Interpreting PFAS Fate to Inform Biosolids Management

APPLICABILITY Growing regulatory scrutiny of per- and polyfluoroalkyl substances (PFAS) is increasing pressure on utilities to reassess biosolids management strategies. Evolving state requirements, uncertain federal timelines, reduced land application options, and the absence of widely deployable PFAS destruction technologies complicate long-term planning for solids treatment and reuse. In this environment, utilities must make risk-informed decisions using incomplete and evolving data. This work describes how PFAS fate and transport data generated through full-scale solids treatment evaluations were interpreted and applied to biosolids management decisions under regulatory uncertainty. The effort draws on a detailed evaluation of PFAS behavior through anaerobic digestion and thermal hydrolysis conducted at Hampton Roads Sanitation District's (HRSD's) Atlantic Treatment Plant (ATP), followed by broader, system-wide PFAS characterization across HRSD's other treatment facilities. Together, these efforts illustrate how site-specific treatment data, when considered within a regional context, can support adaptive biosolids management decisions. DEMONSTRATED RESULTS AND OUTCOMES Methodology A focused, full-scale evaluation of PFAS fate through solids treatment was conducted at ATP during two distinct operating periods. The first occurred in 2019 and was characterized by conventional anaerobic digestion. The second followed implementation of thermal hydrolysis to provide the additional solids treatment capacity required to support consolidation of wastewater flows at ATP after closure of another major treatment plant in the service area. This ATP-specific work examined changes in PFAS concentrations and class distributions through treatment under different operating conditions. Analytical targets for this evaluation included PFAS precursors, intermediate compounds, terminal PFAS (the perfluoroalkyl acids - PFAAs), and total fluorine to assess both quantified and unquantified organofluorine contributions. In parallel with and subsequent to this work, HRSD conducted PFAS characterization of biosolids produced at its other treatment facilities to document PFAS concentrations across the system and compare PFAS levels among facilities. Results At ATP, anaerobic digestion during the 2019 operating period was associated with reductions in PFAS precursors and redistribution toward terminal PFAAs (Figure 1). Following implementation of thermal hydrolysis to support increased wastewater flows at ATP, influent PFAS concentrations and class distributions entering the solids train changed as a result of upstream flow consolidation and routing adjustments. Post-treatment PFAS class distributions observed after thermal hydrolysis and downstream anaerobic digestion differed from those observed during the earlier anaerobic digestion-only period (Figure 1). These differences reflect the combined influence of influent composition, treatment configuration, and matrix effects. Evaluation of PFAS at ATP also indicated that total fluorine concentrations exceeded fluorine attributable to quantified PFAS, highlighting the presence of uncharacterized organofluorine and limitations associated with PFAS-targeted analytical approaches. Separately, PFAS characterization across HRSD's other treatment facilities demonstrated variability in PFOS concentrations among biosolids produced across the region (Figure 2). These results indicate that PFAS-related risk is not uniformly distributed across the regional biosolids program and underscore the importance of facility-specific data when evaluating management options. Conclusions/Outcomes Insights from the ATP solids treatment evaluation, when considered alongside system-wide PFAS characterization and evolving regulatory context, informed adjustments to HRSD's biosolids management strategy (Figure 3). Following release of the U.S. EPA's Draft Sewage Sludge Risk Assessment for PFOA and PFOS in early 2025, biosolids routing was adjusted to maintain operational flexibility while addressing PFAS-related risk considerations. In parallel, HRSD initiated additional efforts to better understand PFAS loadings within the treatment process, including characterization of internal recycle streams and evaluation of post-processing steps such as curing and drying. Together, these efforts reflect an adaptive approach to biosolids management that integrates facility-specific data, system-wide characterization, and cautious interpretation of PFAS fate under conditions of regulatory and scientific uncertainty. RELEVANCE TO AUDIENCE This presentation will benefit utility managers, biosolids professionals, process engineers, and regulators involved in solids treatment, biosolids reuse planning, and PFAS risk management. The work is particularly relevant for utilities navigating PFAS-related decision-making in the absence of finalized regulatory thresholds or widely deployable treatment solutions. Rather than proposing a single technology or prescriptive approach, the presentation emphasizes how PFAS fate data from full-scale operations can be interpreted cautiously and applied alongside system-wide characterization to support practical biosolids management decisions. The lessons presented are broadly applicable to utilities facing similar challenges associated with influent variability, evolving regulations, and uncertainty surrounding PFAS behavior in solids treatment processes.