Within the FP5 European APUSS project (Assessing infiltration and exfiltration on the Performance of Urban Sewer Systems), a methodology has been proposed to account for environmental, technical and economic impacts of infiltration or exfiltration on sewer systems, treatment plants and receiving waters, and to evaluate the consequences of possible investment / rehabilitation strategies. The multi-criteria method Electre III is used to compare and to rank the various strategies, as this method has a high potential adaptability to diverse contexts and objectives. An example of application of the proposed methodology is given for a 245 ha residential catchment equipped with a combined sewer system, including scenarios with different infiltration rates. It shows that the methodology is applicable, but also that validated and site specific information (data, measurements, calibrated models, etc.) is necessary in order to carry out a valuable analysis. Results shall be reviewed carefully and the final decision making process should take into account the context of the study. There is no unique best investment strategy, but several options may be considered depending on priorities and criteria of the sewer operator. The methodology is general, but results of the case study are site specific and cannot be extrapolated.

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