The European Commission has presented a list of priority substances in addition to the Water Framework Directive (WFD) adopted in December 2000. The list of priority substances is a matter of continuous review hence other relevant substances identified as hazardous can be implemented for regulation. In that regard a group of potential hazardous substances, quaternary ammonium compounds (QAC) is selected for further investigation and assessment, as QAC are widely used as disinfectants, biocides, and detergents among a variety of other applications. This paper provides information on a general interdisciplinary approach for assessing the potential significance of chemical substances hitherto not described in a coherent way considering QACs as example. Benzalkonium chlorides (BAC) and dialkyldimethylammonium chlorides (DDAC) were selected as key compounds because of their product and application profiles, as well as their ecotoxicological properties. For basic environmental risk evaluation, QAC usage pattern, emissions from single source polluters, the fate in waste water treatment plants, concentrations in surface water and sediments, as well as ecotoxicological effective concentrations were analyzed in this study. Based on substrate characteristics and use pattern relevant single source polluters were identified and emission concentrations as well as loads discharged into the sewerage were determined. Effluents from hospitals and laundries but also from wellness resorts showed high effluent concentrations compared to municipal waste water. To describe the fate of QACs during waste water treatment, adsorption and degradation behavior were determined. Additionally the influence of QACs on biological processes, especially nitrification was assessed. Partition coefficient values (log k(OC)) for QACs were determined between 4.35 for DDAC-C10 and 5.69 for DDAC-C18 (data not shown) indicating the high adsorption potential of those substances to the activated sludge in the waste water treatment plants. Results for BAC-C 12-18 were found to be in the same range. Concentrations for nitrification inhibition in waste water treatment plants lay above concentrations found in municipal waste water but in the range of concentrations discharged by single source polluters.