PFAS in biosolids: Accumulation characteristics and fate profiles after land application

Abstract

The land application of biosolids as a management practice is considered a beneficial use for improving crop yield and reducing the need for other fertilizers. PFAS enter wastewater treatment plants through collection networks, including industrial discharges, the use of PFAS-containing products, and runoff. Therefore, PFAS may be present in biosolids derived from sewage sludge. The objectives of this study were to evaluate PFAS levels in biosolids samples collected at two wastewater treatment plants operated by the Miami Dade Water and Sewer Department (MDWASD): (1) the South District Wastewater Treatment Plant (SDWWTP) which received landfill leachate and (2) the Central District Wastewater Treatment Plant (CDWWTP). Sludge samples were collected after thickening, anaerobic digestion, and dewatering processes. The samples were subjected to batch leaching tests for 30 days. After the leaching tests, the PFAS levels in the liquid and solid fractions were analyzed for 40 PFAS. The findings show that during the aeration process (i.e., activated sludge process), PFAS are removed from the wastewater and accumulate on the solids. When the thickened sludge is digested, some PFAS are released to the liquid phase as the volatile solids decompose. During the dewatering process by centrifugation, PFAS that are partitioned to the liquid phase are removed, reducing PFAS content in the dewatered biosolids. Of the 40 PFAS analyzed, 24 were detected in leachate or solid residue samples. Samples from the SDWWTP had higher levels of PFAS due to the contribution from landfill leachate discharged to this facility. The partitioning of PFAS between the liquid phase and solid residue after 30 days of mixing indicates that the majority of PFAS in the biosolids are highly soluble and have a high tendency to be mobilized (by runoff, irrigation, precipitation) after land application. The fate profiles of PFAS biosolids were evaluated in terms of their solubility and retardation characteristics.

Keywords: Biosolids; Dewatering; Digested sludge; PFAS fate; Poly- and perfluoroalkyl substances; Wastewater treatment.