Urban wastewater systems should be evaluated and analysed from an integrated point of view, taking all parts of the system, that is sewer system, wastewater treatment plant and receiving waters into consideration. Risk and parameter uncertainties are aspects that hardly ever have been addressed in the evaluation and design of urban wastewater systems. In this paper we present and discuss a probabilistic approach for evaluation of the performance of urban wastewater systems. Risk analysis together with the traditional cost-benefit analysis is a special variant of multi-criteria analysis that seeks to find the most feasible improvement alternative for an urban wastewater system. The most feasible alternative in this context is the alternative that has the best performance, meaning that the alternative has the lowest sum of costs, benefits and risks. The sum is expressed as the Net Present Cost (NPC). To use NPC as a decision variable has the problematic effect, that two alternatives performing completely differently when focusing on environmental cost can have the same NPC. The extreme example is one alternative with high risk and low cost and another with low risk and high cost. In this example it is up to the decision-maker to decide whether she wants to spend the budget on preventive installations or cleaning up after failures in the environment.

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