This paper aims at comparing the cost-effectiveness of the two main types of urban drainage systems, that is, the combined sewer system and the separate sewer system, based on the analysis of simulations. The problem of which of the two systems is better was heavily discussed over the years and the answer given to the question was usually: ‘it depends’. In this work, specific impacts are investigated in terms of a cause–effect analysis. The results are subsequently summarized and can help in the choice of the system to be implemented. Despite earlier reasoning, studies on river water quality strongly indicate that the separate system is not always the preferable solution because the polluted runoff from the street, containing e.g. different heavy metals, is discharged directly into the river. This analysis aims to compare the two different sewer systems on the basis of literature data and simulation of specific cases. The results are evaluated, as suggested in the EU-Water Framework Directive, on the basis of different assessment criteria: river water quality and morphology impacts, emissions and costs.

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