Given the observed negative effects of anthropogenic micropollutants on aquatic organisms and the recent development of advanced wastewater treatment technologies it is now the task to formulate an optimized strategy for minimizing micropollutant discharges to the environment: what are possible criteria, where are critical river sections and where should investment be allocated best on a large (national) scale?

A ban of critical substances or the treatment of wastewater with high micropollutant concentrations would be most effective, e.g. urine separation in hospitals. If such measures are not feasible or not sufficient, micropollutants have to be eliminated at the end-of-pipe, i.e. central wastewater treatment plants (WWTPs).

The model presented in this study comprises all Swiss major WWTPs (760, serving over 95% of the population) all lakes and rivers (over 14’000 of 277’000 river sections contain treated wastewater). Based on sales data, transfer coefficients (metabolism during application and elimination rates in WWTPs) and the flow rates in rivers the model can predict concentrations for all river sections.

For widely applied “down-the-drain” household chemicals with reliable sales data and known transfer coefficients, the model provides realistic mass flows well agreeing with measurements for different catchment sizes (e.g. benzotriazole, well soluble and persistent).

The strength of the model is an algorithm to optimize for immission-based criteria in large river networks: It finds the smallest number of WWTPs which would have to be upgraded with advanced treatment technologies in order not to exceed a certain concentration. Carbamazepine is used as an example to illustrate the effects of different strategies for micropollutant reduction. The model is highly beneficial as screening tool for authorities since the results clearly indicate that local approaches and mere load reduction strategies are not adequate for micropollutants on a large scale.