Biotransformation and fate of a model fluorinated surfactant during wastewater treatment

The fluorinated repellent N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE) can be released to the environment via wastewater treatment plants (WWTPs) potentially causing harm to humans and wildlife. However, predicting its fate in WWTPs is complicated by biotransformation of its nonfluorinated side chains. The extent and rate of biotransformation was determined in this study using batch microcosms and a continuous-flow in situ reactor. Then, a comprehensive fate model was developed to predict emissions of N-EtFOSE and its transformation products from a typical WWTP.Batch biotransformation assays containing aerobic activated sludge yielded six N-EtFOSE transformation products. N-EtFOSE degraded to N-ethyl perfluorooctane sulfonamido acetic acid (N-EtFOSAA) with an aqueous phase pseudo-second order rate coefficient k = 2.7 + 0.3 L/g VSS day-1. Separate assays were performed using each transformation product to determine the reaction pathway. N-EtFOSAA underwent further transformation to N-ethylperfluorooctane sulfonamide (N-EtFOSA). N-EtFOSA then transformed to perfluorooctane sulfonamide (FOSA). FOSA transformed to perfluorooctane sulfinate (PFOSI), and PFOSI transformed to perfluorooctane sulfonate (PFOS). Perfluorooctanoic acid (PFOA) was not detected as a transformation product of any compound.In an effort to generate more realistic biotransformation rates, a novel in situ bioreactor was developed using a continuous flow of sludge from a full-scale activated sludge municipal treatment plant. N-EtFOSAA was detected as the sole transformation product of the six compounds previously-identified N-EtFOSE metabolites. The calculated biotransformation rate coefficients were k = 2.6 and 2.7 L/g VSS day-1 for two independent reactors.To predict the transformation rate during sludge digestion, batch biotransformation studies were conducted in anaerobic digester sludge. N-EtFOSE biotransformed to N-EtFOSAA with an aqueous-phase pseudo-second order rate coefficient kaq 0.13 L/g VSS day -1.A comprehensive fate model of a typical treatment predicted that approximately 71% of N-EtFOSE entering a sewer will be volatilized or stripped to the atmosphere, mostly during secondary treatment. Transformation of approximately 18% of the incoming N-EtFOSE to N-EtFOSAA was predicted during secondary treatment and anaerobic sludge digestion. The primary removal mechanism for N-EtFOSAA is sorption and removal in the waste solids. No additional N-EtFOSE transformation products are expected. The predicted percentage of N-EtFOSE exiting the plant is 4% in the wastewater effluent and 7% in the waste solids. The results of the model suggest that N-EtFOSE discharged to sewers can contribute to fluorochemical contamination in agricultural fields and remote regions, such as the Arctic.