Continuing research is showing that the presence of per- and polyfluoroalkyl substances (PFAS) in the environment is leading to health and safety concerns. The challenge continues to be to find an effective technique for destroying PFAS, which has been proven to be difficult due to the exceptionally high stability of these compounds. The key focus is determining how to efficiently break the very stable, carbon-fluorine bonds present in PFAS.

A previous TLT article¹ discussed the use of a technique known as hydrodynamic cavitation that has been found to remove PFAS from wastewater in a cost-effective/energy-efficient manner. When wastewater containing PFAS is passed through a short channel within a hydrodynamic reactor, bubbles are generated due to a reduction in pressure. Once the bubbles reach a specific size, they will then collapse, releasing a significant amount of energy in the form of mechanical, thermal and chemical effects. These conditions lead to the degradation of several PFAS compounds by 34% on average over a 30-minute period. In pilot trials using wastewater from a European city, degradation rates averaged 50% for two PFAS, PFOS and PFNA. 

Another approach for destroying PFAS is incineration. But this method has been found to be ineffective due to the detection of PFAS remaining in ash, leachate and air near incineration plants. A third method, known as supercritical water oxidation (SCWO), is very effective in destroying all types of PFAS down to the extremely low parts-per-trillion detection limits. 

Rick Gillespie, chief commercial officer for Revive Environmental in Columbus, Ohio, says, “Permitted SCWO facilities are utilized to destroy PFAS using precisely, controlled high-temperature and high-pressure conditions. The SCWO process heats water beyond its critical point (above 374°C and 221 bar pressure) creating supercritical water that behaves like both a liquid and a gas. Under these extreme conditions, water becomes an excellent solvent that can break down even the strongest chemical bonds. When hydrogen peroxide is added as an oxidizer, it creates powerful chemical reactions that completely destroy PFAS molecules. SCWO facilities typically operate at temperatures between 500-600°C and pressures 200-300 times greater than atmospheric pressure, with treatment times of 30 seconds to two minutes. This process breaks down PFAS into harmless byproducts.”

Amy Dindal, PFAS program manager at Batelle Memorial Institute in Columbus, Ohio, says, “We have demonstrated the efficacy of SCWO in destroying PFAS when supercritical water conditions are maintained. Upon completion of PFAS destruction, the finished products are inorganic fluorine, water and carbon dioxide.”

One concerning source of PFAS is aqueous film-forming foams (AFFFs). They were widely used at U.S. Department of Defense facilities, industrial manufacturing facilities, fire departments and airports, among others, to deal with situations where fires could be produced from the use of combustible and flammable liquids. The PFAS-containing AFFF is very effective in controlling dangerous fires but has led to health and safety issues.

A new study has now been published to evaluate the effectiveness of SCWO in destroying PFAS in AFFFs.

PFAS Annihilator^®
Battelle developed a SCWO system known as the PFAS Annihilator. Figure 1 provides a schematic of how the PFAS Annihilator processes incoming PFAS-containing wastewater to eventually produce effluent that is compliant with local regulations. The PFAS Annihilator utilizes a reactor that is electrically heated by a sand bath and is constructed in a six-meter-long shipping container (see Figure 2).


Figure 1. The PFAS Annihilator system will convert PFAS-containing aqueous film-forming foam (AAAF) into a compliant discharge through the use of SCWO. Figure courtesy of Batelle Memorial Institute.


Figure 2. An operating PFAS Annihilator unit is shown. Figure courtesy of Batelle Memorial Institute.
In one demonstration of the technology, three types of AFFF used in commercial applications were diluted with deionized water to varying concentrations. The concentrations of target PFAS analytes were analyzed by liquid chromatography-tandem mass spectrometry both before and after processing through the PFAS Annihilator.
 
Analysis of the PFAS in the AFFFs indicated that the main components were fluorotelomer-based chemistries, which mostly contained fluorotelomer sulfonates and perfluoralkyl carboxylates. Gillespie says, “Based on the analysis of the incoming samples, the concentration of hydrogen peroxide required to provide the proper oxygen dosing was determined. This process was conducted to ensure that the stoichiometrically required concentration of oxygen was available to completely oxidize the sample.”

PFAS destruction performance was measured by calculating the ratio of the effluent concentration of PFAS or total organic carbon to the initial influent concentration of the samples. This is known as the destruction ratio. 

As part of this study, the AFFF were diluted at different ratios (6%-16%) leading to PFAS concentrations ranging from 1.9 to 64 ppm. After analysis of the effluent, destruction ratios of greater than 99.9% were achieved with the PFAS Annihilator. Analysis of the effluent indicated that there was 40% excess fluorine present, which suggests that the AFFFs contained a significant amount of unquantifiable nontarget PFAS precursors. This observation reinforces the ability of SCWO in destroying PFAS and its precursors. 

Due to the acidity of the effluent produced by SCWO, sodium hydroxide is utilized to ensure the effluent is pH neutral according to Gillespie. 

The PFAS Annihilator is now commercially available and is operated by Revive Environmental. Gillespie says, “We are operating two permitted facilities and processing PFAS wastes, including AFFF, and concentrated PFAS waste streams from landfills and industrial wastewater. Organizations trying to eliminate PFAS have faced liability issues when using commercial incineration or landfill disposal. The success of SCWO effectively ends any liability concerns.”

The study demonstrating the efficacy of SCWO in destroying PFAS found in AAAF can be found in a recent article.² Additional information can be obtained by contacting Katy Delaney, media relations manager at Battelle Memorial Institute, at delaneyk@battelle.org.