This New Spin on Decades-Old Technology Can Eliminate PFAS from Wastewater

PFAS (per- and polyfluoroalkyl substances) are a group of manufactured chemicals that have been produced since the 1940s. While they have myriad useful properties and manifest in a range of products from nonstick surfaces to personal care products, concerns over their use are growing. 

Current scientific research suggests exposure to PFAS may lead to a range of adverse health outcomes, including certain types of cancer, according to the U.S. Environmental Protection Agency.

PFAS are also known as “forever chemicals,” because they break down very slowly, if at all, in nature. Consequently, they continue to accumulate in greater concentrations in our environment, and by now they’ve even infiltrated our bloodstreams.

TriplePundit recently reported on new innovations aiming to mitigate the proliferation of PFAS by finding safer alternatives to them. But we need to find ways to remove existing PFAS, too. 

Though this is notoriously difficult, a North Carolina-based company found a way to eliminate these chemicals, somewhat by accident, in its effort to modernize wastewater treatment. “We got lucky in that we responded to the challenge to re-invent the toilet.” Sunny Viswanathan, VP and head of global sales at 374Water, told TriplePundit. 

Meeting that challenge, seeded by a grant from the Bill and Melinda Gates Foundation, focused the team on developing an optimal sanitation system which could be deployed in low-income parts of the world. In that effort, they sought ways to render waste sludges both useful and inert, leading them to consider supercritical water oxidation (SCWO) as a potential solution. 

Historically, SCWO was used to destroy persistent environmental damage resulting from chemical warfare, Viswanathan told us. But his team found the technology translated well to wastewater management, while coincidentally dealing with PFAS. 

What is supercritical water oxidation?

Water reaches the supercritical stage when both its temperature and pressure are increased to a point where it is no longer a liquid, nor is it a gas. Instead, as Viswanathan described it, “It goes into another ‘phase’ of water.”

Supercritical conditions for water arise at 374 degrees Celsius and a pressure of 218 atmospheres, or over 3,200 PSI (pounds-force per square inch). Once supercritical, water develops some interesting properties which are useful for processing organic waste. 

“Water as a liquid can dissolve salts, but it can’t dissolve organic matter,” Viswanathan explained. He used the example of adding pepper to water, which of course won’t dissolve. Why that’s the case has to do with the shape of the water molecule and consequent polarity of water, Viswanathan explained.

A water molecule has a V-shaped structure that includes a single oxygen atom with two hydrogen atoms attached. This structure affords it a positive and negative charge at an atomic level. Because of this, ionic salts can dissolve but, with very few exceptions, most organic matter — like, in this example, pepper — will be unaffected, Viswanathan explained. But the inverse is true under supercritical conditions.

“When you go supercritical, the shape of the water molecule literally changes — which means it loses its polarity and becomes a very good solvent of organic substances and a bad solvent of salts," Viswanathan said. "Salts will come out of the solution, but now your pepper will disappear. Your poop will disappear.”

And here is the important point. Because PFAS substances are organic, “Your PFAS will disappear," he said. 
 
In essence, under supercritical conditions, all the organic matter in wastewater — including PFAS — becomes completely dissociated. When air is added to the mix, an exothermic oxidation reaction takes place, completing the process.

“By introducing air, which has 21 percent oxygen, it will go after the carbon and make CO2 [carbon dioxide]. Once it removes carbon from the material, it becomes inorganic and will form salts and water — and energy, as it is an exothermic process," Viswanathan said.

The last point is important. An exothermic reaction is one which produces heat. 374Water’s AirSCWO system uses the heat produced by the exothermic reaction to perpetuate the process. So long as you continue to put waste sludge in, “the waste is the fuel," Viswanathan said. 

Putting the technology in the field to eliminate PFAS

With this simplified and abstract explanation of the science in mind, what does 374Water’s system look like in the field?

The company’s AirSCWO reactor units are packaged into 40-foot shipping containers (see above). The smallest reactor is a single container, while larger configurations would combine two or more. The company has plans for building-based systems, too.

Household or industrial wastewater comes into the container through a pipe at one end, and inside it, the contents of the pipe are pressurized and heated. Some external energy source is needed initially to start the system.

Wastewater sludge coming into the reactor is typically 80 percent water, and it’s the existing water content of the sludge which goes supercritical. Once that happens, all organic matter within gets dissociated and oxidized, which happens quite rapidly. “It takes four to 40 seconds to go from something that is completely toxic to something that is completely benign, clean and useful," Viswathanan said.

Indeed, it's useful in various ways. The system’s output is distilled water and useful minerals such as phosphorus which can be processed into fertilizer. Meanwhile, surplus energy from the exothermic reaction has the potential to be captured for electricity generation.

As for the PFAS, these are broken down into carbon, fluorine, hydrogen, oxygen and sulfur. As Viswathanan put it, “Just by exposing PFAS to supercritical conditions, you have actually destroyed them.”

What's next for this high-potential PFAS solution?

It’s taken 10 years for 374Water to go from concept to commercialization. The company, now traded on the NASDAQ stock exchange, will see the first of its commercial units go into operation in Orange County, California, next month. 
 
Expansion from there will be carefully undertaken, as 374Water plans to start at a scale that is manageable. But the addressable market is substantial.

Each 40-foot reactor can process up to 1 million gallons of wastewater per day. Of the roughly 17,000 wastewater facilities treating household, commercial and some industrial wastewater in the U.S., only 9,000 of these are in the 1-million-gallon range. In theory, in combination with the larger reactors the company has planned, it would have the capacity to service all of these facilities.

That said, scalability relies to a large extent on the right incentives. The state of Maine offers one such example. 

Because of PFAS, the state has banned the application of wastewater sludge on the land, an increasingly common practice on U.S. farms. That shift means water treatment plants have to spend up to $200 per ton to send their wet sludge out of state. Since 374Water’s method eliminates PFAS and produces no waste sludge, the system would provide a huge cost avoidance opportunity under these circumstances. Consequently, municipal sanitation providers could see payback on a reactor in as few as three years, Viswathanan said. 

As a final point, he emphasized the long-term opportunity this way. “The technologies we are relying on now for waste treatment are nearly 100-year-old, antiquated technologies. We now have a system that is capable of not only treating the waste, but also destroying the recalcitrant waste and taking it out of the ecosystem.”