove TDS during treatment. In regions of the country where the source water has a relatively high salinity content (e.g., Colorado River, some groundwater resources) and where blending with lower TDS water is not an option, this technology may result in water that exceeds the 500 mg level for TDS, which could lead to taste issues that some end users may choose to mitigate. At even higher TDS levels, the reuse water would not be acceptable, so other treatment technologies that remove TDS would be necessary. This can occur in closed groundwater systems, for example. Thus TDS is an important consideration in selecting the preferred treatment technology. For the locations where TDS is not a concern, this TBL analysis has shown that there are considerable cost savings from selecting a GAC-based approach over an RO-based approach at facilities of about 5 MGD (19 MLD) and higher, but RO with ocean disposal appears to be a reasonable alternative for small facilities 5 MGD (19 MLD) or less.

The final qualitative factor relates to human health risks. In general, potable reuse systems are protective of public health based on all drinking water standards and public health criteria. Each of the treatment trains discussed here are also comparable and protective of public health. Each treatment process differs in effectiveness at removing various constituents of emerging concern; whereas some constituents may now be detectable with advances in technology, the concentrations are mathematically small. Any differences in risk perceptions across treatment trains are not based on any differences in known risks. However, to the extent that public risk perceptions are not in line with known risks, it can be important to communicate effectively with the public about human risks, potable drinking water safety, and TBL costs of treatment to support sound decisions.

Conclusions

This research was conducted with the expectation that the need for developing new sources of affordable water supply will grow significantly in the near future in both arid and less arid climates. The focus of this research was to evaluate alternative potable reuse treatment trains applying a cost-benefit analysis and LCA approach toward TBL accounting to better inform this decision, specifically to avoid cases of overtreatment. In the context of this research, overtreatment is defined by spending more than is necessary or causing adverse environmental impacts and social effects without providing counterbalancing benefits. To ensure a fair comparison, the treatment technologies were selected with the aim of providing comparable water quality. Any differences in water quality that remained were discussed in terms of the benefits associated with selecting one treatment technology over another and weighed against the differential in the costs of treatment. To this end, a potable reuse for reservoir augmentation scenario was selected. In addition, this scenario was analyzed at three flows: 5 MGD (19 MLD), 20 MGD (78 MLD), and 70 MGD (265 MLD) to show how TBL costs vary with flow and to determine whether conclusions about the lowest cost treatment train varies with flow. Considerably more treatment trains are available to those utilities considering water reuse than the trains analyzed in this research; however, the trains identified represent reasonable prototypes for options that are widely applicable and serve to illustrate the BCA TBL evaluation framework.

Table 5 summarizes the major financial and environmental costs and associated considerations when evaluating alternative treatment trains for the 20 MGD (78 MLD) plant capacity. All these factors, either directly or indirectly, affect the total net present value (NPV) of each treatment train option. For example, chemical consumption directly affects financial costs through the annual purchase of chemicals and indirectly affects environmental costs through the air emissions related to the production and delivery of the chemicals.

Table 5 Major financial and environmental costs and associated considerations for 20 MGD (78 MLD) plant capacity

Scenario	Capital cost	Annual O&M cost	Annual environ cost^a	Total TBL NPV	Power consumption (MWH per year)	Chemical consumption (dry tons per year)	Air emissions (tons per year)
							CO2e	Other
a These annual environmental costs reflect the first year of operation only. The environmental costs increase over time because of such factors as population growth, growth in real income and increasing concentrations of pollutants.
A: (Coag-Sed-O3-BAC-GAC-UV)	$91M	$4.2M	$0.4M	$173M	4400	1770	2900	11
B: (MF-RO-UVAOP) w/ocean disposal	$120M	$5.9M	$1.6M	$267M	16 000	1860	13 400	30
B: (MF-RO-UVAOP) w/mech evap	$172M	$10.9M	$6.3M	$533M	65 400	3020	44 200	150
B: (MF-RO-UVAOP) w/evap ponds	$303M	$9.0M	$2.2M	$512M	22 000	1860	17 200	49

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A comparison of the GAC-based potable reuse approach (scenario A) to the RO-based potable reuse approach (scenarios B) was conducted. Scenario B includes three concentrate handling approaches: ocean or sewer disposal, mechanical evaporation, and evaporation ponds. The major conclusions from comparing the treatment trains for the potable reuse scenario are as follows:

• The NPV of all TBL costs combined for scenario A GAC-based treatment are the lowest of the four scenarios for all flows analyzed. For example, at the 3% discount rate, the total TBL NPV costs come to $173 million for the 20 MGD (78 MLD) plant. This compares with $267 million for scenario B (RO with ocean disposal); $533 million for scenario B (RO with mechanical evaporation); and $512 million for scenario B (RO with evaporative pond).

• Considering just the life-cycle capital and O&M cost to the utility and ignoring the environmental costs, scenario B with ocean disposal is most competitive with scenario A at low flows. For the 5 MGD (19 MLD) facility, these NPV costs differ by $11.2 million (14%). Therefore, where sewer or ocean disposal of RO concentrate is readily available for plant capacities of 5 MGD (19 MLD) and less, an RO-based treatment train might result in competitive costs. However, for larger plant capacities and when environmental costs are considered, the TBL costs for the RO-based approach increase considerably.

• Because of such dramatic differences in TBL NPV costs, especially for large plant capacities and where sewer or ocean disposal of RO concentrate is not available, one needs compelling benefits to justify RO over GAC-based treatment from a TBL perspective. That benefit is the very low salinity content of RO treated water in locations where the source water has excessively high TDS levels and blending with other less saline water sources is not possible. In these situations, TBL costs for the RO-based approach could possibly be reduced by implementing sidestream RO treatment.

• In situations where TDS removal is required at locations where concentrate disposal via the sewer or ocean is not available (e.g., some inland locations), concentrate management TBL costs can be extremely high and possibly cost prohibitive. For example, a mechanical evaporation approach to concentrate management adds $52 million and $5 million per year in capital and annual O&M costs, respectively, to the ocean disposal approach for a 20 MGD (78 MLD) plant.

• The RO-based train (scenario B) will produce water with a significantly lower TOC concentration than the GAC-based train (scenario A). Alternative treatment approaches using nanofiltration (NF) in lieu of RO or sidestream RO treatment are being considered that will result in higher TOC and lower TBL costs.

• The environmental costs for each scenario primarily caused by power requirements. The major power using and GHG emitting countries of the world have committed to reducing GHG emissions. Except in situations limited by excess TDS in source water, utilities have a clear opportunity to minimize GHG emissions while not sacrificing reuse water quality by choosing scenario A over scenario B. This choice also has the benefit of reducing other air emissions that are harmful to human health.

• Human health risks are not a differentiator among the four scenarios; however, perceptions of human health risks may vary across treatments. This is a risk communication issue of high importance lest uniformed risk perceptions lead to excessive overpayment for reuse water and significant adverse environmental and human health effects without achieving measurable reductions in risk.

• This research was intended to develop the TBL approach as it pertains to selecting water reuse treatment and illustrate the methodology with carefully selected treatments. The analysis of treatment did not exhaust all alternatives. For example, one alternative treatment process, soil aquifer treatment (SAT) for potable reuse, was not included in this research, although it is expected to have relatively low TBL costs. It may well be the preferred TBL alternative in certain locations. For example, the Montebello Forebay groundwater recharge project in Southern California has been successfully recharging a potable aquifer for more than 40 years using soil aquifer treatment via spreading basins.

Acknowledgements

This project was funded by the WateReuse Research Foundation through project WRRF-10-01. More information related to this project can be found in the project's final report, which is available at www.watereuse.org.

Notes and references

1. L. Schimmoller and M. Kealy, Fit for Purpose Water: the Cost of Overtreating Reclaimed Water, WateReuse Research Foundation, 2014.
2. California Office of Administrative Law, California Code of Regulations. Title 22 (Social Security), Division 7 (Water Quality), Chapter 7 (Water Reuse), January 2009, Section 13562.
3. National Research Council (NRC), Water Reuse: Potential for Expanding the Nation's Water Supply through Reuse of Municipal Wastewater, National Academies Press, 2012.
4. U.S. Environmental Protection Agency (EPA), Guidelines for Water Reuse, 2012, EPA/600/R-12/618.
5. S. Kenway, C. Howe and S. Maheepala, Triple Bottom Line Reporting of Sustainable Water Utility Performance, AWWA Research Foundation and CSIRO Land and Water, Australia, ACT, 2007.
6. J. E. Drewes and S. Khan, Water Reuse for Drinking Water Augmentation. In Water Quality and Treatment, American Water Works Association, 2010, vol. 16, pp. 1–14.
7. California Department of Public Health (CDPH), Groundwater Replenishment Reuse DRAFT Regulation, March 2013, Section 60320.200.
8. T. Cristiano and J. Henderson, Evaluating Pricing Levels and Structures to Support Reclaimed Water Systems, Final Report, WateReuse Research Foundation, 2009.
9. U.S. Environmental Protection Agency (EPA), Guidelines for Water Reuse, September 2004, EPA/625/R-04/108.
10. R. Raucher, An Economic Framework for Evaluating the Costs and Benefits of Water Reuse, WateReuse Foundation, 2006.
11. F. Hernández, A. Urkiaga, L. De las Fuentes, B. Bis, E. Chiru, B. Balazs and T. Wintgens, Feasibility Studies for Water Reuse Projects: An Economical Approach, Desalination, 2006, 187, 253–261.
12. C. Kfouri, Water Reuse Cost-Benefit Analysis: The Morocco Example, Presented at Water Week, 2009.
13. Stratus Consulting. El Paso Triple Bottom Line: Desalination and Reuse Water, October 2011, El Paso Water Utilities.
14. F. Hernández-Sancho, M. Molinos-Senante and R. Sala-Garrido, Cost-Benefit Analysis of Water Reuse Importance of Economic Valuation of Environmental Benefits, September 26–29, 2011, International Water Association (IWA) 8th International Conference on Water Reclamation and Reuse, Barcelona, Spain, 2011.
15. S. De Souza, J. Medellin-Azuara, N. Burlye, J. Lund and R. Howitt, Guidelines for Preparing Economic Analysis for Water Recycling Projects, April 2011, State Water Resources Control Board, 2011.
16. Office of Management and Budget (OMB). Circular A-4, September 17, 2003.

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