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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, *Corresponding author. A special issue devoted to and inspired by WaT3R, MEDA WATER International Conference on Sustainable Water Management, Rational Water Use, Wastewater Treatment and Reuse, Marrakech, Morocco, 8–10 June 2006. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.0000.00.000

The available database for QAC environmental risk assessment is still fragmentary, therefore, high assessment factors were necessary for calculation of PNEC values. The investigations of the waste water samples indicated, that the concentrations are below the effect concentrations for microorganisms in WWTPs. The pollution sources could be related to two single source polluters: hospitals and laundries. For surface water, environmental risk could not be excluded in the risk evaluation, as the PEC/PNEC ratio was above 1 for one river (L1) with low discharge and high content of treated waste water. The sediment concentrations are well below the PNEC values.