Water above 374 °C and 22.1 MPa, becomes supercritical, a special state where organic solubility increases and oxidation processes are accelerated. Supercritical water oxidation (SCWO) has been previously shown to destroy hazardous substances such as halogenated compounds. Three separate providers of SCWO technology were contracted to test the efficacy of SCWO systems to reduce per- and poly-fluoroalkyl substances (PFAS) concentrations from solutions of dilute aqueous film-forming foam (AFFF). The findings of all three demonstration studies, showed greater than 99% reduction of the total PFAS identified in a targeted-compound analysis, including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). PFOS was reduced from 26.2 mg/L to 240 μg/L, 30.4 mg/L to 0.310 μg/L, and 190 mg/L to 8.57 μg/L, from the Aquarden, Battelle, and 374Water demonstrations, respectively. Similarly, PFOA was reduced from 930 to 0.14 μg/L, 883 to 0.102 μg/L, and 3,100 μg/L to non-detect in the three evaluations. Additionally, chemical oxygen demand of the dilute AFFF was shown to reduce from 4,750 to 5.17 mg/L after treatment, indicating significant organic compound destruction. In one demonstration, a mass balance of the influent and effluent found that the targeted compounds accounted for only 27% of the generated fluoride, suggesting that more PFAS were destroyed than measured and emphasizing the limitations of targeted analysis alone. As a destructive technology, SCWO may be an alternative to incineration and could be a permanent solution for PFAS-laden wastewaters rather than disposal by injection into a deep-well or landfilling. Additional investigation of reaction by-products remains to be conducted for a complete assessment of SCWO's potential as a safe and effective PFAS treatment technology.
Keywords: PFAS; PFOS; Supercritical water oxidation; aqueous film-forming foam; fluoride.