Science-based pathogen treatment requirements for direct potable reuse

Abstract

Specifying appropriate pathogen treatment requirements is critical to ensure that direct potable reuse (DPR) systems provide consistent and reliable protection of public health. This study leverages several research efforts conducted on behalf of the California State Water Resources Control Board to provide guidance on selecting science-based pathogen treatment requirements for DPR. Advancements in pathogen detection methods have produced new robust, high-quality datasets that can be used to characterize the distribution of pathogen concentrations present in raw wastewater. Such probabilistic distributions should replace the deterministic point estimate approach previously used in regulatory development. Specifically, to calculate pathogen treatment requirements, pathogen distributions should be used in probabilistic quantitative microbial risk assessments that account for variability in concentrations. This approach was applied using the latest high-quality datasets to determine the log reduction targets necessary to achieve an annual risk goal of 1 in 10 000 infections per person as well as a more stringent daily risk goal of 2.7 × 10⁻⁷ infections per person. The probabilistic approach resulted in pathogen log reduction targets of 13-log₁₀ for enteric viruses, 10-log₁₀ for Giardia, and 10-log₁₀ for Cryptosporidium. An additional 4-log₁₀ level of redundancy provides protection against undetected failures while maintaining high degrees of compliance with the daily (99%) and annual risk goals (>99%). The limitations of the use of molecular pathogen data are also discussed. While the recommendations and findings are targeted for California, they are broadly applicable to the development of DPR regulations outside California and the U.S.