Fate of per- and polyfluoroalkyl substances (PFAS) in anaerobic digestion of tannery sludge and their removal from clarified digestate by adsorption

Anaerobic digestion (AD) is a well-established technology for sludge stabilization and energy recovery; however, its application to PFAS-contaminated industrial sludge remains poorly understood. In this study, tannery sludge was treated under mesophilic (37 ◦ C) and thermophilic (55 ◦ C) conditions in continuous stirred tank reactors, to evaluate process performance and assess the fate of per- and polyfluoroalkyl substances (PFAS). Subsequently, adsorption was investigated as a post-treatment for PFAS removal from the clarified digestate (CD) using granular activated carbon (GAC) and a strong-base anion exchange resin (AER). Both reactors achieved stable operation, with thermophilic conditions resulting in higher specific gas production (0.62 vs 0.49 m 3 /kg VS) and volatile solids removal (54% vs 42%) compared with mesophilic digestion. However, PFAS concentrations in the CD remained above 10,000 ng/L; no significant differences were observed between mesophilic and thermophilic conditions, suggesting that AD thermal regime had a limited influence on PFAS occurrence in the liquid fraction. Adsorption experiments showed that AER exhibited a higher adsorption capacity than GAC for both total PFAS and individual congeners. The adsorbent dosages required to achieve target PFAS concentrations (500 and 100 ng/L) were estimated, and a preliminary full-scale assessment highlighted substantial differences in adsorbent inventory requirements and infrastructure footprint between the two technologies. Overall, the results demonstrate that, while AD is effective for energy recovery from tannery sludge, it does not mitigate PFAS contamination and therefore requires dedicated post-treatment. These findings provide new insights into the integrated management of PFAS-contaminated industrial sludge, supporting the development of scalable treatment strategies.