For 25 rain events test runs were performed on a catchment of a small town, where 40% of the person equivalents are connected to a separate sewer system and 60% to a combined one. We investigate the effects of rain events with regard to NH4-loads discharged by the wastewater treatment plant and by a combined sewer overflow. NH4-concentrations in the river caused by these loads are also discussed. NH4 is important as (i) it closely interrelates with NH3 – a fish poison – and (ii) it stands for dissolved compounds that mainly origin from the wastewater. The sewer system is simulated with three different options: with a combined water retention tank connected to the CSO structure, with a sewage retention tank at the interface between the separate and the combined system, and without any retention structure. While the sewage retention tank performs better concerning the integrated load discharged into the river, the combined sewer retention tank is more efficient in decreasing peak values and duration of high concentrations in the river. We also show that NH4 peak concentrations in the river are largely due to a wash-out effect in the main sewer at the beginning of a rain event.

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