From Sludge to Soil

Per- and polyfluoroalkyl substances (PFAS) are subjects of increased regulatory measures, however, current methods and limits focus mainly on a few legacy compounds, such as PFOS and PFOA. Novel PFAS substances that can transform into legacy PFAS, i.e. precursors, are largely excluded from both analysis and regulation, despite their potential risks. In Sweden, 85% of sewage sludge is applied to soil, mainly on farmland. New REVAQ guidelines set limits for 4 and 22 specific PFAS compounds respectively, but do not account for precursors despite several studies indicating that they represent a dominant portion of the PFAS content. This study aimed to assess the PFAS content in digested and undigested sewage sludge from five wastewater treatment plants using both target analysis (49 compounds) and the Total Oxidizable Precursor (TOP) assay. Leachability was evaluated using a leaching test, and short-term degradation of precursors in soil was monitored over four weeks. Results showed that the TOP assay revealed significantly higher PFAS concentrations than target analysis alone, particularly in undigested sludge, indicating a high content of precursors. The dominant PFASs included PFOS and PFHpA, using target analysis and TOP Assay, respectively. No degradation of precursors was observed over the incubation period. The findings suggest that precursors make up a substantial portion of total PFAS in sludge and that current target-based analyses underestimate true PFAS levels. The TOP assay is recommended for inclusion in standard PFAS monitoring protocols for sewage sludge.