Updated Pathogen Log-Reduction Targets for Onsite Non-Potable Water Reuse
Citation:
Jahne, M., M. Schoen, J. Soller, J. Garland, AND S. Thimons. Updated Pathogen Log-Reduction Targets for Onsite Non-Potable Water Reuse. Presented at 2023 WateReuse Symposium, Atlanta, GA, March 05 - 08, 2023.
Impact/Purpose:
Risk-based water management relies on treatment targets derived using quantitative risk assessment. Here, we present updated pathogen log-reduction targets (LRTs) for onsite non-potable water systems reflecting the latest science on pathogen densities in source waters and their respective public health impacts. Participants will: Identify different public health metrics that can be used to quantify “acceptable risk” Learn how disability-adjusted life-years can be used to evaluate a variety of public health impacts See how the selection of risk metrics impacts treatment targets and resulting unit process selection Understand the latest science on dose-response of enteric pathogens relevant to water reuse Recognize the assumptions behind different log-reduction targets for onsite non-potable water systems and how their selection may impact treatment trains
Description:
Onsite non-potable water systems (ONWS) collect and treat locally available source waters (e.g., wastewater, graywater, stormwater, and roof runoff) for end uses such as toilet flushing and irrigation. Quantitative microbial risk assessment has been used to develop pathogen log-reduction targets (LRTs) for these systems, relying on scientific assumptions regarding pathogen densities, exposure levels, and dose-response. These assumptions must be transparently justified and critically evaluated to understand their impact on LRTs and resulting treatment train selections. When new data becomes available, the risk-based framework is adaptable to accommodate this information and produce updated LRTs. Two previous sets of ONWS LRTs have been published to date, reflecting different assumptions on reference pathogens, source water concentrations, and dose-response relationships; both have been adopted in state-level regulatory development. After reviewing these efforts and their key similarities/differences, we present new LRTs that incorporate the latest science on pathogen dose-response and densities in alternative source waters. Importantly, this set includes two potential “acceptable risk” benchmarks for selection; the 10-4 infection per person per year (ppy) target used in previous studies and an alternative target of 10-6 disability-adjusted life-years (DALY) ppy as used by the WHO and Australia in their water reuse guidelines. The DALY metric builds on infection risk by additionally considering the associated likelihood of illness and its resulting health burden (i.e., morbidity and mortality). In doing so, it better captures the full public health impact of management decisions and allows for comparison of impacts across distinct exposure pathways and health outcomes (e.g., microbial vs. chemical risk). The presentation will culminate in a comparison of treatment trains achieving the different LRT sets (i.e., the previous two and new ones using infection and DALY metrics, respectively). Emphasis is placed on how different LRT requirements affect unit process selections, noting where similar LRTs have limited impact and where larger differences result in additional treatments or operational changes (e.g., disinfectant doses). These considerations are critical as stakeholders consider the broader effects of LRT selection, including cost and climate change contributions.