Hydrologically-informed toxicity risks across river networks for mixtures of micropollutants discharged from wastewater treatment plants

Chemicals in the aquatic environment can be harmful to biota and may cause toxic risks to the aquatic ecosystems. A high number of these chemicals originate from households, manufacturing and industries and are released to the aquatic environment as point source when connected to wastewater treatment plants (WWTP´s). A subset of the substances is permanently released and the load is proportional to the number of people connected to WWTPs, while the concentration of these substances shows higher variability. Especially at low discharges of the receiving waters the toxic risk may increase due to reduced dilution.

With a hydrologically informed approach that combines river network hierarchy, river discharge, wastewater loads and spatial allocation of point sources we developed a parsimonious model to calculate the total toxicity risk at each location of wastewater treatment plant (WWTP) discharges. The total toxicity risk was calculated as the sum of individual risks for 42 substances selected from a reference mixture of chemicals being considered as representative for European wastewater treatment plant effluents for a river network in Central Germany with about 300 WWTP´s of various sizes.

The results showed consistent patterns of substance specific cumulative toxicity and allowed an assessment of toxicity risks locally and at catchment scale. Different scenarios were analyzed to evaluate the consequences of different strategies to minimize toxic risks either by (1) source control, (2) relocation of WWTPs or their effluents or (3) end-of-pipe solutions like the 4^th treatment level depending on local conditions. With these capabilities the approach and model may support the implementation of the revised European Urban Wastewater Treatment Directive.