Quantifying Supercritical Water Oxidation Efficiency Treating PFAS Laden Sludge, Ion Exchange Resin and Aqueous Film Forming Foam

Abstract Rising concern of Perfluorinated alkylated substances (PFAS) contamination of our ecosystem has sparked interest in this pollutant 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 elimination 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 waste substrates 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 physical-thermal 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, organics are fully soluble in supercritical water, and with the addition of oxygen, all organics rapidly and completely oxidized to form carbon dioxide, clean water, and inorganic salts. 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 prolific in our ecosystem. 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 was 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. Introduction - Section removed to fit 9,000 character count - Material and Method An AirSCWO 1 system was used for these studies, this system is the smallest in scale, and designed to continuously treat aqueous waste of about 1 m3 or 1 wet ton of sludge at 10-20% dry solids content per day (Figure 2). The system consists of a 25-gallon feed hopper integrated with a homogenization tank with internal recirculation to keep the slurry in suspension. A high-pressure pump transfers the homogenized waste to the SCWO reactor via a heat exchanger, to preheat the incoming slurry. A high-pressure compressor provides air, also preheated using a heat exchanger, followed by a plug flow once through SCWO reactor. Three cooling heat exchangers and depressurizing equipment bring the outputs to near ambient conditions. For treatability studies, a minimum of 5 gallons is required for a representative run, but larger volumes of wastes are needed to simulate real world operating conditions. A typical run lasts about 12-36 hours, during which the process operates at different conditions, until steady state performance is established, at which point samples of all effluents (liquid, gaseous, solids) are collected and analyzed to determine treatment performance, including fate of N, P, organics and key metals. Most operating parameters are monitored in real time while others, such as trace and emerging contaminants are analyzed off-line using grab sampling. Process operating parameters were monitored for energy balances and energy recovery, these data are used to develop detailed modeling for scaled systems. In an effort to demonstrate the efficacy of SCWO for PFAS elimination for varied input slurries, three wastes from three different sources were selected for the studies. The three wastes are (i) municipal wastewater sludge, which was stabilized using lime but contained high concentrations of PFAS; (ii) full strength aqueous film forming foam (AFFF) and (iii) ion exchange (IX) resin from a pump and treat system, which is sequestering PFAS compounds in the adsorption media. These wastes all pose different challenges, for example, the need for AFFF to be diluted, making it calorie poor and requiring cofuel; or IX wastes being generated once every 6 months or so, requiring the waste to be stored and processed over the period between changes. Results The first study was treating PFAS laden lime stabilized sludge, results for which show that greater than 99% conversion of Chemical Oxygen Demand (COD) to energy, and elimination of 99.95% for PFOA and 99.99% for PFOS. Table 1 shows that the removal across all carboxylic acid (PFCAs) compounds was greater than 99.9%, and across all sulfonic acid (PFSA's) compounds was greater than 99.99%. This waste had no detectable precursors and SCWO demonstrated excellent elimination rates for short chained (6 Carbon and less) PFAS compounds. The process achieved greater than 99.99% elimination across other 24 derivatives of PFAS found in the sludge. In this study, PFOS was the predominant compound and constituted 88% of the influent PFAS but only 3% of the effluent concentration. In the second study, the AirSCWOTM process treated AFFF, diluted down to about 30 times. About 25 L of dilute AFFF was processed (Krause, M. et al., 2022), only 9 compounds of the 28 analyzed were detected in the influent. PFOS was reduced over 99.99%, and PFOA nearly complete at 100%. Table 2 shows that the removal across all carboxylic acid (PFCAs) compounds was greater than 99.9%, and across all sulfonic acid (PFSA's) compounds was greater than 99.99%. This waste had detectable precursors and greater than 99.9% removal was achieved. Greater than 99.99% elimination rates were demonstrated for short chained PFAS compounds. Total targeted PFAS elimination was established at 99.99% over the course of the experiment. In this study, PFOS was again the predominant compound, and constituted 72% of the influent PFAS and 45% of the effluent concentration. The third study was a demonstration aiming to process spent ion exchange resin, a waste byproduct of water treatment, specifically targeting PFAS compounds. The results from the study treating IX demonstrated that PFOS was reduced over 98%, and PFOA nearly complete at 100%. Table 3 shows that the removal across all carboxylic acid (PFCAs) compounds was greater than 99.9%, and across all sulfonic acid (PFSA's) compounds was greater than 99%. This waste had detectable precursors and nearly 99% removal was achieved. Greater than 99.9% elimination rates were demonstrated for short chained PFAS compounds . Total targeted PFAS elimination was greater than 99.5% over the course of the experiment. In this study, both PFHXS and PFOS were the predominant compounds, and constituted 71% of the influent PFAS and 90% of the effluent concentration. The reactor residence time within the SCWO reactor ranged between 68 seconds for all the studies. Greater destruction of PFAS can be achieved by extending the reaction time (e.g., to greater than 10 seconds) to the desired log-reduction of specific PFAS compounds, especially in highly concentrated wastes such as aqueous film forming foam and ion exchange. The non-targeted PFAS elimination was verified at laboratory scale at Duke University 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 emission. (Deshusses, M.A., 2020) Summary As the studies demonstrate, supercritical water oxidation using the AirSCWOTM system can be effectively and efficiently utilized to achieve near complete PFAS elimination in various PFAS contaminated substrates, rendering the process outputs - i.e., water and air free of contaminants. Other studies involving aqueous slurries including pharmaceuticals, PFAS concentrates and microplastic have been successfully treated using AirSCWOTM, demonstrating the versatility of the process to treat and eliminate recalcitrant wastes. With full scale units being implemented, these studies offer valuable insights to further optimize the process and achieve even greater performance and cost effectiveness.