Establishing Acceptable Exposure Limits for <i>Escherichia coli</i> in WWTP Tailwater Discharge for Recreational Activities

Tailwater discharged from wastewater treatment plants (WWTPs) can contain pathogenic microorganisms, posing potential health risks during recreational water activities. While Quantitative Microbial Risk Assessment (QMRA) is commonly used to evaluate these risks, its complex outputs were not easily translated into operational standards. To address this gap, this study introduces the concept of a threshold limit value (TLV) defined as the maximum acceptable E. coli concentration in tailwater that ensures compliance with specific health risk benchmarks. TLVs were derived using reverse QMRA for four age groups (children, early teens, teens, and adults) under two risk criteria: the U.S. EPA annual infection risk (10^-4) and the World Health Organization disease burden benchmark (10^-6 DALYs per person per year). Results showed that TLVs decrease with age, as adult individuals inhale or ingest larger volumes, resulting in higher exposure doses under identical conditions. Consequently, lower TLVs indicate stricter health protection requirements. WWTPs with higher treatment capacity and larger receiving water flows exhibited lower TLVs, reflecting more stringent acceptable concentrations due to reduced exposure risk. Swimming TLVs (4.43E+01-7.72E+02 CFU/100 mL) were about three times lower than rowing TLVs (1.25E+02-1.09E+03 CFU/m³), based on WHO and U.S. EPA benchmarks, due to more direct exposure and higher contact frequency. TLVs based on the WHO benchmark were consistently lower than those based on the EPA benchmark, emphasizing the impact of risk criteria on regulatory limits. Sensitivity analysis identified annual exposure frequency as the dominant variable for both rowing and swimming, with exposure time also being a key determinant for swimming exposure. This study provides a practical, risk-based framework for defining site-specific microbial limits, supporting evidence-based water quality management.