Public Health Implications of Short Duration, Off-Specification Conditions at Potable Reuse Water Treatment Facilities

Recycled water is an increasingly important water supply component for many communities. Widespread success with nonpotable reuse, and indirect potable reuse (IPR) such as groundwater injection, has an increasing number of municipalities considering potable reuse without the use of an environmental buffer [direct potable reuse (DPR)]. Previous risk assessment studies have evaluated the microbiological risks associated with potable reuse. However, no studies have rigorously evaluated the potential public health implications of known, infrequent, and short duration, off-specification conditions in advanced water treatment facilities (“off-spec events”). Herein, we couple previously reported data describing the frequency and severity of off-spec events with a microbial risk assessment methodology to quantitatively characterize the public health implications of these off-spec events. The results indicate that these low-probability, short duration, off-spec events in a potable reuse treatment system can drive annual risks; predicted annual median risk differences were increased by up to ∼4 orders of magnitude. The use of accurate monitoring of performance surrogates and diversion [e.g., engineered storage buffer (ESB)] allows for these increased risks to be minimized. These results highlight the importance of understanding water treatment system operations on a real-time basis and point toward managing plant operations and off-spec events in real time to maintain public health protection.