Occurrence of per- and polyfluorinated alkyl substances in wastewater treatment plants in Northern Italy

Wastewater treatment plants are known to be relevant input sources of per- and polyfluoroalkyl substances (PFAS) in the aquatic environment. This study aimed to investigate the occurrence, fate, and seasonal variability of twenty-five PFAS in four municipal wastewater treatment plants (WWTP A, B, C, and D) surrounding the city of Milan (Northern, Italy). Composite 24-h wastewater samples were collected in July and October 2021 and May and February 2022 from influents and effluents of the four WWTPs. PFAS were detected at concentrations ranging between 24.1 and 66.9 μg L^-1 for influent and 13.4 and 107 μg L^-1 for effluent wastewater samples. Perfluoropentanoic acid was the most abundant (1.91-30.0 μg L^-1) in influent samples, whereas perfluorobutane sulfonic acid predominated (0.80-66.1 μg L^-1) in effluent samples. In sludge, PFOA was detected in plant A at concentrations in the range of 96.6-165 ng kg^-1 dw in primary sludge samples and 98.6-440 ng kg^-1 dw in secondary treatment sludge samples. The removal efficiency of total PFAS varied between 6 % and 96 %. However, an increase of PFAS concentrations was observed from influents to effluents for plant D (during July and October), plant A (during October and May), and plant C (during May) indicating that biotransformation of PFAS precursors can occur during biological treatments. This was supported by the observed increase in concentrations of PFOA from primary to secondary treatment sludge samples in plant A. Moreover, the plant operating at shorter hydraulic retention times (plant D) showed lower removal efficiency (<45 %). Seasonal variation of PFAS in influent and effluent appears rather low and more likely due to pulse release instead of seasonal factors.