PFAS Removal During Pyrolysis of Biosolids Is Affected by Initial PFAS Concentration and Pyrolysis Efficiency

Current and pending biosolids regulations related to per- and polyfluoroalkyl substances (PFAS) have triggered the industry to consider emerging biosolids treatment processes. Thermal processes, in particular, have garnered interest for their potential to remove PFAS from the solid phase. Research has documented that pyrolysis can remove PFAS from the solid phase at PFAS concentrations found in non-industrially impacted biosolids, but little information is available on the fate of PFAS in effluent py-liquid and py-gas phases. The objective of this research was to determine the fate of PFAS during pyrolysis of biosolids using non-industrially impacted and industrially impacted biosolids. Bench-scale pyrolysis experiments were conducted in triplicate at 500°C and 800°C. PFAS were quantified in the resulting biochar, py-liquid, and py-gas. PFAS removal was greater than 95% at both temperatures for the non-industrially impacted biosolids, but removal was 24%-88% in the industrially impacted biosolids at 500°C. At 800°C, removal was over 95% as well for the industrially impacted biosolids. The biochar yield was highest for the industrially impacted biosolids at 500°C, indicating less conversion of solids to liquid/gas and that the extent of pyrolysis reaction, in addition to initial PFAS concentration, can impact PFAS removal. The mass of PFAS in the effluent liquid and gas for the non-industrially impacted biosolids was approximately half of the influent PFAS mass, suggesting that a substantial fraction of PFAS is transferred/transformed in the effluent liquid and gas products that would require further polishing for improved PFAS removal.