OC San supercritical water oxidation (SCWO) building location. Courtesy of OC San.
The practice of treating wastewater has existed since the 19th century, beginning with centralized designs to keep sewage discharge out of clean water sources. Around this time, efforts to separate storm runoff from wastewater was also introduced in many communities.
Fast forward to today, decades of focus on the environmental impact of waste treatment and disposal have led to advanced technologies and dedicated efforts to remove contaminants and protect our waterways.
However, most of today’s widely used processes, including anaerobic digestion and activated sludge, do not break down all chemicals and pharmaceuticals. Some of these compounds of emerging concern (CECs) have the ability to evade mainstream treatment technologies and return to the environment.
Supercritical water oxidation, also known as SCWO, is a process that has been around for 30 years. 374Water’s proprietary AirSCWO™ technology relies on the power of supercritical water oxidation to offer a reliable, cost-effective, and sustainable solution for water and wastewater treatment to eliminate nearly all CECs.
SCWO process. Courtesy of 374Water.
THE CHALLENGE
The US EPA acknowledges the risk of CECs in the environment and the need for reliable treatment and disposal. Their 2019 biosolids report gathered information from 2,200 large U.S. facilities, finding that between 4.75 and 7 million dry metric tons of biosolids were produced annually across the country.
Some biosolids may contain very small amounts of CECs that aren’t broken down by biological treatment processes, including pathogens, inorganic chemicals, pharmaceuticals, microplastics, and PFAS. These man-made chemicals are all around us, found in our blood and household dust, and over the last 40 years have accumulated in our lives and never fully break down.
ADDITIONAL SCWO OPPORTUNITIES
Another opportunity for improvement is greenhouse gas emissions from conventional solid waste treatment. The EPA estimates only 10% of food waste generated is managed by co-digestion and anaerobic digestion. A major byproduct of anaerobic digestion is methane, which accounts for 50-75% of the biogas produced during the process.
Although methane is often captured and used as a biofuel, methane escapes during processing and transport. Methane has a Global Warming Potential 21-28 times higher than carbon dioxide, which means that one ton of methane emissions over a given period of time, will absorb 21-28 more energy than 1 ton of carbon dioxide emissions.
In addition, small to medium water resource recovery facilities operate as end-of-pipe treatment, using an activated sludge process to degrade organics and nitrify ammonia to nitrite and nitrate. While some plants perform denitrification or recover ammonia and nutrients, most facilities do not. These approaches are very energy intensive, using 2300 kWh of power per one million gallons of wastewater treated – roughly 3% of the nation’s energy load.
WHAT IS SUPERCRITICAL WATER OXIDATION?
Supercritical water oxidation offers unique value since the physical-thermal method can treat a wide range of organic wastes, in particular wet wastes, such as biosolids, sludges, food waste, and chemical wastes.
Supercritical water oxidation is a transformative technology that taps into the unique properties of water at its critical point: 374°C and 221 bar (705°F and 3200 psi). In these conditions and with oxygen present, organics are rapidly converted to water, inert minerals, gasses, and reusable heat. In many cases, supercritical water oxidation can eliminate more than 99.9% of organic compounds.
Supercritical water oxidation has received increased attention as the challenges caused by CECs grow. The ability to eliminate nearly all traces of contaminants in residuals and biosolids is groundbreaking. However, since its introduction, commercial development of supercritical water oxidation has been slow due to technical challenges.
The complex nature of a high-pressure, high-temperature process can lead to corrosion, plugging and fouling. These logistical issues have driven a need for change to critical design elements of supercritical water oxidation processors – including experimenting with changes to the reactor’s materials, shape, and size. What’s more, the pure oxygen required for prior design attempts must be handled and stored safely, creating a need for expensive safety designs and control processes.
Early attempts to commercialize supercritical water oxidation were also sidelined by unrealistic expectations that it could produce power at a competitive retail rate (approximately $0.03-0.05 per kWh). This goal proved unobtainable, making supercritical water oxidation commercialization and adoption more difficult.
374Water’s AirSCWO™ Technology. Courtesy of 374Water.
A PIONEERING METHOD
Using technology invented at Duke University, 374Water has developed AirSCWO™, a patented waste processing technology that is the world’s first self-sustaining, continuous flow supercritical water oxidation omniprocessor. AirSCWO™ was labeled “the third generation of supercritical water oxidation” during recent PFAS elimination testing performed by the EPA PFAS Innovation Technology Team (PITT).
The technology is in commercial production and has the potential to shift wastewater management from traditional centralized, end-of-pipe treatment and disposal to innovative, decentralized pollutant elimination and resource recovery systems.
The AirSCWO™ process is fast, clean and the units serve as net energy producers leveraging a unique, proprietary energy recovery unit called the “Expander.” This allows air to serve as the oxidant, a much safer alternative than pure oxygen and chemicals.
Further, internal mixing brings waste to supercritical conditions rapidly, minimizing corrosion and risks of waste charring and plugging. With AirSCWO, all organics and CECs found in the sludge are reduced mainly to water, carbon dioxide, and inorganics in seconds with the heat of the process recovered to pre-heat the influent.
The system features a multi-stream tubular reactor configuration, which enables energy efficient and sustainable treatment while helping overcome corrosion and plugging.
FUTURE OF RESOURCE RECOVERY
To date, 374Water’s AirSCWO™ solution has been demonstrated with success at industrial scale in over 100 test campaigns, and over 1000 hours of operation.
In one case, AirSCWO™ was installed in a small-scale facility in Maine to treat biosolids and test the extent of PFAS elimination. During the trial, the system was found to effectively treat lime stabilized sludge contaminated with PFAS. AirSCWO™ successfully destroyed PFAS below the proposed regulatory limits without any signs of enhanced corrosion.
In this demonstration, the influent concentrations of PFOS and PFOA were 110,000 ng/l and less than 6200 ng/l, respectively. AirSCWO™ was found to reduce these concentrations to 0.65 ng/l and 3.15 ng/l, respectively – confirming a 99.95% PFAS elimination capability.
When deployed at a WRRF, AirSCWO™ has the potential to transform the facility from one that solely treats wastewater to a more holistic resource recovery site.
Not only can AirSCWO™ reduce the volume of solids by more than 97%, but the process offered by 374Water generates energy and clean water while recovering safe minerals and resources that can be reused or sold.
ABOUT THE AUTHORS
Sudhakar Viswanathan is an environmental engineer and Vice President of Wastewater Residuals and Biosolids.
Doug Hatler is an environmental engineer and Chief Revenue Officer for 374Water, a global, Durham, North Carolina-based social impact, cleantech company.
My Profile
