Potable Water Reuse: What Are the Microbiological Risks?

Persistence of sewage-associated genetic markers in advanced and conventional treated recycled water: implications for microbial source tracking in surface waters

Sewage contamination of environmental waters is increasingly assessed by measuring DNA from sewage-associated microorganisms in microbial source tracking (MST) approaches. However, DNA can persist through wastewater treatment and reach surface waters when treated sewage/recycled water is discharged, which may falsely indicate pollution from untreated sewage. Recycled water discharged from an advanced wastewater treatment (AWT) facility into a Florida stream elevated the sewage-associated HF183 marker 1,000-fold, with a minimal increase in cultured Escherichia coli. The persistence of sewage-associated microorganisms was compared by qPCR in untreated sewage and recycled water from conventional wastewater treatment (CWT) and AWT facilities. E. coli (EC23S857) and sewage-associated markers HF183, H8, and viral crAssphage CPQ_056 were always detected in untreated sewage (6.5-8.7 log10 GC/100 mL). Multivariate analysis found a significantly greater reduction of microbial variables via AWT vs CWT. Bacterial markers decayed ~4-5 log10 through CWT, but CPQ_056 was ~100-fold more persistent. In AWT facilities, the log10 reduction of all variables was ~5. In recycled water, bacterial marker concentrations were significantly correlated (P ≤ 0.0136; tau ≥ 0.44); however, CPQ_056 was not correlated with any marker, suggesting varying drivers of decay. Concentrations of cultured E. coli carrying the H8 marker (EcH8) in untreated sewage were 5.24-6.02 log10 CFU/100 mL, while no E. coli was isolated from recycled water. HF183 and culturable EcH8 were also correlated in contaminated surface waters (odds ratio β1 = 1.701). Culturable EcH8 has a strong potential to differentiate positive MST marker signals arising from treated (e.g., recycled water) and untreated sewage discharged into environmental waters.

IMPORTANCE

Genes in sewage-associated microorganisms are widely accepted indicators of sewage pollution in environmental waters. However, DNA persists through wastewater treatment and can reach surface waters when recycled water is discharged, potentially causing false-positive indications of sewage contamination. Previous studies have found that bacterial and viral sewage-associated genes persist through wastewater treatment; however, these studies did not compare different facilities or identify a solution to distinguish sewage from recycled water. In this study, we demonstrated the persistence of bacterial marker genes and the greater persistence of a viral marker gene (CPQ_056 of crAssphage) through varying wastewater treatment facilities. We also aim to provide a tool to confirm sewage contamination in surface waters with recycled water inputs. This work showed that the level of wastewater treatment affects the removal of microorganisms, particularly viruses, and expands our ability to identify sewage in surface waters.

A unit process log reduction database for water reuse practitioners

Pathogen reduction for the purpose of human health protection is a critical function provided by water reuse systems. Pathogen reduction performance potential is dependent on a wide range of design and operational parameters. Poor understanding of pathogen reduction performance has important consequences-under treatment can jeopardize human health, while over treatment can lead to unnecessary costs and environmental impacts. Documented pathogen reduction potential of the unit processes that make up water reuse treatment trains is based on a highly dispersed and unstructured literature, creating an impediment to practitioners looking to design, model or simply better understand these systems. This review presents a database of compiled log reduction values (LRVs) and log reduction credits (LRCs) for unit processes capable of providing some level of pathogen reduction, with a focus on processes suitable for onsite non-potable water reuse systems. Where reported, we have also compiled all relevant design and operational factors associated with the LRVs and LRCs. Overall, we compiled over 1100 individual LRV data entries for 31 unit processes, and LRCs for 8 unit processes. Results show very inconsistent reporting of influencing parameters, representing a limitation to the use of some of the data. As a standalone resource, the database (included as Supplemental Information) provides water reuse practitioners with easy access to LRV and LRC data. The database is also part of a longer-term effort to optimize the balance between human health protection, potential environmental impacts and cost of water reuse treatment trains.

Water, Water Everywhere, but Every Drop Unique: Challenges in the Science to Understand the Role of Contaminants of Emerging Concern in the Management of Drinking Water Supplies

The protection and management of water resources continues to be challenged by multiple and ongoing factors such as shifts in demographic, social, economic, and public health requirements. Physical limitations placed on access to potable supplies include natural and human-caused factors such as aquifer depletion, aging infrastructure, saltwater intrusion, floods, and drought. These factors, although varying in magnitude, spatial extent, and timing, can exacerbate the potential for contaminants of concern (CECs) to be present in sources of drinking water, infrastructure, premise plumbing and associated tap water. This monograph examines how current and emerging scientific efforts and technologies increase our understanding of the range of CECs and drinking water issues facing current and future populations. It is not intended to be read in one sitting, but is instead a starting point for scientists wanting to learn more about the issues surrounding CECs. This text discusses the topical evolution CECs over time (Section 1), improvements in measuring chemical and microbial CECs, through both analysis of concentration and toxicity (Section 2) and modeling CEC exposure and fate (Section 3), forms of treatment effective at removing chemical and microbial CECs (Section 4), and potential for human health impacts from exposure to CECs (Section 5). The paper concludes with how changes to water quantity, both scarcity and surpluses, could affect water quality (Section 6). Taken together, these sections document the past 25 years of CEC research and the regulatory response to these contaminants, the current work to identify and monitor CECs and mitigate exposure, and the challenges facing the future.

The Protective Effect of Virus Capsids on RNA and DNA Virus Genomes in Wastewater

Virus concentrations measured in municipal wastewater help inform both the water treatment necessary to protect human health and wastewater-based epidemiology. Wastewater measurements are typically PCR-based, and interpreting gene copy concentrations requires an understanding of the form and stability of the nucleic acids. Here, we study the persistence of model virus genomes in wastewater, the protective effects provided by the virus capsids, and the relative decay rates of the genome and infectious viruses. In benchtop batch experiments in wastewater influent at 25 °C, extraviral (+)ssRNA and dsDNA amplicons degraded by 90% within 15-19 min and 1.6-1.9 h, respectively. When encapsidated, the T90 for MS2 (+)ssRNA increased by 424× and the T90 for T4 dsDNA increased by 52×. The (+)ssRNA decay rates were similar for a range of amplicon sizes. For our model phages MS2 and T4, the nucleic acid signal in untreated wastewater disappeared shortly after the viruses lost infectivity. Combined, these results suggest that most viral genome copies measured in wastewater are encapsidated, that measured concentrations are independent of assay amplicon sizes, and that the virus genome decay rates of nonenveloped (i.e., naked) viruses are similar to inactivation rates. These findings are valuable for the interpretation of wastewater virus measurements.

Non-potable water reuse and the public health risks from protozoa and helminths: a case study from a city with a semi-arid climate

The study estimated the risk due to Cryptosporidium, Giardia, and Ascaris, associated with non-potable water reuse in the city of Jaipur, India. The study first determined the exposure dose of Cryptosporidium, Giardia, and Ascaris based on various wastewater treatment technologies for various scenarios of reuse for six wastewater treatment plants (WWTPs) in the city. The exposure scenarios considered were (1) garden irrigation; (2) working and lounging in the garden; and (3) consumption of crops irrigated with recycled water. The estimated annual risk of infection varied between 8.57 × 10-7 and 1.0 for protozoa and helminths, respectively. The order of treatment processes, in decreasing order of annual risk of infection, was found to be: moving-bed bioreactor (MBBR) technology > activated sludge process (ASP) technology > sequencing batch reactor (SBR) technology. The estimated annual risk was found to be in this order: Ascaris > Giardia > Cryptosporidium. The study also estimated the maximum allowable concentration (Cmax) of pathogen in the effluent for a benchmark value of annual infection of risk equal to 1:10,000, the acceptable level of risk used for drinking water. The estimated Cmax values were found to be 6.54 × 10-5, 1.37 × 10-5, and 2.89 × 10-6 (oo) cysts/mL for Cryptosporidium, Giardia, and Ascaris, respectively.

Enteric pathogen reduction targets for onsite non-potable water systems: A critical evaluation

Onsite non-potable water systems (ONWS) collect and treat local source waters for non-potable end uses such as toilet flushing and irrigation. Quantitative microbial risk assessment (QMRA) has been used to set pathogen log10-reduction targets (LRTs) for ONWS to achieve the risk benchmark of 10-4 infections per person per year (ppy) in a series of two efforts completed in 2017 and 2021. In this work, we compare and synthesize the ONWS LRT efforts to inform the selection of pathogen LRTs. For onsite wastewater, greywater, and stormwater, LRTs for human enteric viruses and parasitic protozoa were within 1.5-log10 units between 2017 and 2021 efforts, despite differences in approaches used to characterize pathogens in these waters. For onsite wastewater and greywater, the 2017 effort used an epidemiology-based model to simulate pathogen concentrations contributed exclusively from onsite waste and selected Norovirus as the viral reference pathogen; the 2021 effort used municipal wastewater pathogen data and cultivable adenoviruses as the reference viral pathogen. Across source waters, the greatest differences occurred for viruses in stormwater, given the newly available municipal wastewater characterizations used for modeling sewage contributions in 2021 and the different selection of reference pathogens (Norovirus vs. adenoviruses). The roof runoff LRTs support the need for protozoa treatment, but these remain difficult to characterize due to the pathogen variability in roof runoff across space and time. The comparison highlights adaptability of the risk-based approach, allowing for updated LRTs as site specific or improved information becomes available. Future research efforts should focus on data collection of onsite water sources.

Bayesian estimation of seasonal and between year variability of norovirus infection risks for workers in agricultural water reuse using epidemiological data

Norovirus infections are among the major causes of acute gastroenteritis worldwide. In Germany, norovirus infections are the most frequently reported cause of gastroenteritis, although only laboratory confirmed cases are officially counted. The high infectivity and environmental persistence of norovirus, makes the virus a relevant pathogen for water related infections. In the 2017 guidelines for potable water reuse, the World Health Organization proposes Norovirus as a reference pathogen for viral pathogens for quantitative microbial risk assessment (QMRA). A challenge for QMRA is, that norovirus data are rarely available over long monitoring periods to assess inter-annual variability of the associated health risk, raising the question about the relevance of this source of variability regarding potential risk management alternatives. Moreover, norovirus infections show high prevalence during winter and early spring and lower incidence during summer. Therefore, our objective is to derive risk scenarios for assessing the potential relevance of the within and between year variability of norovirus concentrations in municipal wastewater for the assessment of health risks of fieldworkers, if treated wastewater is used for irrigation in agriculture. To this end, we use the correlation between norovirus influent concentration and reported epidemiological incidence (R²=0.93), found at a large city in Germany. Risk scenarios are subsequently derived from long-term reported epidemiological data, by applying a Bayesian regression approach. For assessing the practical relevance for wastewater reuse we apply the risk scenarios to different irrigation patterns under various treatment options, namely "status-quo" and "irrigation on demand". While status-quo refers to an almost all-year irrigation, the latter assumes that irrigation only takes place during the vegetation period from May - September. Our results indicate that the log-difference of infection risks between scenarios may vary between 0.8 and 1.7 log given the same level of pre-treatment. They also indicate that under the same exposure scenario the between-year variability of norovirus infection risk may be > 1log, which makes it a relevant factor to consider in future QMRA studies and studies which aim at evaluating safe water reuse applications. The predictive power and wider use of epidemiological data as a suitable predictor variable should be further validated with paired multi-year data.

Pathogen reduction by ozone-biological activated carbon-based advanced water reclamation for reuse

Ozone-biological activated carbon (ozone-BAC)-based technologies are emerging as an appealing option for potable reuse systems; however, uncertainty remains regarding the reduction of waterborne pathogens. Common log reduction requirements have been modeled after California Department of Drinking Water's 12-10-10 log reduction value (LRV) for enteric virus, Cryptosporidium, and Giardia, respectively. The objective of this research was to investigate appropriate LRVs of pathogens that can be achieved in ozone-BAC-based treatment systems and to assess the applicability of employing drinking water pathogen guidelines for potable reuse applications. A pilot scale ozone-BAC-based treatment train was operated at two water reclamation facilities in Reno, Nevada, USA. Virus, Cryptosporidium, Giardia, and bacterial indicators were monitored across individual and combined treatment processes. Pathogen barriers investigated include conventional filtration, ozonation, and ultraviolet disinfection. Based on sampling and treatment validation strategies, the three pathogen barriers can provide minimum LRVs of 13-9-9.5 for virus, Giardia, and Cryptosporidium. Secondary biological treatment can provide additional pathogen LRVs with site-specific sampling. The present study addresses regulatory uncertainties associated with ozone-BAC pathogen reduction. PRACTITIONER POINTS: Ozone-biological activated carbon-based advanced treatment can meet pathogen LRV requirements with a minimum of three pathogen barriers. Successfully applied drinking water pathogen reduction guidelines for potable reuse applications verified by operational criteria. Low presence of pathogens requires surrogates and indicator analyses and variety of monitoring techniques to verify pathogen log reduction.

The status of potable water reuse implementation

A review of the current status of direct and indirect potable water reuse (DPR/IPR) implementation has been conducted, focusing on the regulatory and practical aspects and with reference to the most recent published literature. The review encompasses (a) the principal contaminant types, their required removal and the methods by which their concentration is monitored, (b) regulatory approaches and stipulations in assessing/ratifying treatment schemes and maintaining treated water quality, and (c) existing full-scale installations. Analytical methods discussed include established in-line monitoring tools, such as turbidity measurement, to more recent polymerase chain reaction (PCR)-based assay methods for microbial detection. The key risk assessment tools of quantitative microbial risk assessment (QMRA) and water safety plans (WSPs) are considered in relation to their use in selecting/ratifying treatment schemes, and the components of the treatment schemes from 40 existing IPR/DPR installations summarised. Five specific schemes are considered in more detail. The review reveals:Whilst there are a number of ongoing projects where RO is not used because of the challenge imposed by disposal of RO concentrate, the prevalence of the sequential RO-UV combination implies the importance of quantifying the impact of process upsets on these unit operations.

Understanding Microbial Loads in Wastewater Treatment Works as Source Water for Water Reuse

Facing challenges in water demands and population size, particularly in the water-scarce regions in the United States, the reuse of treated municipal wastewater has become a viable potential to relieve the ever-increasing demands of providing water for (non-)potable use. The objectives of this study were to assess microbial quality of reclaimed water and to investigate treatability of microorganisms during different treatment processes. Raw and final treated effluent samples from three participating utilities were collected monthly for 16 months and analyzed for various microbial pathogens and fecal indicator organisms. Results revealed that the detectable levels of microbial pathogens tested were observed in the treated effluent samples from all participating utilities. Log₁₀ reduction values (LRVs) of Cryptosporidium oocysts and Giardia cysts were at least two orders of magnitude lower than those of human adenovirus and all fecal indicator organisms except for aerobic endospores, which showed the lowest LRVs. The relatively higher LRV of the indicator organisms such as bacteriophages suggested that these microorganisms are not good candidates of viral indicators of human adenovirus during wastewater treatment processes. Overall, this study will assist municipalities considering the use of wastewater effluent as another source of drinking water by providing important data on the prevalence, occurrence, and reduction of waterborne pathogens in wastewater. More importantly, the results from this study will aid in building a richer microbial occurrence database that can be used towards evaluating reuse guidelines and disinfection practices for water reuse practices.

Droplet digital PCR quantification of norovirus and adenovirus in decentralized wastewater and graywater collections: Implications for onsite reuse

Risk-based treatment of onsite wastewaters for decentralized reuse requires information on the occurrence and density of pathogens in source waters, which differ from municipal wastewater due to scaling and dilution effects in addition to variable source contributions. In this first quantitative report of viral enteric pathogens in onsite-collected graywater and wastewater, untreated graywater (n = 50 samples) and combined wastewater (i.e., including blackwater; n = 28) from three decentralized collection systems were analyzed for two norovirus genogroups (GI/GII) and human adenoviruses using droplet digital polymerase chain reaction (ddPCR). Compared to traditional quantitative PCR (qPCR), which had insufficient sensitivity to quantify viruses in graywater, ddPCR allowed quantification of norovirus GII and adenovirus in 4% and 14% of graywater samples, respectively (none quantifiable for norovirus GI). Norovirus GII was routinely quantifiable in combined wastewater by either PCR method (96% of samples), with well-correlated results between the analyses (R² = 0.96) indicating a density range of 5.2-7.9 log₁₀ genome copies/L. These concentrations are greater than typically reported in centralized municipal wastewater, yet agree well with an epidemiology-based model previously used to develop pathogen log-reduction targets (LRTs) for decentralized non-potable water systems. Results emphasize the unique quality of onsite wastewaters, supporting the previous LRTs and further quantitative microbial risk assessment (QMRA) of decentralized water reuse.

Comparison of Predicted Microbiological Human Health Risks Associated with de Facto, Indirect, and Direct Potable Water Reuse

Increasing interest in recycling water for potable purposes makes understanding the risks associated with potential acute microbial hazards important. We compared risks from de facto reuse, indirect potable reuse (IPR), and direct potable reuse (DPR) scenarios using a previously published quantitative microbial risk assessment methodology and literature review results. The de facto reuse simulation results are compared to a Cryptosporidium spp. database collected for the Long Term 2 Enhanced Surface Water Treatment Rule's information collection rule (ICR) and to a literature review of norovirus (NoV) densities in ambient surface waters. The de facto simulation results with a treated wastewater effluent contribution of 1% in surface waters and a residence time of 30 days most closely match the ICR dataset. The de facto simulations also suggest that using NoV monitoring data from surface waters may overestimate microbial risks, compared to NoV data from raw sewage coupled with wastewater treatment reduction estimates. The predicted risks from IPR and DPR are consistently lower than those for the de facto reuse scenarios assuming the AWTFs are operating within design specifications. These analyses provide insight into the microbial risks associated with various potable reuse scenarios and highlight the need to carefully consider drinking water treatment choices when wastewater effluent is a component of any drinking water supply.

Occurrence of coliphage in raw wastewater and in ambient water: A meta-analysis

Coliphage have been proposed as indicators of fecal contamination in recreational waters because they better reflect the persistence of pathogenic viruses in the environment and through wastewater treatment than traditional fecal indicator bacteria. Herein, we conducted a systematic literature search of peer-reviewed publications to identify coliphage density data (somatic and male-specific, or MSC) in raw wastewater and ambient waters. The literature review inclusion criteria included scope, study quality, and data availability. A non-parametric two-stage bootstrap analysis was used to estimate the coliphage distributions in raw wastewater and account for geographic region and season. Additionally, two statistical methodologies were explored for developing coliphage density distributions in ambient waters, to account for the nondetects in the datasets. In raw wastewater, the analysis resulted in seasonal density distributions of somatic coliphage (SC) (mean 6.5 log₁₀ plaque forming units (PFU)/L; 95% confidence interval (CI): 6.2-6.8) and MSC (mean 5.9 log₁₀ PFU/L; 95% CI: 5.5-6.1). In ambient waters, 49% of MSC samples were nondetects, compared with less than 5% for SC. Overall distributional estimates of ambient densities of coliphage were statistically higher for SC than for MSC (mean 3.4 and 1.0 log₁₀ PFU/L, respectively). Distributions of coliphage in raw wastewater and ambient water will be useful for future microbial risk assessments.