Comparison of dilution factors for German wastewater treatment plant effluents in receiving streams to the fixed dilution factor from chemical risk assessment

Abstract Incomplete removal during wastewater treatment leads to frequent detection of compounds such as pharmaceuticals and personal care products in municipal effluents. A fixed standard dilutionfactor of 10 for effluents entering receiving water bodies is used during the exposure assessmentof several chemical risk assessments. However, the dilutionpotential of German receiving waters under low flow conditionsis largely unknown and information is sparse for other European countries. We calculated dilutionfactors for two datasets differing in spatial extent and wastewater treatment plant (WWTP) size: a national dataset comprising 1225 large WWTPs in Central and Northern Germany and a federal dataset for 678 WWTPs of a single state in Southwest Germany. We found that the fixed factor approach overestimates the dilutionpotential of 60% and 40% of receiving waters in the national and the federal dataset, with median dilutionfactors of 5 and 14.5, respectively. Under mean flow conditions, 8% of calculated dilutionfactors were below 10, with a median dilutionfactor of 106. We also calculated regional dilutionfactors that accounted for effluent inputs from upstream WWTPs. For the national and the federal dataset, 70% and 60% of calculated regional dilutionfactors fell below 10 under mean low flow conditions, respectively. Decrease of regional dilutionpotential in small receiving streams was mainly driven by the next WWTP upstream with a 2.5 fold drop of median regional dilutionfactors. Our results show that using the standard dilutionfactor of 10 would result in the underestimation of environmental concentrations for authorised chemicals by a factor of 3–5 for about 10% of WWTPs, especially during low flow conditions. Consequently, measured environmental concentrations might exceed predicted environmental concentrations and ecological risks posed by effluents could be much higher, suggesting that a revision of current risk assessment practices may be required.