First-flush-effect of dissolved and particle-bound antibiotics and resulting environmental risk during combined sewer overflows

Background. Excreting antibiotics in the sanitary system is unavoidable, caused by the metabolism of the human body and the characteristics of antibiotics itself. Therefore we have to ensure drainage safety starting from the input in the sewer system until the wastewater treatment plant for both cases, dry-and rain weather. The hydraulic capacity of the drainage system is however limited during extreme rainfall events. In this particular case, polluted excess water (including micro pollutants) may be directly discharged into the adjacent surface water body via combined sewer overflow structures. Hence, it is necessary to minimize this impact with appropriate arrangements. Methods. The focus of our analysis was put on a sampling point near a main pumping station in the catchment area of Briesnitz of the city of Dresden. The loads of antibiotics within the sewage was estimated through field sampling using an automatic sampler and mobile flow sensors. The laboratory analysis was accomplished by the medicine faculty of Technische Universitat Dresden. In the course of the analysis we installed an online monitoring station, to allow a better monitoring of flow and pollutant dynamics during the dry and wet weather conditions. Results. The occurrence of high pollutant loads within the frontal part of the wave ('first flush phenomenon') was observed during online measurements. Due to low biodegradability of antibiotics and the fact that antibiotic-concentrations within sewage temporarily exceed PNEC-values, it is important to minimize discharges into surface waters to protect the adjacent aquatic ecosystem. Conclusion. Driven by these aspects it is necessary to implement technically and economically efficient arrangements, e.g. constructing in-sewer retention tanks or implement an online control system to ensure drainage safety with a minimum environmental impact.