N. Ojeda, E. Hart, K. Liberty, A. Wells Carpenter
AxNano,
United States
Keywords: solids, SCWO, technoeconomic, pilot, destruction
Summary:
The rapid expansion of sorption‑based treatment systems for per‑ and polyfluoroalkyl substances (PFAS) in groundwater and drinking water applications is generating increasing quantities of PFAS‑laden solid residuals, including spent granular activated carbon (GAC), ion‑exchange resin, biosolids, and membrane‑treatment concentrates. Current disposal pathways—landfilling and thermal treatment—pose significant limitations. Landfilling retains PFAS in the environment with uncertain long‑term risks, while thermal destruction technologies face challenges related to incomplete mineralization, high energy demand, and the logistical and economic burdens of transporting large waste volumes off‑site. There is a clear need for modular, onsite destruction solutions capable of treating solid PFAS wastes reliably, efficiently, and without extensive preprocessing. Supercritical water oxidation (SCWO) offers a promising path to complete PFAS mineralization but has historically been constrained by its inability to handle solids and heterogeneous waste slurries while maintaining stable continuous operation. To address these limitations, AxNano developed a proprietary Automated Feed System (AFS) designed to preprocess and continuously deliver PFAS‑bearing solids and slurries to a pilot‑scale SCWO reactor. Two pilot demonstration programs were conducted to evaluate whether integrating the AFS with SCWO enables stable continuous processing of realistic, variable solid waste streams at throughputs relevant for future full‑scale deployment. The AFS homogenizes and size‑reduces solid wastes to SCWO‑compatible particle sizes (≤250 µm) and solids loadings (≤15 wt%), creating a consistent feedstock capable of supporting uninterrupted reactor operation. Eleven integrated AFS–SCWO tests were performed using five real waste materials sourced from full‑scale groundwater pump‑and‑treat systems, surface‑water remediation sites, and centralized wastewater treatment facilities. Wastes included biosolids, spent GAC, exhausted ion‑exchange resin, and reverse‑osmosis (RO) reject streams with high suspended‑solids content. Analytical characterization encompassed PFAS via EPA Method 1633, metals, VOCs, TOC/COD, anions, and gaseous emissions, alongside technoeconomic assessments comparing SCWO to existing disposal practices. Across all pilots, the AFS successfully produced stable SCWO‑compatible feeds, enabling continuous reactor operation at slurry flowrates up to 3 GPM. The system rapidly reduced GAC particle sizes from approximately 4 mm to <45 µm—achieving in hours what historically required months of milling—eliminating plugging events that previously forced shutdowns. PFAS destruction efficiencies reached 99.995% for GAC, with a strong correlation between waste BTU content and PFAS removal (R² = 0.98). High‑salinity RO reject did not negatively impact reactor stability or corrosion, and PFAS concentrations in gas‑phase emissions were at or below background levels. TOC and COD reductions exceeded 90–99% across all waste types. Economic modeling indicates that onsite SCWO utilizing the AFS can reduce long‑term operating costs by 77–83% for facilities capable of continuous operation, primarily due to avoided transportation and reduced diesel consumption. Pilot‑scale results confirm that coupling AxNano’s AFS with SCWO overcomes historical barriers to treating solid PFAS wastes and offers a viable pathway toward scalable, onsite PFAS destruction. Ongoing work includes comparative technoeconomic analyses of SCWO versus thermal regeneration and continued optimization of feed metering, temperature control, and vapor/foam management to support full‑scale deployment.