Risk assessment and distribution of PFAS in a biological phosphorus recovery sludge used for agriculture

The growing demand for phosphorus recovery from wastewater has increased interest in Enhanced Biological Phosphorus Removal (EBPR) processes. However, the fate of per- and polyfluoroalkyl substances (PFAS) in EBPR-derived biosolids remains poorly understood. This study investigated the adsorption behavior of four PFAS: perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorooctanesulfonamide (PFOSA), and perfluorotetradecanoic acid (PFTDA) in EBPR sludge and assessed the long-term soil risks following biosolid application. Results showed that PFAS adsorption was significantly influenced by the matrix type, with EBPR sludge exhibiting stronger adsorption than particulate matter in wastewater. Adsorption affinity followed the order: PFOA < PFOS < PFTDA < PFOSA, and was not affected by anaerobic and aerobic conditions or spiking levels. Log K_d values in EBPR sludge were 2.6 (PFOA), 3.3 (PFOS), 3.5 (PFTDA) and 4.1 (PFOSA), in L kg^-1 VS. A 100-year risk assessment using relative potency factors (RPFs) for 19 PFAS compounds revealed that cumulative soil risk quotients (RQs) remained below 1 for both EBPR and baseline scenarios, with median RQs declining from 0.61 in year-1 to 0.43 and 0.38 by year-100 for baseline and EBPR scenarios, respectively. Sensitivity analysis identified initial soil concentrations, PFAS half-lives, and application rates as key risk drivers. Although EBPR increased PFAS partitioning into solids, it did not result in elevated soil risk compared to conventional treatment. These findings support the safe reuse of EBPR-derived biosolids under current application guidelines and underscore the importance of integrated risk assessments in wastewater resource recovery.