Quantitative risk assessment of water supply systems from source to tap

In the 3rd edition of the Guidelines for Drinking-water Quality, the World Health Organisation concludes that an integrated management of risks in source waters, treatment systems and distribution networks is the most efficient way to guarantee safe drinking water to consumers. The integrated approach is fundamental to avoid sub-optimisation of risk-reduction efforts. This paper presents an application of an integrated and quantitative risk model for comparing risk-reduction alternatives to support decisions for reaching specified water safety targets. A fault tree approach is used for structuring the risk analysis and for estimating the risk, expressed as Costumer Minutes Lost (CML). Input information is a combination of hard data and expert judgements. Uncertainties in input information are considerable and modelled by a Bayesian statistical approach. The Göteborg drinking water system is used to exemplify model application. Quantitative safety targets have been confirmed at the political level as a basis for long-term planning of investments and reinvestments. Four different risk-reduction alternatives concerning additional raw water supplies and increased treatment capacity were compared. A combination of increased treatment capacity and additional raw water sources was shown to provide the greatest risk reduction. The paper describes how a structured and thorough analysis of risk-reduction options can facilitate transparency and long-term planning of drinking water systems.