Destruction of Military Spent Media PFAS with SuperCritical Water Oxidation(SCWO)
Destruction of Spent Media PFAS with Supercritical Water Oxidation (SCWO)
Per- and polyfluoroalkyl substances (PFAS) are a large, complex group of manufactured organofluorine chemical compounds used in the production of heat, oil, stains, grease, and water resistant products.
The carbon-fluorine bond that forms the main structure of PFAS compounds is one of the strongest, making it nearly impossible to naturally degrade in the environment. PFAS compounds transport and contaminate soils, waters and are known to bioaccumulate.
The historical use of aqueous film-forming foams (AFFF) in military use and training exercises has resulted in at least 710 known contaminated military sites in the US requiring clean up. In 2022, the National Defense Authorization Act (NDAA) recommended the Department of Defense (DoD) ban PFAS waste incineration.
DoD started investigating alternative destructive processes and technologies, that does not qualify as incineration, to aid in their site remediations. Supercritical Water Oxidation is one of those technologies.
Supercritical Water Oxidation (SCWO) is a physical thermal process that is powered by water above its critical point (374°C and 221 bar) and an oxidant. Under these conditions, in the presence of oxygen, all organics rapidly and completely oxidize to form carbon dioxide, clean water, and inorganic salts, essentially converting organic wastes into water, energy and nutrients in a matter of seconds. Named “3rd Generation SCWO” by the U.S. EPA, 374Water’s AirSCWO™ process uses a scalable tubular reactor, and air rather than pure oxygen to achieve oxidation. Multi-stream feed- stock mixing and preheating prevents fouling and charring, and an efficient energy recovery system converts thermal heat generated in the process to electricity that self-powers the unit.
Typical site clean-up is undertaken by concentrating PFAS contaminated soil washings or groundwater into absorptives such as Granular Activated Carbon (GAC) or Anion Exchange Resin (AIX). These media are excellent at adsorption and sequestering PFAS, but without destruction the medias only allow for the aggregation and movement of PFAS from one place (i.e. groundwater) to another (landfill).
In a joint study with DoD supported by WSP, 374ater’s AirSCWO process was used to destroy samples of spent contaminated GAC and AIX collected from a full-scale PFAS treatment systems for groundwater remediation. All spent media samples contained high amounts of PFAS per kilogram of spent media and were processed separately through the AirSCWO system. Samples of the system effluents were grabbed at regular intervals and analyzed for PFAS via EPA 1633. The analysis showed that the spent media and PFAS contained within were completely mineralized to fluoride, water, carbon dioxide (CO2) and a negligible amount of residual salts.
Results are presented by courtesy and in collaboration with WSP, USA.
| Spent IX Feedstock | Effluent | Effluent control | Corrected effluent | Rate-based removal | Rate-based (corrected) removal | |
| ng/kg | ng/L, ppt | ng/L, ppt | ng/L, ppt | % | % | |
| Total PFCA | 260,218.52 | 1.04 | 1.02 | 0.02 | 99.97% | 100.00% |
| Total PFSA | 1,092,234.96 | 70.83 | 98.674 | 7.256 | 99.52% | 100.00% |
| Precursors | 33,256.92 | 5.13 | ND | 1.39 | 98.85% | 99.70% |
| Short chain( C<6) | 84,383.95 | 0.97 | 0 | 0.97 | 99.91% | 99.90% |
| Total | 1,385,892.83 | 77 | 104.664 | 8.666 | 99.59% | 99.95% |
| Spent GAC Feedstock | Effluent | Effluent control | Corrected effluent | Rate-based removal | Rate-based (corrected) removal | |
| ng/kg | ng/L, ppt | ng/L, ppt | ng/L, ppt | % | % | |
| Total PFCA | 90,451.61 | 99.24 | 51.417 | 47.823 | 91.82% | 96.06% |
| Total PFSA | 103,805.09 | 344.91 | 177.94 | 166.97 | 75.22% | 88.01% |
| Precursors | 20,097.50 | 103.77 | 76.52 | 29.6 | 61.50% | 89.02% |
| Short chain (C<6) | 41,436.58 | 114.69 | 51.66 | 63.03 | 79.36% | 88.66% |
| Total | 214,354.20 | 547.92 | 305.88 | 244.393 | 80.94% | 91.50% |
PFAS concentrations in the SCWO system effluents were either non-detectable or were in the nanogram-per-liter range (ng/L). This study demonstrated that SCWO is an effective PFAS destruction technology for solid wastes, such as spent GAC and AIX, derived from site clean up.
What next for Supercritical Water Oxidation?
In Q4 2023 374Water, in collaboration with WSP, will deploy an AirSCWO 6 to Fort Sumter in Charleston, SC. Over the course of one month the system will destroy Ion Exchange resin that were produced by the bases active groundwater pump and treat facility. The IEX destroyed by the system of PFAS that was removed from Fort Sumter’s drinking water.
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