PFAS Destruction Through Supercritical Water Oxidation
In an era of increasingly aware and health-conscious people, it has been well known for a while that some easy to use or easy to consume products such as Teflon pans and packaged food can be detrimental to human health. A relatively recent addition to products of concern are PFAS. Per- and polyfluoroalkyl substances, also known as PFAS, or commonly referred to as “forever chemicals” are a group of man-made chemicals that include a combination of individual chemicals such as PFOA, PFOS, and GenX. These chemicals have been around since the 1940s and are increasingly present in the environment and in the human body – because they never break down and thus accumulate over time. As aforementioned, PFAS can be found in packaging for food, sometimes the packaged food in itself if it is grown in PFAS contaminated soil or water, even commercial household products such as nonstick pans, pizza boxes, stain and water-repellent fabrics, and most importantly in drinking water. 
So why does this sound so scary? There is evidence that exposure to PFAS can lead to adverse health outcomes in humans. Studies indicate that PFOA and PFOS (two PFAS species commonly found in water) can cause reproductive and developmental, liver and kidney, and immunological effects in laboratory animals. Both chemicals have caused tumors in animals. The most consistent findings are increased cholesterol levels among exposed populations, with more limited findings related to – low infant birth weights, effects on the immune system, cancer (for PFOA), and thyroid hormone disruption (for PFOS). Studies suggest that up to 110 million Americans could have PFAS in their water. 
While PFAS remain largely unregulated at the federal level, the federal government has taken steps to develop guidelines to protect human health from PFAS contamination. In 2009, the EPA set non-binding advisories for PFOA at 400 parts per trillion (ppt) and PFOS at 200 ppt which was later revised to 70 ppt for both compounds. In February 2019, the EPA issued its PFAS Action Plan that after several recent revisions, called for long- and short-term actions including – issuing preliminary determinations to regulate PFOA and PFOS in drinking water, issuing proposals to prevent manufacturing and/or importing PFAS chemicals in surface coatings, publishing new methods to accurately test drinking water for the presence of PFAS, among several other actions.  Over the past several years, 17 states including California, Massachusetts, New York, Minnesota, Wisconsin, and Vermont have also either implemented their own PFAS rules and regulations or have announced their intentions to do so. The incoming Biden administration has set its intentions to tackle PFAS by setting enforceable limits in the Safe Drinking Water Act. 
These regulatory developments are unequivocally supported by technology companies in developing and implementing technologies to eliminate PFAS, predominantly including – granular activated carbon technology, ion-exchange resins, and reverse osmosis.  However, each of these technologies have significant caveats. Granular activated carbon has been long used to remove a variety of contaminants from water by making water flow through packed beds of the material so that the carbon absorbs compounds such as PFAS. However, its strength poses as a limitation as well since other compounds can also be removed by the granular carbon, thus reducing their capacity for PFAS. Additionally, smaller-chain PFAS molecules are often not well removed.
In the second technology, ion-exchange resins are also installed in packed beds. The working principle is that positively charged resins bind the negatively charged PFAS molecules as they pass through. However, this technology is defeated by the same caveat as that of the granular carbon. Reverse osmosis on the other hand, has a different working principle. Here, contaminated water is pressurized, forcing it through a semipermeable membrane that filters out PFAS. However, reverse osmosis is energy intensive. Additionally, the method strips drinking water of its essential minerals. In all cases, the PFAS are not actually destroyed but instead concentrated on a matrix (the granular activated carbon or ion exchange resin) or in a brine (for reverse osmosis), which requires treatment or disposal.
Supercritical Water Oxidation, on the other hand, is an advanced oxidation method that can treat a wide range of organic wastes, in particular wet wastes and slurries such as biosolids, sludges, agricultural wastes, chemical wastes and many more. It is a transformative technology that utilizes the unique properties of water above its critical point: 374°C and 221 bar (705 °F and 3200 psi). At these conditions, in the presence of oxygen, organics are rapidly converted to water, inert minerals and gases, and reusable heat with generally more than 99.9 percent destruction of all organic compounds.
The magic number of 374°C, which is the critical point of water, is the foundation of the company name ‘374Water’. The company’s name reflects the innovative engineering of 374Water’s waste processors, now known as the AirSCWOs. 374Water is the first company in the world to develop a SCWO sanitation treatment system with patented self-sustaining reactor technology. 374Water offers prefabricated, compact and efficient waste processors that convert organic waste into clean water within seconds. Invented by engineers from Duke University in the United States, the AirSCWO provides a small footprint, clean, energy-neutral and sustainable solution at an affordable cost.
CaseStudy: Municipality–elimination of PFAS in Lime Stabilized Sludge
The 374Water solution has been successfully demonstrated at industrial scale in over 100 test campaigns and over a 1000 hours of operation. One such trial was conducted with biosolids from a small-scale facility in Maine with an objective to test the extent of PFAS elimination from lime stabilized sludge that was heavily contaminated with PFAS. A successful trial demonstrated that the SCWO system can effectively treat contaminated sludge and destroy PFAS below the proposed regulatory limits with no sign of enhanced corrosion. The influent concentrations of PFOS and PFOA were 110,000 ng/l and less than 6200 ng/l, respectively, and were sharply reduced to 0.65 ng/l and 3.15 ng/l, respectively highlighting a 99.95 percent PFAS destruction capability.
374Water’s SCWO technology is truly disruptive as it is one of the few technologies that will truly destroy PFAS. With greater concerns about emerging contaminants such as PFAS, 374Water’s SCWO technology can contribute to a cleaner and healthier environment.
Marc A. Deshusses