Quantitative microbial risk assessment to evaluate failure assumptions and excess treatment requirements for direct potable reuse

There is ongoing debate regarding the necessity of excess pathogen treatment to address potential public health risks from direct potable reuse (DPR) schemes. One approach in quantitative microbial risk assessment (QMRA) has been to incorporate conservative treatment failure scenarios, reflecting the assumption that DPR systems may have shorter response times to treatment failures than indirect potable reuse (IPR) systems. In this study, deterministic and stochastic QMRA approaches were used to determine minimum log reduction value (LRV) requirements for potable reuse schemes, explicitly accounting for process failures of varying magnitude, duration, and frequency while meeting health-based targets specified by World Health Organization (WHO) and Australian guidelines. Evaluation of multiple failure scenarios demonstrates a direct and quantifiable link between selected failure assumptions and the resulting excess pathogen treatment requirements. The results highlight that failure assumption selection represents a critical risk management decision that should be considered holistically within a risk-based regulatory framework. This work also provides insight into how excess pathogen removal contributes to system robustness by offering protection against a range of treatment failure conditions.