Impact of granular activated carbon, voltage application and sewage sludge pretreatment on mesophilic anaerobic digestion performance and per- and polyfluoroalkyl substances removal

Sammendrag

Four laboratory-scale mesophilic anaerobic digesters were operated in parallel to investigate the impact of granular activated carbon (GAC) addition, voltage application, and thermal and ultrasound pretreatment on anaerobic digestion (AD) performance, as well as the distribution and removal of seven per- and polyfluoroalkyl substances (PFAS). When raw sludge was used as feed, there were no statistically significant differences in volatile solids (VS) and soluble chemical oxygen demand (sCOD) removal, biogas yield or methane content between the reactors. Ultrasound or thermal sludge pretreatment increased the solubilisation of organic compounds but did not enhance methane production. The highest sCOD and VS removal (79 ± 11 % and 52 ± 17 %, respectively) was observed in bioreactors containing GAC after sludge thermal pretreatment. The addition of GAC into biochemical methane potential tests accelerated the consumption of VFAs and enhanced maximum methane production. PFAS distribution analysis showed that an increase of perfluorocarbon chain length and the presence of a sulfonic acid group reduced the fraction of PFAS found in the dissolved phase. Thermal and ultrasound pretreatment of sludge slightly increased the fraction of longer perfluorocarboxylic acids and perfluorooctane sulfonic acid in the dissolved phase, without affecting the total concentrations of PFAS in the pretreated sludge or digestate. Batch and continuous-flow experiments demonstrated significant total PFAS removal in bioreactors containing GAC, suggesting possible biotransformation, while the application of voltage did not affect their concentrations. Genera found in higher abundance in GAC systems, but not in systems without GAC, included Defluviicoccus, Limnobacter, Romboutsia and Methanomethylovorans.