Development of a Risk-Based Decision Model for Prioritizing Microbial Risk Mitigation Measures in Drinking Water Systems
Waterborne outbreaks of gastrointestinal diseases can cause large costs to the society. Risk management needs to be holistic and transparent in order to reduce these risks in an effective manner. Microbial risk mitigation measures in a drinking water system were investigated using a novel approach combining probabilistic risk assessment and cost-benefit analysis. Lake Vomb in Scania, the southern part of Sweden, was used to exemplify and illustrate the risk-based decision model. Four mitigation alternatives were investigated, where the tree first alternatives, A1-A3, represented connecting 25, 50 and 75 %, respectively, of on-site wastewater treatment systems in the catchment to the municipal wastewater treatment plant. The fourth alternative (A4) represented installing a UV-disinfection unit in the drinking water treatment plant. Quantitative microbial risk assessment was used to estimate the positive health effects in terms of quality adjusted life years (QALY), resulting from the four mitigation measures. The health effects were monetised using a unit cost per QALY. The results showed that only A4 can reduce the risk (probability of infection) below the World Health Organization guidelines of 10-4 infections per person per year (looking at the 95th percentile). Furthermore, all alternatives result in a negative net present value. However, the net present value would be positive (looking at the 50th percentile using a 1 % discount rate) if non-monetised benefits, such as increased property value and reduced microbial risks posed to animals, were estimated to 800-1200 SEK per connected on-site wastewater treatment system per year. This risk-based decision model creates a robust and transparent decision support. It is flexible enough to be tailored and applied to local settings of drinking water systems. The model provides a clear and holistic structure for decisions related to microbial risk mitigation. Nevertheless, detailed descriptions of the decision model compartments are suggested as further research and development.
water quality modelling
quantitative microbial risk assessment