Mass flows of per- and polyfluoroalkyl substances (PFASs) in a Swedish wastewater network and treatment plant

Abstract: Per- and polyfluoroalkyl substances (PFASs) are man-made substances that hold unique properties. They are not only oil- and water repellants but also very resistant to degradation. Due to these properties, the applications are endless and PFASs can be found in a wide range of industrial applications and commercial products. The effluents of wastewater treatment plants (WWTPs) have been pointed out as one of the major sources of PFASs in the environment. The main aim of this project was to evaluate the sources and the occurrence of PFASs in a wastewater network in a Swedish city and in the different treatment steps at the connected WWTP. Another objective was to use these data to calculate mass flows and to investigate the fate of PFASs within the WWTP. The city of Uppsala and the WWTP Kungsängsverket were selected as study objects. Both wastewater and sludge were sampled and analyzed. In the wastewater network, a total of 15 pumping stations (PSTs) were sampled for wastewater, and at the WWTP, a total of 10 wastewater and 10 sludge samples were taken. The samples consisted of grab samples (n = 24), time-integrated samples (100 mL every 20 min during 24 hours, n = 2) and flow proportional samples (24 hours, n = 9). The aqueous and sludge samples were prepared for analysis using solid-phase and solid liquid extraction, respectively, and then analyzed by liquid chromatography tandem mass spectrometry (LC/MS/MS). The PFAS concentrations and composition profiles varied greatly in the network. High concentrations of 6:2 fluorotelomer sulfonate (6:2 FTSA) were generally found in the wastewater, which indicates increased usage of 6:2 FTSA in industrial processes and applications as replacement for perfluorooctane sulfonate (PFOS) and/or leaching from fire-training sites. A hot spot was detected (Sågargatan PST: ΣPFAS = 55,000 ng L-1 = 110,000 mg d-1) with elevated discharges of C3 – C8 perfluoroalkyl carboxylates (PFCAs). The studied WWTP was ineffective in removing C4, C6, C8 perfluoroalkyl sulfonates (PFSAs), C3 – C8 PFCAs and 6:2 FTSA from wastewater. Longer chained C9 – C17 PFCAs tended to partition to sludge more effectively than shorter chained C3 – C8 PFCAs, where PFCAs with an even amount of perfluorated carbon atoms had a higher affinity for sludge than those with an odd amount. The PFAS concentrations and mass flows tended to increase across the second clarifier in both wastewater and sludge, probably due to precursor degradation. PFSAs and PFCAs tended to be at similar or lower concentrations in the effluent compared to the influent. This shows that these substances enter the WWTP from an upstream source and are not formed or added in the WWTP. The transformation of precursors is therefore not the most important source of PFASs in Kungsängsverket. PFASs in wastewater at a large scale municipal WWTP may origin to a large extent from both industrial applications and domestic sources, such as daily life products. The new knowledge generated within this project will help Uppsala Vatten to protect drinking water supplies and the receiving aquatic environment from PFAS contamination.