The present study proposes an approach for obtaining a candidate value of a water hygiene standard using water quality monitoring datasets and acceptable infection risks of waterborne pathogenic microbes. In the proposed approach, posterior predictive distributions of the concentration of an indicator microorganism and a pathogen-derived gene are estimated separately based on monitoring datasets, and then the probabilistic distribution of concentration ratio between the indicator microorganism and the pathogen-derived gene is obtained. In addition to it, an acceptable concentration of a pathogen-derived gene in water is derived from an acceptable infection risk (10
-4 infections per person per year) by quantitative microbial risk assessment (QMRA). The candidate value of water hygiene standard is determined by collating the calculated acceptable concentration of a pathogen-derived gene and the estimated distribution of concentration ratio. When a safety factor value is set at 10, 150 most probable number/ 100mL of
Escherichia coli was obtained as a candidate value of water hygiene standard for river water that is used as a drinking water source. Although this study identified research areas that have to be investigated further, such as the validation of QMRA assumptions, the proposed approach makes it possible to determine water hygiene standard values based on the scientific basis, including accumulated datasets of environmental water monitoring.
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