A wastewater treatment plant operated by Orange County (Calif.) Sanitation District is the site of the first installment, at a commercial facility, of a new supercritical water oxidation (SCWO) technology capable of producing clean water and minerals from waste slurries. The clean-technology startup 374Water Inc. (Durham, N.C.; ), originating from Duke University, developed the technology, known as AirSCWO. Supercritical water processes for waste destruction involve elevating water above its critical point (heated above 374°C and pressurized above 221 bars) to take advantage of the unique properties of water under those conditions. “Water beyond its critical point exhibits properties that are opposite those of water at ambient temperature and pressure,” explains Naomi Senehi, applications engineer at 374Water. “For example, organic materials and gases like oxygen are highly soluble in supercritical water, while inorganic materials are not.”

In the AirSCWO process, a pumpable slurry of waste is mixed with ambient air and preheated before entering a pipe reactor, where rapid oxidation reactions dissociate organic waste into inorganic compounds, CO₂ and water. A pressurized stream of vaporized water exits the reactor and enters an expander, which generates distilled water and electricity, while a liquid stream from the reactor carries minerals, such as phosphates, sulfates and metal oxides for potential recovery. Excess heat, generated by the exothermic oxidation reactions, is used to preheat the slurry.

374Water’s process aims to overcome certain technical challenges associated with existing SCWO systems. The company holds patents on a corrosion-resistant alloy for the reactor that allows the reactor to withstand corrosion that has proved problematic for some previous SCWO efforts. Also, the AirSCWO process is engineered for the use of ambient air as the oxidant, eliminating the need for compressed oxygen, Senehi points out, which reduces cost and improves safety. AirSCWO units are housed in 40-ft shipping containers, which can be stacked together to increase scale. Each unit is capable of processing 6 wet tons of sludge per day, Senehi says.