This is an investigation into possible changes to quantity and composition of sewage in the combined sewer system and the waste water treatment plant (WWTP) caused by new stormwater management strategies (mainly by disconnecting areas from the sewer system). Various scenarios were developed and the consequences on the water systems were calculated by using the simulation model KOSMO, in a fictional catchment.

The results of the combined sewer overflow-calculation show an enormous decrease of the overflow annual load for different parameters (COD, ammonia) by the reduction of impervious surfaces. Thus, the overflow concentrations of some parameters increase dramatically under these conditions, in particular the ammonia concentration might cause critical situations (e.g. sensitive receiving water).

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