Equivalency of Indirect and Direct Potable Reuse Paradigms Based on a Quantitative Microbial Risk Assessment Framework

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Microbial Risk Analysis

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Due to rapid growth in the potable reuse industry, there is a need to better characterize the associated pathogen risks and degree of attenuation achieved by various treatment trains. It is also important to understand how emerging treatment frameworks compare to the historical practice of de facto reuse. As such, the goals of this study were to (1) evaluate the equivalency of indirect potable reuse (IPR) and direct potable reuse (DPR) systems; (2) compare alternative treatment trains; and (3) identify the design components and operational conditions that are most critical to minimizing public health risks. To this end, we developed a static quantitative microbial risk assessment (QMRA) for Cryptosporidium, norovirus, adenovirus, and Salmonella. Treatment process performance (including failure scenarios) and resultant public health risks were estimated using a Stella 10.1 system dynamics model. The combined annual risk of infection was lower in DPR systems with no surface water influence [median risk = 1.1 × 10−8 for ozone-based DPR and 3.9 × 10−6 for DPR with reverse osmosis (RO)] compared with IPR systems or DPR with raw water augmentation (median risk = 9.0 × 10−4 to 3.8 × 10−3). Although de facto reuse, planned IPR, and DPR with raw water augmentation exceeded the common 10−4 annual risk benchmark, the risks were generally dominated by the pathogen concentrations in the upstream surface water, thereby highlighting the importance of source water characterization in all drinking water systems. Moreover, the risk calculation for each system was often dominated by a particular pathogen (e.g., adenovirus with a maximum risk of 3.7 × 10−2 for RO-based DPR during a compound failure). Sensitivity analyses demonstrated that storage time and temperature were important for de facto reuse and during compound failures and that risks generally decreased with greater recycled water contributions (RWC) due to the robustness of advanced treatment and/or attenuation in the environmental buffer.


Quantitative microbial risk assessment (QMRA); Potable reuse; De facto reuse; Full advanced treatment (FAT); Virus; Cryptosporidium


Environmental Microbiology and Microbial Ecology | Virology | Water Resource Management



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