Carefully conducted model-based simulation studies have become an important part of studies evaluating control strategies for wastewater treatments plants. During the past four years a consortium of researchers and practitioners developed a standard test methodology called the Benchmark, a simulation procedure defined around a simulation model, a plant layout, realistic influent loads and a test protocol that provides an objective measure of performance. It is this measure of performance that

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... the focus of this paper. First, a total cost index, TCI, is developed that makes a weighted sum of the different performance evaluation criteria (energy consumption, sludge production, effluent quality, ...). In addition a robustness index is proposed that aims to evaluate the sensitivity of the performance to the WWTP characteristics. It requires some well-designed additional simulations that evaluate the performance under process conditions that differ slightly from the ones defined in the benchmark protocol.