PFAS Elimination Rates in Sludge, Ion Exchange Resin, and Aqueous Film Forming Foam Using Supercritical Water Oxidation: A Quantitative Review

Rising concern of Perfluorinated alkylated substances (PFAS) contamination of our ecosystem has sparked interest in this class of pollutants as it pertains to water and waste management. While PFAS sources are numerous, it is widely believed that firefighting foam and extensive industrial uses are common pathways for PFAS compounds to proliferate into our ecosystem. In an attempt to better understand the fate and transport of PFAS compounds undergoing supercritical water oxidation (SCWO), a one (1) wet ton per day scale SCWO system was employed to study the treatment efficiencies of this process. There is a pressing need to develop and validate advanced treatment technologies that can destroy PFAS in a variety of substrates. In this paper, we report on how three distinct PFAS containing wastes from three different sources were treated using SCWO process. The waste includes lime stabilized sludge from a municipal wastewater resource recovery facility; aqueous film forming foam (AFFF) from a DoD facility; and spent ion exchange resin from a ‘pump and treat’ water treatment facility. Supercritical water oxidation or SCWO for short, is a physicalthermal process that relies on the unique reactivity and transport properties of water above its critical point of 374 °C and 218 atm. At these conditions, organic compounds and oxygen are highly soluble and often completely miscible in supercritical water (Phillip E. Savage 1999), and with the addition of oxygen, they rapidly and completely oxidized to form carbon dioxide, clean water, and inorganic salts (Thomason, T.B. et al., 1984)( Marrone, P.A. et al., 2004). The studies examined a variety of PFAS compounds, both targeted and non-targeted, but most specifically focused on PFOA and PFOS. These two compounds are the most studied PFAS compounds, they are highly toxic and most commonly detected in our ecosystem (McCarthy, C. et al., 2017). SCWO, on average, was able to eliminate 99.95% of PFOA and 99.99% of PFOS across all waste substrates, and greater than 99.9% elimination of all other PFAS compounds combined. Non-targeted PFAS were accounted for using laboratory scale verification tests by employing fluorine mass balance. No hydrogen fluoride was found in the effluent gas, and all the fluorine from the destroyed PFAS was accounted for as fluoride in the effluent water, with no low molecular weight or volatile PFAS compounds in the emission. This paper will cover the pre-treatment, pre-preparation and supercritical water oxidation requirements for the various waste inputs to ensure complete destruction of recalcitrant wastes without producing any undesired byproducts. The studies produced valuable data and design parameters to support design and deployment of SCWO for real world applications.