Per- and Polyfluoroalkyl Substances (PFAS) in Sewage Sludge: A Journey from Global Occurrence to Adsorption and the Fate During Conventional Disposal Pathways

Wastewater treatment plants (WWTPs) not only function as receivers of per- and polyfluoroalkyl substances (PFAS) from influent streams, but also act as PFAS contributors to the environment via effluent and sewage sludge, attributed to their strong adsorption affinity and resistance to conventional wastewater treatment processes. Thus, this review aims to comprehensively analyze the occurrence, adsorption mechanisms, and environmental fate of PFAS in sewage sludge, with a particular focus on their fate during traditional sludge disposal, namely, land application, landfill, and incineration. Long-chain perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) dominate in sewage sludge from worldwide WWTPs, with increasing prevalence of short-chain compounds. Adsorption of PFAS by sewage sludge is influenced by PFAS chain length and function group, pH, ionic strength, and organic matter. Enhanced adsorption is achieved under acidic conditions and with divalent cations. In terms of PFAS behavior in conventional sludge management, land application facilitates PFAS uptake by crops, leading to bioaccumulation in the food chain and potential human exposure. Prolonged exposure to PFAS via landfilling not only significantly alters soil microbial community structures, but also yields bacterial toxicity. Effective incineration typically requires temperatures above 850–1000°C. Incomplete thermal decomposition releases toxic fluorinated by-products into the environment and generates persistent fluorinated radicals, which result in fluorinated compounds formation in further thermal treatment. In conclusion, this work highlights the significant accumulation and persistence of PFAS in sewage sludge and their concerning behavior during conventional sludge disposal. They underscore the necessity for advanced treatment technologies and managing their impact on the ecosystem.