Study on the removal of PFAS during hydrothermal carbonization of sewage sludge

Poly- and perfluoroalkyl substances (PFAS) are frequently detected in sewage sludge and pose significant health risks due to their persistence and bioaccumulation. Hydrothermal carbonization (HTC) offers a promising thermochemical method to manage sewage sludge while destroying embedded PFAS. This study investigated PFAS fate during HTC of PFAS-spiked sewage sludge under varied conditions (200–300 °C, 2 h), including tests on anaerobically digested sludge at 250 °C with different pressures and pH adjustments. Removal efficiencies for seven targeted PFAS were quantified across solid, liquid, and gas phases. The C5–C10 perfluorocarboxylates were removed by over 94% under all experimental conditions included different temperatures (200–300 °C), pressures (38.3 to 68.3 bar) and pH rates (7 – 9) with NaOH and KOH, while C11 perfluoroundecanoic acid (PFUnDA) and perfluorooctane sulfonate (PFOS) showed only 55–75% removal, indicating greater recalcitrance. Mass balance calculations confirmed extensive PFAS degradation, with less than 0.001% transferred to the gas phase, as verified by UPLC/QTOF analysis. Notably, trace amounts of trifluoroacetic acid (TFA) and perfluoropentanoic acid (PFPeA) were detected in the gas phase at neutral pH but were absent at pH 9. Overall HTC performance (mass balances, pH, Chemical Oxygen Demand (COD)) was evaluated, and phytotoxicity tests highlighted that the resulting hydrochar is unsuitable for use as a soil amendment. • HTC of sewage sludge reduced PFAS levels in all product phases • C5–C10 PFAS achieved >94% removal via HTC of sewage sludge. • PFOS showed moderate removal, ranging from 55 to 75%. • TFA was detected in gases but eliminated after pH adjustment to 9 • Hydrochar is unsuitable for use as soil amendment