Detailed mass flows and removal efficiencies for biocides and antibiotics in Swedish sewage treatment plants

Highlights: • Detailed mass flows for eleven antimicrobials in different sections of three STPs. • First mass balance reported for chlorhexidine and hexadecylpyridinium chloride. • No biodegradation of chlorhexidine. • The biological treatment step was the most important for degradation. Antimicrobial compounds, such as biocides and antibiotics, are widely used in society with significant quantities of these chemicals ending up in sewage treatment plants (STPs). In this study, mass flows and removal efficiency in different treatment steps at three Swedish STPs were evaluated for eleven different biocides and antibiotics. Mass flows were calculated at eight different locations (incoming wastewater, water after the first sedimentation step, treated effluent, primary sludge, surplus sludge, digested sludge, dewatered digested sludge and reject water). Samples were collected for a total of nine days over three weeks. The STPs were able to remove 53–>99% of the antimicrobial compounds and 0–64% were biodegraded on average in the three STPs. Quaternary ammonium compounds were removed from the wastewater >99%, partly through biodegradation, but 38–96% remained in the digested sludge. Chlorhexidine was not biodegraded but was efficiently removed from the wastewater to the sludge. The biological treatment step was the most important step for the degradation of the studied compounds, but also removed several compounds through the surplus sludge. Compounds that were inefficiently removed included benzotriazoles, trimethoprim and fluconazole. The study provides mass flows and removal efficiencies for several compounds that have been seldom studied.