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. Author manuscript; available in PMC: 2019 Oct 7.
Published in final edited form as: Curr Environ Health Rep. 2018 Jun;5(2):283–292. doi: 10.1007/s40572-018-0195-y

Potable Water Reuse: What Are the Microbiological Risks?

Sharon P Nappier 1,*, Jeffrey A Soller 2, Sorina E Eftim 3
PMCID: PMC6779056  NIHMSID: NIHMS1049147  PMID: 29721701

Abstract

Purpose of Review

With the increasing interest in recycling water for potable reuse purposes, it is important to understand the microbial risks associated with potable reuse. This review focuses on potable reuse systems that use high level treatment and de facto reuse scenarios that include a quantifiable wastewater effluent component.

Findings

In this article, we summarize the published human health studies related to potable reuse, including both epidemiology studies and quantitative microbial risk assessments (QMRA). Overall, there have been relatively few health-based studies evaluating the microbial risks associated with potable reuse. Several microbial risk assessments focused on risks associated with unplanned (or de facto) reuse, while others evaluated planned potable reuse, such as indirect potable reuse (IPR) or direct potable reuse (DPR).

Summary

The reported QMRA-based risks for planned potable reuse varied substantially, indicating there is a need for risk assessors to use consistent input parameters and transparent assumptions, so that risk results are easily translated across studies. However, the current results overall indicate that predicted risks associated with planned potable reuse scenarios may be lower than those for de facto reuse scenarios. Overall, there is a clear need to carefully consider water treatment train choices when wastewater is a component of the drinking water supply (whether de facto, IPR, or DPR). More data from full-scale water treatment facilities would be helpful to quantify levels of viruses in raw sewage and reductions across unit treatment processes for both culturable and molecular detection methods.

Keywords: microbial risk assessment, de facto reuse, indirect potable reuse, direct potable reuse, risk review

I. Introduction

Population growth, urbanization, and drought have stressed water supplies worldwide. To strengthen water portfolios, municipalities are increasingly considering innovative water management strategies, including water reuse. Potable reuse, where the drinking water source includes recycled wastewater, can be categorized as planned reuse and unplanned reuse [1, 2].

Planned Reuse

Planned potable reuse encompasses indirect potable reuse (IPR), which typically includes an environmental buffer such as a reservoir or groundwater basin; and direct potable reuse (DPR), which does not include an environmental buffer [2]. Until very recently, IPR has appealed to most communities because passage through an environmental buffer can further improve water quality through pathogen removal/inactivation and provide dilution of treated wastewater contaminants. Additionally, retention in the environment provides time for treatment excursions to be detected and corrected before water reaches the public [2, 3]. Several notable IPR project examples include groundwater recharge and direct injection projects in California (Orange County Sanitation District) and surface water augmentation in the Republic of Singapore (NEWater plants) [2].

Until recently, DPR projects were extremely rare. However, DPR is gaining acceptance among water professionals, State regulators, and the public. In some cases environmental buffers are unavailable, in other cases DPR may be more cost-efficient than IPR options or provide the ability to expand available water quantity [4]. Several existing or proposed DPR projects include those in Windhoek, Namibia; Cloudcroft, New Mexico; and El Paso, Texas [5, 6••]. Of note, Namibia has the most extensive DPR experience in the world (50 years). Additionally, the project in El Paso, Texas includes a DPR scheme in which the DPR product water will enter the potable water distribution system directly from the Advanced Water Treatment Facility [7, 8].

Unplanned reuse

Unplanned reuse, also known as “de facto” reuse, is the incidental presence of treated wastewater in a downstream water supply source [2, 9] or septage recharge of ground water supplying a public well [10]. The magnitude of de facto reuse is geographically widespread and increasing [2, 11]. Recently, Rice and Westerhoff [9] reported that upstream WWTP discharges potentially contribute to source waters used at 50% of the DWTPs that serve greater than 10,000 people and that low streamflow conditions could be of concern, particularly for smaller systems and under drought conditions [12].

Current Recommendations for Potable Reuse

No federal regulations in the United States specifically address potable reuse [6••] or the co-location of DWTPs in relation to WWTP discharge sites upstream of the same water source [9]. But, the states may choose to implement potable reuse practices using the Safe Drinking Water Act and Clean Water Act as a foundation [6••]. For example, IPR projects in California apply a “12/10/10 Rule,” where viruses should be reduced by 12-logs, and Cryptosporidium and Giardia by 10-logs each [13, 14]. These log-reduction values are intended to achieve a benchmark level of public health protection of 1 infection in 10,000 people per year (10−4), and were initially derived from reported densities of culturable enteric viruses, G. lamblia, and Cryptosporidium spp. found in raw sewage [14, 15, 16]. California is now considering the same microbial log-reduction requirements for DPR projects [17•]. The World Health Organization (WHO) published pathogen reduction target recommendations using a health burden (Disability Adjusted Life Year -DALY) risk-based approach, resulting in slightly less stringent log-reduction estimates - enteric viruses should be removed by 9.5-logs, bacteria by 8.5-logs, and protozoa by 8.5-logs [18•].

With the increasing interest in using recycled water for potable reuse purposes, it is important to understand the microbial risks associated with potable reuse. In this review, we address this need through the following objectives: (1) discuss common microbial pathogens associated with raw sewage; (2) summarize the published health assessments related to potable reuse, including both QMRA and epidemiologic studies; and (3) synthesize the potable reuse risk literature in terms of overall conclusions and considerations.

II. Pathogens of Concern

Given the nature of the source water, risks from microbial constituents are of particular concern when potable reuse is considered [1] and it is critical pathogens are removed sufficiently to prevent acute adverse health effects [2]. In general, the classes of microbes that can cause infection in humans include pathogenic bacteria, viruses, and protozoa, most of which are transmitted via a fecal-oral route.

Bacteria

Pathogenic bacteria commonly associated with raw wastewater include, but are not limited to, Salmonella spp., Campylobacter spp., and pathogenic Escherichia coli. These bacteria generally result in self-limiting, acute gastroenteritis. However, more severe illness outcomes, such as hemolytic uremic syndrome (HUS) caused by E. coli O157:H7, can also occur.

Due to their size and cell wall composition, bacteria are generally more susceptible to treatment and disinfection than viruses and protozoa. Spore-forming bacteria are an exception that can withstand high temperatures and disinfection treatments [19]. Additionally, some bacteria, such as Legionella spp., can proliferate in drinking water distribution systems and biofilms [20]

Viruses

Human enteric viruses of concern in raw wastewater include noroviruses (NoV), enteroviruses (e.g., poliovirus, hepatitis A virus, rotaviruses), and adenoviruses (AdV) [21]. Viruses, particularly non-enveloped viruses, are of interest in potable reuse applications because of their small size (20–30 nm), resistance to disinfection, and highly infectious nature [22]. Smaller pore sizes, such as those in membrane technologies are needed to significantly remove viruses found in wastewater [23]. Additionally, many non-enveloped viruses are resistant to commonly used disinfectants, such monochloromine; and AdV are particularly resistant to ultraviolet disinfection (UV) [21].

Human enteric viruses are present in raw and undisinfected secondary effluent [24••, 25••], to a lesser degree in disinfected effluent [26], and can even persist in effluents after some advanced treatment [27, 28]. Most waterborne illnesses are thought to be associated with viruses [29], with NoV causing the greatest number of gastrointestinal illnesses in the US (>20 million episodes, 569 death per year) [29]. However, NoV are difficult to culture [30], which creates challenges in understanding their rate of inactivation through treatment and disinfection processes.

Parasitic Protozoa

The pathogenic protozoa most commonly associated with raw wastewater are Cryptosporidium spp. and Giardia lamblia. Pathogenic protozoans are of concern in potable reuse applications because of their resistance to treatment and disinfection (particularly the cysts and oocysts of the protozoan life cycle), and ability to cause infection at low doses of exposure (i.e. one oocyst) typical of environmental exposure [31•]. Cryptosporidium oocysts and Giardia cysts are often detected in secondary wastewater effluent [32] and may persist in disinfected effluent after granular media or membrane filtration [33]. Some recent studies highlight the lack of published correlations between removal/inactivation of Cryptosporidium and secondary effluent water quality as evaluated via traditional fecal indicator bacteria [34].

Potable Reuse Reference Pathogens

Soller et al. [35••] summarized the reported densities of key pathogens in raw wastewater, which included NoV, AdV, Giardia spp., Cryptosporidium spp., Campylobacter spp., and Salmonella enterica. Together, these reference pathogens comprise more than 75% of illnesses from all known pathogens in the United States [29, 36], and are representative of other pathogens potentially of concern from the waterborne exposure route [37–39]. Since these pathogens cause the vast majority of known illnesses, it is assumed these same pathogens will be present in raw sewage. Soller et al. [35••] also summarizes the range of reductions of each of the key reference pathogens across individual unit treatment processes.

III. Review of Epidemiology and QMRA Studies for Potable Reuse

It is impractical to make inferences about public health safety via monitoring of microbial pathogens in the product water from advanced water treatment facilities because practical detection limits are much higher than the densities corresponding to benchmark levels of risk [40••]. Therefore, studies evaluating adverse human health impacts associated with exposure to pathogens from potable reuse can help fill this gap. Epidemiological studies have been a traditional approach for this purpose, and more recently QMRA, is also being used, particularly as a comparative risk tool [22, 41]. Following is a summary of epidemiology and risk assessment studies that have been completed to-date evaluating the safety of potable water reuse, considering the microbial risks.

Epidemiology Studies

DPR began in Windhoek, Namibia1 in 1968. Retrospective epidemiological evaluations of that population have found no relationships between drinking water source and diarrheal disease, jaundice, or mortality [2, 42], however, the limitations of that study including the lack of power, preclude extrapolations to other populations in industrialized countries [2, 43].

Recycled water, Colorado River water and local stormwater runoff, have been used since 1962 as sources for replenishing the Central Groundwater Basin at the Montebello Forebay in Los Angeles County [2]2. Three ecologic epidemiology studies have been conducted to assess the health significance of the use of recycled water at this location. The studies focused on a broad spectrum of health concerns that could potentially be attributed to constituents in drinking water and compared the health outcomes for 900,000 people who received some recycled water in their household water supplies to a control area of 700,000 people with similar demographic and socioeconomic characteristics, but who did not receive recycled water. The results from the collective studies [44–46] found no association between recycled water and higher rates of cancer, mortality, infectious disease or adverse birth outcomes. A fourth health study conducted in the Montebello Forebay [47] assessed the rates that residents consulted with primary care physicians for gastroenteritis, respiratory complaints, and dermatological complaints (conditions that could be related to reclaimed water exposure) as well as two conditions unrelated to water reuse or waterborne disease exposure. No differences in consultation rates between the two groups were reported.

Chanute, Kansas (USA) experienced a drought in 1956 and 1957 requiring the implementation of an emergency indirect potable reuse scheme involving a dam on the Neosho River. A portion of intake to the drinking water plant was municipal wastewater, and an epidemiology study was completed investigating the number of cases of stomach and intestinal illness during this period. The study concluded that a fewer number of cases of stomach and intestinal illness were reported when recycled water was being consumed versus the number of cases reported during the following winter when the conventional water supply was being utilized [6••, 48].

Microbial Risk Assessments

The National Academies of Science [43] compared the risk associated with planned potable groundwater reuse projects to a de facto reuse scenario. Hypothetical scenarios evaluated the relative risk from exposure to pathogens in a: 1) de facto reuse scenario (assumed 5% disinfected wastewater effluent upstream in drinking water plant intake); 2) planned reuse scenario of groundwater recharge to a potable aquifer via surface spreading basins with subsequent soil aquifer treatment; and 3) planned reuse scenario of groundwater recharge to a potable aquifer by direct injection of reclaimed water receiving subsequent advanced water treatment. The reference pathogens included AdV, NoV, Salmonella enterica, and Cryptosporidium. The results indicate that both planned groundwater recharge scenarios have lower predicted risks relative to the de facto scenario, especially from viral pathogens in water receiving soil aquifer treatment, and from all four pathogens in the water receiving advanced water treatment. The authors concluded that predicted microbial risks from the planned potable reuse scenarios are much less than those from de facto reuse.

Tanaka et al. [49] evaluated the predicted risks associated with groundwater recharge in California using secondary effluent data for culturable enteric viruses. The scenario assumed 1L of water ingestion daily, a six-month aquifer retention time, direct chlorination of secondary effluent, a rotavirus dose-response model, and a first order decay constant of 0.1/day [50]. The predicted mean annual risks associated with the groundwater recharge scenario were less than 10−10 for all four treatment facilities evaluated. These results are consistent with a prior and similar groundwater recharge scenario assessment that assumed 1 and 111 viruses/100L in chlorinated tertiary effluent, which estimated annual median and maximum risks at 10−9 and 10−7, respectively [50].

Ander and Forss [51] evaluated the predicted annual risk of infection from NoV, Giardia and Cryptosporidium for drinking water consumers under nine different scenarios that represent nominal, outbreak and failure scenarios in Windhoek, Namibia. The median raw water densities used in the risk assessment were 0.002 NoV gene copies/L, 0.250 Giardia cysts/L, and 0.1 Cryptosporidium oocysts/L. Overall, the QMRA results indicated that the predicted risk of infection was driven by Cryptosporidium risks in all the modelled scenarios. The median Cryptosporidium risks peaked at approximately 10−9. The 95th-percentile annual Cryptosporidium risk for most scenarios was roughly 10−5, and the 95th-percentile annual Cryptosporidium risk for the highest risk scenario was close to 10−4.

Barker et al. [52] conducted a QMRA, using NoV, Giardia and Campylobacter as reference pathogens, to determine the level of treatment required to meet the annual disease burden of 10−6 DALYs per person per year, using Davis Station in Antarctica as an example of a small remote community. Two scenarios were compared: 1) published municipal sewage pathogen densities and 2) estimated pathogen densities during a gastroenteritis outbreak. Median densities in municipal raw sewage were approximately 106 PCR units/L of NoV; 103 cysts/L of Giardia; and 103 cfu/L of Campylobacter. However, the densities increased by orders of magnitude under the assumed outbreak conditions. For example, the NoV outbreak sewage density was five orders magnitude higher than the municipal sewage density. Using the WHO-based 10−6 DALY health target, the required LRVs were calculated to be 6.9, 8.0 and 7.4 for NoV, Giardia and Campylobacter using municipal sewage density estimates, and 12.1, 10.4 and 12.3 for estimated outbreak conditions [18•]. Under outbreak conditions, the LRVs were much higher as a direct result of the higher sewage pathogen densities. The authors highlight that greater understanding of sewage pathogen densities from small communities is needed if DPR is to be considered in small communities.

Soller et al. [53••] evaluated the potential risks associated with four different DPR treatment trains (TT) [34]. In three of the TTs, the DPR product water was assumed to directly enter the drinking water distribution system after advanced water treatment. In the fourth TT, DPR product water was assumed to receive further treatment via a DWTP, prior to entering the drinking water distribution system. Peer-reviewed data were summarized to characterize densities of six reference pathogens (NoV, AdV, Cryptosporidium spp., G. lamblia, C. jejuni, and S. enterica) in raw wastewater and the reduction of each across the individual unit treatment processes under consideration. Key findings include: 1) TTs can be configured to be effective in removing reference pathogens to levels that present low public health risk; 2) health-based advantages exist for DPR projects in which product water from an advanced water treatment facility is introduced into the raw water supply immediately upstream of a conventional DWTP compared to those in which advanced treatment product water is introduced directly into a potable distribution system; 3) predicted annual risk estimates for any particular TT are driven by the highest daily risks for any of the individual reference pathogens; 4) proposed DPR project designs need to carefully consider reduction of both Cryptosporidium spp. and NoV; and 5) expected performance of individual unit treatment processes within an integrated TT can be critical for consistently achieving the benchmark level of public health protection. Of particular note, when UV unit treatment processes are operated at the higher UV dose (800 millijoule per square centimeter (mJ/cm2) compared to 12 mJ/cm2), the associated relative pathogen risk is significantly less than the risk associated with the lower UV dose.

In a follow-up analysis, Soller et al. [35••] conducted a series of sensitivity analyses; evaluated the predicted risks associated with log credit allocations in the United States; and identified reference pathogen reductions needed to consistently meet currently applied benchmark risk levels. The simulation results illustrated changes in cumulative annual risks estimates, the significance of which depended on the pathogen group driving the risk for a given TT. For example, updates to NoV raw wastewater values [24••] and use of a NoV dose-response approach capturing the full range of uncertainty [54•], increased risks associated with one of the TTs evaluated, but not the other. The most striking results from this study were that viruses need to be reduced by 14-logs or more to consistently achieve currently applied benchmark levels of protection associated with DPR.

Pecson et al. [3] collected treatment performance data (i.e., total organic carbon, UV intensity) over a one-year period from a DPR demonstration facility in California3 to characterize the predicted risk associated with consumption of the product water. Modeled raw water reference pathogens evaluated included culturable enterovirus (geometric mean = ~25 MPN/L) and Cryptosporidium (geometric mean = ~15 oocysts/L) [26]. Based on the modeled log reduction for each of the unit processes in the TT, they estimated that the median log reduction the plant achieves is 14-logs for enteric viruses, 16-logs for Cryptosporidium and ~19-logs for Giardia. Using previously published risk assessment methods [55], the predicted median annual risks for Cryptosporidium were 10−11 and median culturable enteric virus risks were 10−14.

Chaudhry et al. [40••] compared a de facto water reuse scenario with four hypothetical DPR scenarios for NoV, Cryptosporidium, and S. enterica. They assessed the predicted microbial risks of using: 1) source water impacted by 0–100% treated wastewater effluent and 2) different blending ratios (0–100%) of surface water with advanced treated water, assuming that the surface water consisted of 50% wastewater effluent. Median raw wastewater densities used in the analysis were approximately 9000 NoV gene copies/L (based on results from North America only reported by Eftim et al. [24••]), 1300 Salmonella spp. MPN/L, and 10 Cryptosporidium oocysts/L. Predicted de facto reuse risks exceeded the annual 10−4 infection benchmark risks, while all modeled DPR risks were significantly lower. The risks were dominated by Cryptosporidium and NoV; however, the rank order varied depending upon the TT and dose-response model used for NoV. The authors concluded that contributions of 1% or more of wastewater effluent in the source water, or blending 1% or more of wastewater-impacted surface water into the advanced-treated DPR water had substantial impacts on the modelled risks.

Lim et al. [56••] evaluated the predicted risk associated with de facto wastewater reuse in the Trinity River, Texas for exposure to Cryptosporidium and NoV. The Trinity River supplies over 50% of the water to Lake Livingston, a Houston drinking water reservoir. Under base flow conditions, the Trinity River is dominated by wastewater effluent, but occasionally effluent constitutes 100% of the river flow [2]. Both the fraction of wastewater effluent in the lake and the water residence time vary significantly with season. Empirical cumulative probability distributions from literature-based pathogen density data for Cryptosporidium and NoV in conventional activated sludge WWTP effluent were used in the QMRA model. Environmental pathogen decay rates of 0.0155 log10/day for Cryptosporidium [57] and 0.01 log10 /day (based on murine norovirus) for human NoV were used [58]. The annual infection risks due to Cryptosporidium and NoV as a function of different wastewater effluent proportions (15, 30, and 45%) and residence times (270, 315, and 360 days) were computed. No scenario achieved the risk benchmark of 10−4 infections per year and predicted median annual risks were 10−2 for both Cryptosporidium and NoV. Additionally, WHO DALY guideline values were met only when storage residence times reached 360 days. Based on these results, they infer that human exposure to drinking water produced from de facto reuse presents considerably higher infection risks than the benchmark level of often used for safe drinking water, and suggested that the community gastrointestinal illness levels are possibly a reflection of the disease burden from de facto reuse. Additionally, the authors concluded that prolonging source water storage may be more important than controlling the percent of effluent entering the drinking water source, assuming a constant temperature and pathogen decay rate.

Amoueyan et al. [59•] evaluated the reliability and equivalency of three different potable reuse scenarios for the predicted risk of infection from Cryptosporidium: 1) surface water augmentation via de facto reuse with conventional wastewater treatment; 2) surface water augmentation via planned IPR with ultrafiltration, pre-ozone, biologically activated carbon, and post-ozone; and 3) DPR with ultrafiltration, ozone, biologically activated carbon, and UV disinfection. The Cryptosporidium LRVs assumed for the scenarios were 4.0–6.5 for de facto reuse; 14.1–16.6 for IPR; and 13 for DPR. Predicted mean annual risks of infection were ~10−4 for the de facto scenario and the planned IPR system, and 10−11 for the DPR scenario. Because the IPR system was highly effective in reducing Cryptosporidium, the associated risks were generally dominated by the pathogen loading in the surface water. As a result, predicted IPR risks generally decreased with higher recycled water contributions. Similar to Chaudhry et al. [40••], storage time in the environment was important for the de facto reuse system with a critical storage time of approximately 105 days, although the results were temperature-dependent and also increased with higher raw wastewater Cryptosporidium densities (e.g. when outbreak conditions were assumed). Storage times shorter than the critical value resulted in significant increases in predicted risk and potential impacts of unit process treatment failures were noted for the IPR system [59•].

IV. Synthesis, Considerations, and Future Needs

Synthesis

Overall, there have been relatively few health-based studies evaluating the microbial risks associated with potable reuse. The few published epidemiology studies to-date have not identified any patterns of illness related to ongoing water reuse projects [2, 43–46], however many lacked sufficient power to detect such low level adverse health effects. Future epidemiology studies could be designed to evaluate the human health risks associated with potable reuse, particularly to understand the risks associated with de facto reuse scenarios. Studies should consider sample size, locations with known upstream wastewater effluent impacts into drinking source waters (i.e., Trinity River) [56••], and newer infectivity detection methods, such as saliva-antibody multiplex immunoassays [60].

Regarding published microbial risk assessments, several studies focused on risks associated with de facto reuse, while others evaluated planned potable reuse. Results from the collective QMRA studies are compiled and summarized in Figure 1 (Refer to Supplemental Materials for more details).

Figure 1.

Figure 1.

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Summary of unplanned (de facto) and planned (IPR and DPR) potable reuse QMRA results

In the QMRA studies evaluating de facto reuse (green symbols, Figure 1), risks above the 10−4 benchmark (1 infection per 10,000 people per year) are consistently predicted [40••, 56••, 59•]. Lim et al. [56••] and Amoueyan et al. [59•] found that residence time can be a key factor in mitigating de facto reuse risks. Lim et al. [56••] reported that no scenarios evaluated (up to 360 days retention) achieved the risk benchmark of 10−4 infections per year, and WHO DALY guideline values were met only when storage residence times reached 365 days. Amoueyan et al. [59•] reported a critical storage time of approximately 105 days for Cryptosporidium risks, although the results were temperature-dependent and a function of the raw wastewater pathogen densities. Unfortunately, in many situations residence times in an environmental buffer are difficult to control. These findings suggest that engineered storage buffers are also likely to be an important component of any DPR scheme. Additionally, Chaudhry et al. [40••] reported that as little as 1% (or more) wastewater effluent in the source water can substantially elevate drinking water risks. The above findings from de facto reuse QMRA studies are consistent with the epidemiological results reported by Colford et al. [61], who measured a high illness rate attributable to drinking water at a site which may have included a de facto reuse drinking water component in a suitably powered randomized control trial epidemiological study [62].

With respect to planned potable reuse (IPR (red) and DPR (blue) symbols, Figure 1), the reported predicted risks vary substantially, but overall the results indicate that the predicted risks associated with the planned potable reuse scenarios may be lower than those for the de facto reuse scenarios [2, 40••, 56••]. Figure 1 also illustrates the importance of the raw sewage pathogen density estimates applied in various risk models and their impact on overall risk estimates. For example, Ander and Forss [51] reported extremely low risks in almost all DPR scenarios, but they also assumed very low viral pathogen densities. Similarly, using relatively low pathogen densities in their risk model, Pecson et al. [3] reported annual median culturable virus risks of ~10−14 and annual median risks from Cryptosporidium ~10−11. Using virus data from a recent comprehensive meta-analysis [24••], Soller et al. [35••, 53••] reported annual median risks for typical DPR TTs of approximately 10−11 to 10−8 in scenarios using high UV doses (800 mJ/cm2). However, when low UV doses (12 mJ/cm2) were assumed, the median risks were elevated to approximately 10−7 to 10−3. Using data from the same meta-analysis, Chaudhry et al. [40••] reported risks estimates similar to those reported by Soller et al. [35••, 53••] for high UV dose systems.

Based on the results of this review, it can be inferred that the inputs and assumptions used to conduct health studies are critical determinants of the reported studies. The most important aspects include choices made relative to:

  • Pathogens included and methods used to enumerate those pathogens (i.e., cell culture vs molecular methods such as qPCR);

  • The pathogen densities in raw wastewater and whether distributions or point estimates are used;

  • Pathogen dose-response models, and in the case of non-culturable viruses, whether or not aggregation is included in the dose-response model;

  • Pathogen reduction estimates across unit treatment processes and assumed distributions around those estimates;

  • Starting point for the analysis (i.e., raw sewage or elsewhere in the TT); and

  • Risk approach and chosen benchmark (i.e., DALY or infection per year).

Because of the rapidly growing fields of QMRA and water reuse, it would be prudent for risk scientists to identify consistent input parameters for the above metrics, so that results are easily translated across studies.

Considerations

Achieving the Target Risk Level

Potable reuse TTs employ multiple barriers to remove the diverse pathogens found in wastewater. Currently, the WHO suggests a 10−6 DALY per year health endpoint [18•], and an endpoint of 10−4 infections per year is currently used in the United States to ensure sufficient pathogen reduction occurs across potable reuse water treatment [1, 14, 63]. Severe drought and emergency conditions aside, these benchmarks appear to be generally accepted but could be challenged in the future. Early risk-based calculations for potable reuse in the United States were derived from total culturable virus densities in raw sewage [14, 50, 49]. The results of this current review suggest that NoV should now be considered specifically in the evaluation of DPR and other recycled water projects. The primary factors influencing this inference include: 1) total culturable viruses account for only a small portion of known viral enteric illnesses [30]; 2) updated estimates for viruses in raw sewage are now available [24••, 25••]; 3) NoV more closely resemble newly recognized viruses than culturable viruses [65•]; 4) seasonal variability and outbreaks can influence pathogen densities in raw wastewater [24••, 52, 64•]; 5) updated NoV dose-response models are now available [54•, 65•, 66••]; 6) the similarity between NoV in wastewater and clinical trials reduces uncertainty in adopting clinical results to environmental exposure [56•]; 7) new estimates show statistical correlations between NoV and coliphage removal in treatment processes [25••]); and 8) potential decreases in water usage from water conservation measures may allow for increased raw sewage viral densities in the future [64•]. The majority of the water reuse risk analyses published since 2010 include NoV as part of their risk models [2, 35••, 40••, 52, 53••, 56••, 67, 68] and the inclusion of NoV in potable reuse risk-based analyses has revealed significant uncertainty about whether the current log reduction recommendations in the United States (i.e., “the 12/10/10 rule”) are adequate to achieve the intended benchmark risk level [53••, 64•].

De facto Reuse

The results of this summary indicate that there a number of important factors that can influence the human health risk associated with de facto reuse scenarios and that to understand the risk associated with any particular scenario, the factors need to be considered together. Some of those factors include the fraction of water derived from wastewater effluent, the wastewater treatment that occurs (or lack thereof), temperature, retention time in the environment (above or below the surface) [40••, 56••, 59•], and effluent disinfection or lack thereof, as many WWTPs disinfect on a seasonal basis [69]. Since de facto is such a common occurrence in the United States [9, 11], conventional DWTPs may need to consider treatment schemes more like planned IPR, if current the source water includes a de facto component and risk-based health benchmarks are to be consistently achieved.

Use of Surrogates

It is important to differentiate between the actual level of pathogen removal and that which can be rapidly and continuously demonstrated [3]. In lieu of direct pathogen measurements, surrogates are frequently used to provide continuous evaluation of system performance. Typically, treatment facilities receive less reduction credit than actually occurs through the unit treatment processes [35••]. These circumstances create opportunities, such as the use of non-pathogenic microbial surrogates in lieu of pathogens or fine-tuning log-credits for an individual unit process based on microbial or non-microbial surrogate measurements [70, 71]. However, before log credit allocations for individual unit processes are adjusted or alternative unit treatment processes are substituted, it will be important to ensure confidence and accuracy in the actual reductions consistently provided by a treatment process.

Antimicrobial Resistance

The presence of antimicrobial resistant bacteria and genes in treated wastewater is an issue which is currently emerging as an important consideration for potable reuse [18•]. Resistance to key clinical antibiotics is a substantial worldwide public health problem, and concerns have been raised about the exposure to antimicrobial resistant bacteria and genes in water [17•, 18•]. While bacteria are largely removed or inactivated during drinking water treatment, the extent to which resistant genes are present during the various stages of treatment and removed by individual unit processes has largely not been determined, and thus is considered a future research need.

Conclusions and Future Needs

The compilation of information presented here highlights the need to carefully consider water treatment choices when wastewater is a component of the drinking water supply (whether de facto, IPR, or DPR). Overall, we found that QMRA is a useful tool for predicting risks in the various potable water reuse scenarios, but there is a need for consistent input assumptions, particularly for pathogen densities and dose-response models. For example, risk assessments that used lower pathogen density values typically reported lower predicted risks, as compared to studies that used more recent and/or higher pathogen densities (Figure 1). More full-scale data from water treatment facilities using both culturable and molecular detection methods to compare levels of viruses in raw sewage and reductions across unit treatment processes would provide more confidence in microbial risk estimates.

Despite differences in QMRA assumptions across studies, the reported predicted risks from planned potable reuse scenarios were consistently lower than those for the de facto reuse scenarios [2, 40••, 56••] (Figure 1). Since wastewater effluents discharged into environmental waters are not always required to be treated by advanced water treatment, surface waters containing conventionally treated effluent can potentially contain higher densities of pathogens than advanced-treated waters [40••].

Finally, given the limited amount of epidemiological data, epidemiology studies specifically evaluating the risks related to de facto reuse are warranted. Studies should consider sample size, locations with known upstream wastewater effluent impacts into drinking source waters (i.e., Trinity River) [56••], and newer infectivity detection methods [60].

Supplementary Material

Sup 1

Acknowledgements

The research described in this article was funded by the U.S. EPA Office of Water, Office of Science and Technology under contract number EP-C-16-011 to ICF, LLC. This work has been subject to formal Agency review. The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the U.S. EPA. The authors gratefully acknowledge Philip Berger, Robert Bastian, and Jamie Strong for their insightful comments and critical review of the manuscript.

Footnotes

1

The DPR treatment train consists of powdered activated carbon, pre-ozonation, enhanced coagulation and flocculation, dissolved air flotation, dual media filtration, ozonation, biological activated carbon, granular activated carbon, ultrafiltration, chlorination, and stabilization with caustic soda.

2

Recycled water is provided by three tertiary water reclamation plants, each provide sedimentation, activated sludge secondary treatment, coagulation, flocculation, granular activated carbon, disinfection with chlorine, and dechlorination.

3

The advanced treatment portion of the TT consisted of ozone, BAC, MF, RO, high dose UV with an advanced oxidation process, and chlorination with free chlorine.

References

Papers of particular interest, published recently, have been highlighted as:

• Of importance

•• Of major importance

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