Perfluoroalkyl and polyfluoroalkyl substances in sewage sludge: challenges of biological and thermal treatment processes and potential threats to the environment from land disposal

Sewage treatment plants are considered as important pathways for the transfer of perfluoroalkyl and polyfluoroalkyl substances (PFAS) to the environment. In conventional wastewater treatment, some of these compounds accumulate in sewage sludge via sorption onto suspended solids posing a potential environmental threat during sludge disposal and reuse. This review paper summarizes data for the occurrence of 182 PFAS from different classes in sludge matrices. Most of these monitoring data originate from Europe, Asia and North America, while limited data are available from Africa and South America. The most commonly studied classes of PFAS are perfluoroalkyl carboxylic acids, perfluoroalkyl sulfonic acids, and perfluoroalkyl sulfonamides, while few results are available for the occurrence of new generation PFAS such as GenX. The range of the observed concentrations varies between 0.01 ng/g d.w. for perfluorobutane sulfonic acid to some μg/g, depending on the compound, with the highest concentrations found for perfluorooctane sulfonic acid (8.2 μg/g d.w.). Limited information exists for the fate and removal of these substances in full-scale anaerobic digesters while recent articles indicate that some PFAS can be biotransformed under strictly anaerobic conditions. The exact mechanism remains unclear; additional data are needed to identify transformation products, apply mass balances, and understand the role of specific microorganisms. As regards novel thermal processes, encouraging results were recently published for the destruction of specific PFAS in hydrothermal liquefaction and carbonization, ranging between 55 and 100%, depending on the compound and the experimental conditions applied. Future studies should expand the list of studied compounds and focus on the mechanisms of their removal. PFAS are transferred to agricultural lands during sewage sludge reuse and their concentrations in soil are related to the applied biosolids’ loading rates. The parameters that affect leaching and desorption of PFAS from sludge, their plant uptake and possible threats for the environment and human health from the use of sewage sludge as soil amendment should be examined in long-term studies. ZeroPM project aims to address some of the literature gaps regarding the behavior of PFAS during sludge treatment focusing to the modification of anaerobic digestion and to the use of hydrothermal carbonization.