Evaluation of a pilot system coupling thermal and ultrasound pretreatment, anaerobic digestion and hydrothermal carbonization for sewage sludge treatment and per- and polyfluoroalkyl substances removal

The presence of per - and polyfluoroalkyl substances (PFAS) in sewage sludge underscores the need to develop effective methods for their removal. As part of the ZeroPM EU project, we constructed and operated for 11 months a pilot-scale system coupling sludge pretreatment, anaerobic digestion and hydrothermal carbonization. Thermal or ultrasound pretreatment led to a significant increase in biogas production from 16 ± 2 NL d −1 to 27 ± 2 NL d −1 and 26 ± 2 NL d −1 , respectively. Calculations of energy consumption showed that heating of the bioreactor accounted for the largest fraction of total energy use, ranging up to 56%, while hydrothermal carbonization contributed by 27%. The application of a fully validated analytical method in the dissolved and particulate phase of raw sludge showed that 25 out of 27 monitored PFAS were detected, and the highest total concentrations were found for 6:2FTS (258.8 ± 34.4 ng L −1 ), PFDA (208.1 ± 10.7 ng L −1 ), and PFOS (155.8 ± 27.0 ng L −1 ). Sludge pretreatment did not affect the total concentrations of PFAS but slightly increased the presence of some analytes in the dissolved fraction. No PFAS removal was observed during anaerobic digestion whereas the majority of the PFAS were removed, to a significant extent, during hydrothermal carbonization. The removal rate varied depending on the target substance and the class of PFAS. The highest removal was achieved for perfluoroalkyl sulfonamides, exceeding 95%, while the removal of perfluoroalkyl carboxylic acids and perfluoroalkyl sulfonic acids ranged from 40% to 100%, and 56% to 100%, respectively. • Thermal or ultrasound pretreatment enhanced biogas production. • AD heating and HTC were responsible for 54% and 28% of the total energy consumption. • 25 PFAS from 6 groups detected in raw sludge; mainly in the particulate phase. • No total PFAS removal during sludge pretreatment and anaerobic digestion. • Significant removal of most PFAS in HTC; 13 of them were removed by more than 80%.