Nowadays, the protection of the ecosystem and the responsible use of natural resources are of high importance. Therefore, it is of high priority to conserve or re-establish a good chemical and ecological state for all surface water bodies (EC, 2000). In cities that mainly use a combined sewer system, pollutant loads into receiving water bodies by combined sewer overflows (CSOs) and wastewater treatment plant (WWTP) discharge should be minimized with the aim to reduce the total emission.
This paper deals with the management of a newly constructed storage tunnel in Graz (Austria) together with the already existing sewer system and possible control actions (Schuetze et al., 2002) that can be taken to raise the efficiency of the whole system. Only an integrated approach is able to deliver information about the overall efficiency of the whole drainage system (Rauch et al., 2005). Consequently, an integrated simulation model is used that combines a rainfall model, a grey-box rainfall-runoff model of the sewer system on city scale, a detailed hydrodynamic hydraulic model of the controllable storage tunnel and a dynamic activated sludge model for the wastewater treatment plant (WWTP).