Risk Assessment and Decision Support for Managing Drinking Water Systems

Doctoral thesis, 2010

The vital importance of a reliable and safe drinking water supply makes efficient risk management necessary for water utilities. Risks must be assessed and possible risk-reduction measures evaluated to provide relevant decision support. The World Health Organization emphasises the use of an integrated approach where the entire drinking water system, from source to tap, is considered when assessing and managing risks. Integrated risk assessments are important in order to avoid overlooking interactions between subsystems and events and to minimise sub-optimisation of risk-reduction measures. Methods for integrated risk assessment are, however, limited. A dynamic fault tree method is presented that enables quantitative, integrated risk assessment of drinking water systems. An approach for approximate dynamic fault tree calculations has been developed to minimise computational demand. It is shown how the method can be used to evaluate uncertainties and provide information on risk levels, failure probabilities, failure rates and downtimes of the entire system and its subsystems. The fault tree method identifies where risk-reduction measures are needed most and different risk-reduction alternatives can be modelled, evaluated and compared. The method is combined with economic analysis to identify the most cost-effective risk-reduction alternative. Integrated risk assessments of drinking water systems are commonly performed using risk ranking, where the probability and consequence of undesired events are assessed using discretised scales. There is, however, no common, structured way of using risk ranking to prioritise risk-reduction measures. Two alternative models for risk-based, multi-criteria decision analysis (MCDA) for evaluating and comparing risk-reduction measures have therefore been developed. The MCDA models are based on risk ranking, they can consider uncertainty in estimates and include criteria related to, for example, different risk types and economic aspects. In summary, this thesis provides methods for integrated risk assessment that make it possible to prioritise risk-reduction measures. It is concluded that the methods provide relevant decision support for efficient risk management in water utilities.