PFAS in Swedish wastewater: Occurrence, transformation, and fate in biological treatment processes

Per - and polyfluoroalkyl substances (PFAS) are widely present and highly persistent contaminants that pose increasing challenges for wastewater treatment plants (WWTPs). This study investigates the occurrence, transformation, and fate of PFAS and their precursors in five Swedish WWTPs using activated sludge (AS) and membrane bioreactor (MBR) technologies. We applied a multi-analytical approach, including targeted PFAS, extractable organofluorine (EOF), total oxidizable precursor (TOP) assay, and suspect screening analyses to assess the fluorine mass balance and precursor transformation during treatment. Results show high detection frequencies of short-chain PFAS (e.g., PFBA, PFHxA, PFHpA) across all influent and effluent samples, while long-chain compounds were rarely observed in water. No consistent removal trend was identified across treatment systems. In some cases, significant increases in effluent concentrations were observed, indicating possible transformation of both recognized and unrecognized precursors into measurable PFAS. The TOP assay results revealed distinct transformation dynamics, with more pronounced conversion in the MBR systems. Trifluoroacetic acid (TFA), analyzed separately, accounted for 90 % and 93 % when compared to the sum of the targeted PFAS fluorine concentrations in the influent and effluent, highlighting its dominant contribution to the overall fluorine load. EOF analysis showed that only 6.9 % in influent and 4.2 % in effluent could be explained by targeted PFAS, indicating a substantial unidentified organofluorine fraction. Future studies need to address and include analysis of fluorinated pharmaceuticals as these can be potential key contributors to the unidentified fraction. The observed formation of terminal PFAS from precursors in MBR systems highlights a potential intervention point earlier in the treatment chain. Reducing precursor inputs before biological treatment may limit the generation of persistent PFAS in the effluent. Conversely, in treatment trains that include quaternary processes, the transformation to stable terminal PFAS could be advantageous, as these compounds may be more readily removed than precursors. These findings emphasize the need for broader analytical coverage and treatment strategies tailored to specific process configurations to improve PFAS management in wastewater systems. • Short-chain PFAS were more frequently detected than long-chain PFAS. • No consistent PFAS removal trends across WWTPs or treatment types. • Precursor transformation differed between activated sludge and MBR systems. • More than 70 to 93 % of EOF remained unexplained after analyses. • Trifluoroacetic acid (TFA) was the most abundant PFAS, reaching 800 ng/L.