Supercritical Water Oxidation (SCWO) is an advanced thermal oxidation technology operating above water’s thermodynamic critical point (374.15°C and 22.1 MPa / 221 bar). At these extreme conditions, water ceases to exhibit liquid-gas phase boundaries; its density drops to 100–300 kg/m³, and its dielectric constant collapses from 80 to below 5. Organic molecules and oxygen become 100% miscible in the supercritical fluid without interphase mass-transfer resistance, enabling rapid and complete mineralization of toxic wastes, chemical weapons, and refractory per- and polyfluoroalkyl substances (PFAS) into CO₂, H₂O, and harmless mineral fluorides within 30 to 90 seconds.

An SCWO reactor operates autothermally when the exothermic heat released by the complete oxidation of organic Chemical Oxygen Demand (COD) is sufficient to preheat the incoming aqueous feed and oxygen to the supercritical ignition temperature without requiring auxiliary fuel or external electric preheaters. The autothermal threshold typically corresponds to a feed COD concentration between 40 and 65 g/L (4% to 6.5% organic matter by weight).

Because the dielectric constant of supercritical water drops to that of a non-polar solvent (similar to hexane), the solubility of inorganic salts (such as NaCl, Na₂SO₄, and CaF₂) plunges from ~30 wt% in liquid water down to less than 10 to 100 mg/L in supercritical water. Salts instantly crash out of solution as sticky, solid particulates that adhere to hot reactor tube walls, forming impenetrable plugs that cause sudden pressure spikes and emergency shutdowns within minutes unless transpiring-wall liners or mechanical scrapers are employed.

When chlorinated or fluorinated organics (such as PFAS or pesticides) oxidize, they form hydrochloric acid (HCl) and hydrofluoric acid (HF). In the supercritical core (T > 400°C), acids remain largely undissociated and relatively non-corrosive. However, in the subcritical transition zones (250°C to 350°C) during preheating and product cooling, water density and dielectric constant rise, causing extreme ionic dissociation. Hot concentrated mineral acids violently attack even high-nickel superalloys (Inconel 625, Hastelloy C-276), causing severe pitting and stress corrosion cracking (SCC).

DRE measures the percentage reduction of the targeted contaminant between reactor feed and effluent: DRE = [ (Mass_in - Mass_out) / Mass_in ] × 100%. Under EPA and international environmental hazardous waste incineration regulations, thermal destruction of toxic organics and persistent halogenated compounds requires a verified DRE of at least 99.99% ("four nines") to 99.9999% ("six nines").