paper This aims at comparing the cost effectiveness of the two mainly used wastewater system, that is the combined sewer system and the separate sewer system, based on the analysis of simulations. The problem which of the 2 systems is better was heavily discussed over the years. The answer to the question is usually the same: it depends. In this paper specific answers are given for specific cases. The results are subsequently summarized and can help in the system choice to be implemented. Despite earlier reasoning, studies on river water quality strongly indicate that the separate system is not always the better solution because the polluted runoff from the street, containing e.g. different heavy metals, is discharged directly into the river. This paper 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. A specifically tailored weighting system is used for the evaluation of the better solution for each analysed case. The analysis is based on simulation results where pollution fluxes and hydraulic impacts are computed by means of the software program City Drain. The catchment configuration is simple, with one CSO per catchment having a reduced area smaller than 300 ha. For larger catchments these were subdivided in identical sub-catchments. The dimension and population density of the catchment was varied. The related dimension of CSO basin and WWTP inflow are calculated according to the Austrian regulation. The definition of the different wastewater streams is based on a literature review. The concentration of the different pollutants for the two systems were chosen according to the "ATV DVWK Datenpool 2001", described in (Fuchs et al., 2004). Here data on concentration for different pollutants from different world countries is reported. From this database the minimum, median and maximum value is chosen for the simulation. The investigated pollutants are biochemical oxygen demand, chemical oxygen demand, total phosphorus, total nitrogen and un-ionised ammonia, as well as the heavy metals cadmium, lead, copper and zinc. The output of the simulation is evaluated in terms of ecological economic and performance using the above mentioned assessment criteria. Four important types of impacts are investigated: morphological impacts, acute toxic impacts, accumulating impacts and impacts on the oxygen household by oxygen depleting substances and eutrophication. For such assessment a classification method has been developed to determine the sensitivity of a particular receiving water regarding the above mentioned impacts. The indication parameters used for the evaluation are erosion frequency, mass balances for the different pollutants and exceedance of limits for these pollutants in the receiving water. Like suggested in the approach of the EU-WFD the best solution is here evaluated not only from an ecological point of view but also economical. To allow for cost comparison of the different devices only the costs from one literature source, that is (Gnthert and Reicherter, 2001) is used. For some parameters combined and separate system have a similar performance: the loads of BOD, COD and phosphorus discharged to the receiving water are similar in both systems. The hydraulic impact caused by the separate system is usually lower than the impact of the combined system. This is due to the fact that separate systems usually contain more overflow structures and thus lower maximum discharge pro device, and less changes to the natural hydraulic regime because the water remains in its hydrological cycle. The loads of the different heavy metals reaching the receiving water are bigger with separate systems than for combined systems (see figure). Depending on the catchment considerable loads of heavy metals can be discharged to the receiving water, especially if in the catchment there is a high percentage of metallic roofs (copper, zinc) or high traffic density. The answer to the question which system is better - combined or separate - cannot be given easily. The answer is case specific, depending on the characteristics of the system like e.g. the pollution of stormwater or the resilience of the receiving water. In the final version of the paper an attempt will be made to outline the relations in sufficient detail to help decision makers to identify the advantages and disadvantages of the two systems in ecological and economical terms for their specific case and thus evaluate which of the two systems they want to apply.