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7 Evaluating the Risks of Potable Reuse in Context In this chapter, the committee summarizes the ily because the Corps had not included all the tiers of findings of previous National Research Council (NRC) toxicological testing recommended (NRC, 1984). committees as they examined the question of the In the years that followed, more extensive toxi- safety of water reuse. Building on the risk assessment cological tests were conducted for other proposed methodologies presented in Chapter 6, the committee potable reuse projects, particularly in Tampa (CH2M then presents a comparative analysis of potential health Hill, 1993) and Denver (Lauer and Rogers, 1996). In risks of potable reuse in the context of the risks of the reviewing those data (see Box 6-2), which did not dem- common contemporary circumstance of a conventional onstrate any adverse health effects, a third NRC com- drinking water supply derived from a surface water that mittee concluded that such tests provide a database too receives a small percentage of treated wastewater (see limited to draw general conclusions about the safety of Chapter 2). By means of this analysis, the committee potable reuse (NRC, 1998). NRC (1998) also pointed compares the estimated risks of a drinking water source out new concerns that had arisen since the earlier NRC generally perceived as safe (i.e., de facto potable reuse) reports and outlined new testing techniques that had against the estimated risks of two other potable reuse been developed. The committee also recommended scenarios. t hat new toxicity tests be conducted, particularly long-term fish exposure testing, which were partially implemented in subsequent evaluations in Orange PREVIOUS NRC ASSESSMENTS County and Singapore. Advice on testing methods OF REUSE RISKS will continue to evolve as science advances, but based The 1982 Committee on Quality Criteria for on the progression of this research, it is evident that, Water Reuse, citing the many unknowns in making an although such testing might be used to show evidence assessment of the health effects of potable reuse, ad- of potential health risk, it cannot be used to establish opted the view that “the quality of reused water could be the absence of risk. compared to that of conventional drinking water sup- Examining this history from the vantage point plies, which are assumed to be safe” (NRC, 1982). That of 2011, the most profound contribution of the 1982 committee was providing advice for an extensive testing committee was the idea that the quality of the water program being undertaken by the U.S. Army Corps of in potable reuse scenarios should be compared with Engineers on the treatment of an effluent-dominated the quality of conventional drinking waters, which are Potomac River as part of an evaluation of the future assumed safe. Advice on specific tests that might be water supply for Washington, D.C., and it outlined useful in making these comparisons will continue to specific testing procedures for evaluating the treated follow developments in the underlying science, but it water based on the state of science in 1982. A second is unlikely that any laboratory test will ever establish NRC committee reviewed the results of the Corps’ the absence of health risk in a drinking water from testing program and found them inconclusive, primar- any source. This chapter builds on that foundation, 123

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124 WATER REUSE comparing the estimated health risk of water from potable reuse projects with the risk associated with potable reuse projects with conventional supplies using de facto reuse scenarios that are representative of the established tools of risk assessment. supplies that are widely experienced today. The com- The 1982 committee went on to say that the com- mittee chose to construct a “risk exemplar” to examine parison should be made with the highest quality water how these comparisons might be made. The analysis that can be obtained from that locality even if that in this exemplar uses the quantitative risk assessment source may not be in use. In a similar vein, the 1998 methods originally proposed for organic chemicals by committee, after concluding that planned potable reuse the NRC (1983) as expanded for microbial contami- is viable, suggested that planned potable reuse should nants (Haas et al., 1999) and more recently updated be, “an option of last resort—to be adopted only if all (NRC, 2009b). Other methods recently developed to the alternatives are technically or economically infea- address pharmaceuticals, personal care products, and sible” (NRC, 1998). All three committees (NRC, 1982, other anthropogenic contaminants (Rodriquez et al., 1984, and 1998) also took the view that U.S. drinking 2007a,b; Snyder et al., 2008a; Bull et al., 2011) are water regulations were not intended to protect public also used in the analysis to address the risk of classes health when raw water supplies were heavily contami- of contaminants for which rigorous toxicological data nated with municipal and industrial wastewater. are lacking. In the committee’s judgment, these risk In the committee’s judgment, current circum - assessment techniques represent the best means avail- stances call for a reassessment of those views. First, as able at this time for estimating the relative risk in such shown in Chapter 1, the United States has been operat- circumstances (see Chapter 6) and offer a method for ing near the limit of its water supply for several decades evaluating the relative merits of various options for since about the time of the first study. As a result of managing health risks from chemical and microbial further stress from continued population growth and contaminants in reclaimed water. climate change, this report is being written with a view The committee chose to develop an exemplary to providing useful advice to the nation as it comes to comparison of risks associated with various potable terms with this new world where pristine water is ever reuse scenarios, including de facto potable reuse, mod- less abundant, even as the domestic wastewater from eled upon circumstances currently encountered in the an increasing population is discharged into the nation’s United States today. Based on the discussion in Chapter waterways. Second, as demonstrated in Chapter 2, the 2, the committee concluded that it would be appropri- committee concludes that de facto reuse (i.e., when ate to compare the quality of the water in potable reuse a drinking water source consists of some significant scenarios with the quality of a de facto reuse scenario percentage of treated wastewater effluent from an up- where a conventional water supply has an average an- stream discharger) is becoming increasingly common nual wastewater content of 5 percent. This situation is in the United States. Finally, it has become evident commonly found among current surface water supplies to the committee that, in many communities, today’s (see Box 2-4). As shown in the figure in Box 2-4, there drinking water regulations are already being employed are many circumstances where de facto reuse exceeds to address the quality of drinking water prepared from 5 percent, and the committee discussed at length the water supplies that have substantial wastewater content appropriate wastewater content for use in the exem- (see also Chapter 10 for a discussion of regulations). plar. In the end, 5 percent was selected as a wastewater Although this fact does not imply that the regulations content that can be reasonably viewed as commonplace are adequate for that charge, it does reflect a notable and not exaggerated. Swayne et al. (1980) reported that shift in perspectives since the prior NRC reuse reports more than 24 million people of the 76 million people were written. surveyed were using drinking water supplies with a wastewater content of 5 percent or more in low-flow conditions (see figure in Box 2-4). Although no data ex- THE RISK EXEMPLAR ist, anecdotal evidence based on the population growth Under these conditions, the committee judges that in urban areas suggests that wastewater content is often it is appropriate to compare the risk associated with higher today.

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125 EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT The comparative risk approach used in this analy- percent contribution of disinfected wastewater efflu- sis was designed to examine the presence of selected ent upstream of the intake for the drinking water plant pathogens and trace organic chemicals in final product (Figure 7-1a). The two reuse cases describing drinking waters from de facto reuse and two potable reuse sce- water supplies derived from planned potable reuse proj- narios. Contaminant occurrence data, compiled from ects include one with groundwater recharge to a potable several sources, were critically evaluated for each sce- aquifer via surface spreading basins with subsequent nario. The data were then analyzed to assess whether soil aquifer treatment (SAT; Scenario 2, Figure 7-1b) there are likely to be significantly greater human health and one with groundwater recharge to a potable aquifer concerns from exposure to contaminants in these hypo- by direct injection of reclaimed water that has received thetical reuse scenarios, compared with a common de advanced water treatment (Scenario 3, Figure 7-1c). facto reuse scenario. For the chemicals in each of the scenarios, a risk-based action level was used, such as Scenario 1: De Facto Reuse (Common Surface Water the U.S. Environmental Protection Agency’s maximum Supply) contaminant levels (MCLs), Australian drinking water guidelines, or World Health Organization drinking In Scenario 1, a surface water supply that serves as water guideline values. Also, a margin of safety is re- a drinking water source receives discharge from second- ported, defined as the ratio between a risk-based action ary treated wastewater effluent that is disinfected and level (such as an MCL) and the actual concentration dechlorinated prior to discharge to meet a standard of of a chemical in reclaimed water. The resulting ratio 200 fecal coliform/100 mL. The surface water is as- between these two values (i.e., the margin of safety) can sumed to be free of pathogens with no measurable trace be used to characterize potential health risks associated organic chemicals prior to effluent discharge. with exposure to a chemical (Illing, 2006). For mi- Attenuation of contaminants after wastewater crobials, the dose-response relationships were used to discharge can vary widely as a function of distance compute risk from a single day of exposure. Additional between discharge point and raw drinking water with- underlying assumptions are described below. drawal (i.e., retention time), streamflow geometry (i.e., It was beyond the means of the committee to depth, mixing), and environmental conditions such as conduct an analysis of every possible contaminant in temperature, ultraviolet light penetration, particulate reclaimed water. In addition, certain assumptions were matter, biological activity. In this scenario, a worst made to simplify the analysis. The committee focused case is assumed where no inactivation or attenuation on four pathogens commonly of concern in reuse appli- of pathogens or chemicals occurs in the surface water cations and selected 24 chemicals representing different body. The wastewater discharged constitutes 5 percent classes of contaminants (i.e., nitrosamines, disinfection of the flow in the source at the point where water is byproducts (DBPs), hormones, pharmaceuticals, anti- abstracted for the drinking water treatment plant. microbials, flame retardants, and perfluorochemicals), Subsequently, this water is extracted by a conventional for which occurrence and toxicological data were avail- drinking water plant employing coagulation/floccula- able in the published literature. tion, followed by granular media filtration with disin- fection designed to meet the requirements of the Long Term-2 Surface Water Treatment Rule (EPA, 2006a). Potable Reuse Scenarios Considered in the Exemplar Scenario 2: Soil Aquifer Treatment Three hypothetical scenarios were evaluated to compare the relative risk from exposure to pathogens In Scenario 2, a potable aquifer is augmented with and trace organic chemicals in the conventional water reclaimed water via groundwater recharge by surface supply and potable reuse scenarios. Scenario 1, the spreading. Advanced wastewater treatment in this conventional water supply scenario, considers de facto exemplar assumes secondary treatment, followed by reuse with a conventional drinking water treatment nitrification, partial denitrification and granular media plant drawing water from a supply that receives a 5 filtration but no disinfection. The reclaimed water is

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126 WATER REUSE FIGURE 7-1 Summary of scenarios examined in the risk exemplar. (a) Scenario 1—A conventional water plant drawing from a source that is 5 percent treated wastewater in origin; (b) Scenario 2—A deep well in an aquifer fed by reclaimed water via a soil aquifer treatment system and (c) Scenario 3—A deep well drawing from an aquifer fed by injection of reclaimed water from an advanced water treatment (AWT) plant. R02129 Figure 7-1

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127 EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT applied to surface spreading basins with subsequent mg-min/L requirement in California’s Title 22 regu- SAT (see Chapter 4). It is assumed that the water lations was considered, but was not included because remains in the subsurface for 6 months with no dilu- sufficient data could not be obtained to estimate the tion from native groundwater. The assumption of no impact of disinfection on contaminant concentrations. dilution is a worst case, being more conservative than From a qualitative perspective, this scenario would re- the real-world hydrogeological characteristics of typi- sult in significantly lower levels of microbial exposure, cal subsurface systems. This condition was selected to particularly for Salmonella, but higher levels of DBPs assign removal credits only to physicochemical and would be present—trihalomethanes and haloacetic ac- biological attenuation processes occurring during SAT. ids if free chlorine is used or N-nitrosodimethylamine Subsequently, the water is abstracted and disinfected (NDMA) if combined chlorine is used. A review of the with chlorine at the wellhead prior to consumption, literature shows that these DBPs are typically removed assuming that no blending with other source waters during the SAT, particularly NDMA (Kaplan and occurs in the distribution system. This assumption de- Kaplan, 1985; Yang et al., 2005; LACSD, 2008; Zhou scribes a scenario where all the drinking water that is et al., 2009; Nalinakumari et al., 2010; Patterson et al., consumed originates from the reclaimed water source. 2011), but vigilance is called for when disinfected efflu- This assumption is conservative, given that most exist- ent is used for SAT because as shown in the following ing potable reuse projects blend their product water section, the margin of safety is smaller with DBPs than with other sources, providing additional dilution. with most other trace organic chemicals. Further details for Scenarios 1 through 3 are pro- vided in Appendix A with respect to water quality char- Scenario 3: Microfiltration, Reverse Osmosis, Advanced acteristics, attenuation, and generation of contaminants Oxidation, Groundwater Recharge during various treatment steps. In Scenario 3, a potable aquifer is augmented with advanced-treated wastewater followed by groundwater Contaminants Considered in the Risk Exemplar recharge by direct injection. The advanced treatment train includes secondary treatment with chloramina- The committee considered a broad cross section tion, followed by microfiltration, reverse osmosis, and of common pathogenic bacteria, viruses, and protozoa, advanced oxidation using UV irradiation in combina- as well as regulated and nonregulated trace organic tion with hydrogen peroxide (UV/H2O2). It is assumed chemicals that have been reported in reclaimed water that the water remains in the subsurface for 6 months or surface water receiving wastewater discharge, to with no dilution from native groundwater. Again, this determine the contaminants to be considered in the scenario assumes that any attenuation of pathogens and risk exemplar. The contaminants that were ultimately trace organic chemicals in the aquifer is achieved only selected met the following criteria: (1) sufficient infor- by physicochemical and biological processes rather than mation was available on their occurrence, health effects, dilution. Subsequently, the groundwater is abstracted fate and transport, and behavior in treatment systems and disinfected at the wellhead with chlorine prior such that reasonable calculations could be made for to consumption. Again, this case describes a scenario each scenario; and (2) they are either recognized to be where 100 percent of the drinking water consumed of concern based on possible health effects or they are originates from reclaimed water after advanced water of interest to the public. treatment and direct injection into a potable aquifer. Four pathogens were selected: adenovirus, norovi- This assumption is also conservative, given that most rus, Salmonella, and Cryptosporidium. All of these or- existing potable reuse projects blend their product ganisms are transmitted by the fecal-to-oral route, and water with other sources, providing additional dilution. they all play an important role in waterborne illness in the United States. All four pathogens have been stud- ied in effluents, and, for each, dose-response data are Other Scenarios Considered available. Salmonella is a classic bacterial pathogen as- The construction of an additional scenario, similar sociated with both food- and waterborne disease, while to Scenario 2, but with disinfection similar to the 450 the significance of the other three pathogens has only

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128 WATER REUSE Assumptions Concerning Fate, Transport, become clear in recent decades. Toxigenic Escherichia Removal, and Estimates of Risk coli was originally considered as well, but sufficient dose-response data were not available. The assumptions in the exemplar concerning the Potential adverse health effects associated with fate, transport, and removal of the pathogens and trace organic chemicals in drinking water are an im- chemicals considered in each scenario of the exemplar portant concern among stakeholders and the public. are discussed in detail in Appendix A. Literature ref- As noted in Chapter 3, reclaimed water can contain erences are provided for the sources of the data that chemicals originating from consumer products (e.g., make up each scenario, including expected densities or personal care products, pharmaceuticals), human waste concentrations following the various treatment steps (e.g., natural hormones), and industrial and commercial (including both engineered treatment systems and discharges (e.g., solvents). The reclaimed water, itself, engineered natural systems), to characterize the fate can also contain compounds that are generated during of contaminants from the initial water or wastewater water treatment (e.g., DBPs). Collectively, the number source to the product water at the consumer’s tap. of potential compounds present in reclaimed water is Quantitative microbial risk assessment methodologies in the thousands. For the risk exemplar, 24 chemicals are described that are used to estimate the risk of dis- were selected (see Box 7-1) that represent different ease that results. Chemical risk assessment techniques classes of these contaminants (i.e., DBPs, including used in the exemplar are also described that detail nitrosamines; hormones; pharmaceuticals; antimicrobi- methods to derive risk-based action levels for chemicals als; flame retardants; and perfluorochemicals). in reclaimed water. The starting concentrations for the microbial and chemical contaminants were selected on the basis of a Exemplar Results review of contaminant occurrence data in the scientific literature. More details are provided in Appendix A. The goal of the exemplar exercise is to illustrate the relative risk among the scenarios. In Appendix A, the qualities of the surface and reclaimed water and the final water qualities are described for each case as well BOX 7-1 Chemicals Selected for Evaluation in the as the rest of the assumptions behind the exemplar. Risk Exemplar Scenario 1 represents a scenario to which the public is already commonly exposed in many locations through- Hormones and out the United States and which is generally regarded Disinfection Byproducts Pharmaceuticals as safe, whereas Scenarios 2 and 3 represent planned 17β-Estradiol Bromate potable reuse projects. Because of the nature of the risk Bromoform Acetaminophen characterization tools employed, risks from pathogens Chloroform (paracetamol) Dibromoacetic acid (DBCA) Ibuprofen are displayed in a different form than the risks from Dibromoacetonitrile (DBAN) Caffeine chemicals. The pathogen risks are calculated as an esti- Dibromochloromethane (DBCM) Carbamazepine mate of the risk of increased gastroenteric illness. These Dichloroacetic acid (DCAA) Gemfibrozil Dichloroacetonitrile (DCAN) Sulfamethoxazole data also can be usefully displayed as a relative risk—the Haloacetic acid (HAA5) Meprobamate risks of the potable reuse Scenarios 2 and 3 relative to Trihalomethanes (THMs) Primidone the risks of Scenario 1—de facto reuse (Figure 7-2). N-Nitrosodimethylamine Others Figure 7-2 presents a summary of the relative (NDMA) Triclosan risk of illness from exposure to norovirus, adenovirus, Tris(2-chloroethyl)phosphate Salmonella, and Cryptosporidium as a result of drink- (TCEP) ing water from each of the three scenarios. All of the Perfluorooctanesulfonic acid (PFOS) risks have been normalized to Scenario 1, the de facto Perfluorooctanoic acid potable reuse example. As shown, both potable reuse (PFOA) scenarios have reduced risks, especially where viruses

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129 EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT particular process trains produce water of an acceptable quality. Note that these assessments were based on an ingestion scenario. For other end uses, such as shower- ing, some modification in the analyses would need to be made. For the pharmaceuticals, triclosan, and TCEP, the margin of safety ranges from 1000 to 1,000,000 for all three scenarios (a margin of safety lower than 1 poses potential concern). The perfluorinated chemicals (PFOA and PFOS) have lower margins of safety, but have margins of safety exceeding 1 for all three sce- narios. With one exception, the DBPs are shown to have margins of safety above 1. For NDMA, the data for all three scenarios show that it was below the limit of detection, but the detection limit (2 ng/L) exceeds the 10–6 lifetime cancer risk level used for the risk- based action level for this compound in the exemplar (0.7 ng/L). As a result the margin of safety for NDMA can only be established as greater than 0.35 for all FIGURE 7-2 Relative risk of illness (gastroenteritis) to persons three scenarios. These results have not identified any drinking water from each of the reuse scenarios relative to de chemical that presents a health risk of concern in any facto reuse (Scenario 1). The smaller the number, the lower the relative risk of the reuse applications for each organism. For of the scenarios studied, although further research is example in Scenario 2, the risk of illness due to Salmonella is warranted to ensure confidence in these assessments estimated to be less than 1/100th of the risk due to Salmonella (see Chapter 11). Despite uncertainties inherent in in Scenario 1. NOTES: *The risks for Salmonella and Cryptosporidium in the analysis, these results demonstrate that following R02129 Scenario 3 were below the limits that could be assessed by the proper diligence and employing appropriately designed Figure 7-2 model. treatment trains (see Chapter 5), potable reuse systems bitmapped can provide protection from trace organic contaminants comparable to what the public experiences in many drinking water supplies today. As a general rule, DBPs are concerned with the SAT supply, and with all four and perfluorinated chemicals deserve continued scru- organisms where the microfiltration/reverse osmosis/ tiny in all drinking water supplies. UV supply is concerned. In the latter instance, the For microbial agents, if one illness or infec- densities of Salmonella and Cryptosporidium are esti- tion/10,000 persons per year is used as a benchmark, it mated to be reduced to such low levels that the model is apparent that the risks from bacterial and protozoan was unable to calculate a risk. On the basis of these exposure are below this benchmark for all the scenarios, calculations the committee concludes that microbial with the exception of Scenario 1, the de facto reuse ex- risks from these potable reuse scenarios are much less ample (see Appendix A, Table A-6). In this particular than those from de facto reuse. instance, it is likely that the risks for the viruses are Table 7-1 summarizes the estimates of the margin overestimated, perhaps as a result of the conversion of of safety for each of the 24 organic compounds studies the genome copy density to the density of infectious in the exemplar. These are also displayed graphically units (IU) and/or because predation and die-off in the in Figure 7-3. stream was neglected. In any case, the consistent use of conservative assumptions throughout all three scenarios Findings of the Risk Exemplar assures that the assessment of the relative risk of one scenario over the other is robust. The relative analysis The results of the risk assessment (Table 7-1, Fig- makes it clear that the potable reuse scenarios exam- ures 7-2 and 7-3) can be used to ascertain whether the

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130 WATER REUSE TABLE 7-1 Summary of Margin of Safety (MOS) Estimates for the Three Scenarios Analyzed by the Committee Risk-Based MOS Scenario 1, MOS Scenario 2, MOS Scenario 3 Chemical Action Levela de Facto Reuse SAT, No Disinfection MF/RO/UV Nitrosamines >0.4 >0.4 >0.4 NDMA 0.7 ng/L Disinfection byproducts >2 Bromate 10 µg/L N/A N/A >160 Bromoform 80 µg/L 27 160 Chloroform 80 µg/L 16 80 16 >60 >60 >60 DBCA 60 µg/L >54 >140 DBAN 70 µg/L N/A >80 >160 DBCM 80 µg/L N/A >60 >60 DCAA 60 µg/L 12 >20 >20 DCAN 20 µg/L N/A HAA5 60 µg/L 6 12 12 THM 80 µg/L 2.7 16 8 Pharmaceuticals >350,000,000 >350,000,000 >35,000,000 Acetaminophen 350,000,000 ng/L >120,000,000 >280,000,000 Ibuprofen 120,000,000 ng/L 56,000,000 >190,000,000 Carbamazepine 186,900,000 ng/L 10,000,000 1,200,000 >140,000,000 Gemfibrozil 140,000,000 ng/L 8,600,000 2,300,000 >80,000,000 >160,000,000 Sulfamethoxazole 160,000,000 ng/L 720,000 >930,000,000 Meprobamate 280,000,000 ng/L 17,000,000 8,800,000 >58,000,000 Primidone 58,100,000 ng/L 10,000,000 450,000 Others >70,000,000 >23,000,000 Caffeine 70,000,000 ng/L 3,500,000 17-β Estradiol >35,000,000 >35,000,000 >35,000,000 3,500,000 ng/L >3,500,000 >2,100,000 Triclosan 2,100,000 ng/L 840,000 >84,000 >210,000 TCEP 2,100,000 ng/L 5,800 >200 PFOS 200 ng/L 17 4 >80 PFOA 400 ng/L 36 19 NOTES: > indicates that the assumed concentration was below detection, and only an upper limit on the risk calculation was determined. See Appendix A for further detail. aSources of the risk-based action limits are provided in Table A-11 of Appendix 11. ined here represent a reduction in microbial risk when CONCLUSIONS compared with the de facto scenario that has become a It is appropriate to compare the risk from water common occurrence throughout the country. produced by potable reuse projects with the risk as- It should be emphasized that the committee pres- sociated with the water supplies that are presently in ents these calculations as an exemplar. This should use. The committee conducted an original compara- not be used to endorse certain treatment schemes tive analysis of potential health risks of potable reuse or determine the risk at any particular site without in the context of the risks of a conventional drinking site-specific analyses. For example, the presence of a water supply derived from a surface water that receives chemical manufacturing facility in the service area of a a small percentage of treated wastewater. By means of wastewater utility being used for potable reuse would this analysis, termed a risk exemplar, the committee dictate scrutiny of chemicals that might be discharged compared the estimated risks of a common drinking to the sanitary sewer. In addition, the various inputs water source generally perceived as safe (i.e., de facto and assumptions of this risk assessment contain sources potable reuse) against the estimated risks of two other of variability and uncertainty. Good practice in risk potable reuse scenarios. assessment would require full consideration of these The results of the committee’s exemplar risk factors, such as by a Monte Carlo analysis (Burmaster assessments suggest that the risk from 24 selected and Anderson, 1994). chemical contaminants in the two potable reuse sce-

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131 EVALUATING THE RISKS OF POTABLE REUSE IN CONTEXT FIGURE 7-3 Display of the Margin of Safety (MOS) calculations for the 24 chemicals analyzed for each of the three scenarios. MOS <1 is considered a potential concern for human health. NOTE: Bars with diagonal stripes are for MOS values represent the lower limit of the actual value, considering that the concentration of the contaminant was below the detection limit. R02129 Figure 7-3 bitmapped degree of uncertainty, the committee’s analysis narios does not exceed the risk in common existing great water supplies. The results are helpful in providing suggests the risk from potable reuse does not appear to be any higher, and may be orders of magnitude perspective on the relative importance of different lower than currently experienced in at least some groups of chemicals in drinking water. For example, current (and approved) drinking water treatment DBPs, in particular NDMA, and perfluorinated chemi- systems (i.e., de facto reuse). State-of-the-art water cals deserve special attention in water reuse projects be- cause they represent a more serious human health risk treatment trains for potable reuse should be adequate than do pharmaceuticals and personal care products, to address the concerns of microbial contamination given their lower margins of safety. Despite uncertain- if finished water is protected from recontamination ties inherent in the analysis, these results demonstrate during storage and transport and if multiple barriers that following proper diligence and employing tailored and quality assurance strategies are in place to ensure advanced treatment trains and/or natural engineered reliability of the treatment processes (see Chapter 5). treatment, potable reuse systems can provide protec- The committee’s analysis is presented as an exemplar tion from trace organic contaminants comparable to (see Appendix A for details and assumptions made) what the public experiences in many drinking water and should not be used to endorse certain treatment supplies today. schemes or determine the risk at any particular site With respect to pathogens, although there is a without site-specific analyses.

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