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

Abstract

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.

Keywords: Decentralized; Onsite; QMRA; Risk assessment; Wastewater; Water reuse.

Fig. 1.

Modeled pathogen concentrations in greywater (left) and stormwater containing a 10⁻³ wastewater contribution (right) from the 2017 LRT Guidance and 2021 LRT Update. Greywater symbols denote pathogen occurrence rates in the epidemiology-based simulation, i.e., values below these are zero. Onsite wastewater and stormwater 10⁻¹ distributions follow the same patterns as greywater and stormwater 10⁻³, respectively. Roof runoff reference pathogens did not align for comparison.

Fig. 2.

Pathogen log-reduction targets (LRTs) for indoor use (left) and unrestricted irrigation (right) from the 2017 LRT Guidance (solid bars) and 2021 LRT Update (hatched bars). Symbols denote recommended values for enteric viruses (asterisks) and parasitic protozoa (circles) for each source water. Pathogen dose-response models are noted where multiple options were used (HG, hypergeometric; FP, fractional Poisson; EX, exponential). Roof runoff reference pathogens did not align for comparison.