Supercritical water oxidation or
SCWO is a process that occurs in
water at
temperatures and
pressures above a mixture's thermodynamic
critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantage in the destruction of
hazardous wastes such as
PCBs. The fluid has a
density between that of water
vapor and liquid at standard conditions, and exhibits high
gas-like diffusion rates along with high
liquid-like collision rates. In addition, the behavior of water as a solvent is altered (in comparison to that of subcritical liquid water) - it behaves much less like a polar solvent. As a result, the
solubility behavior is "reversed" so that
chlorinated hydrocarbons become soluble in the water, allowing
single-phase reaction of aqueous waste with a dissolved
oxidizer. The reversed solubility also causes
salts to
precipitate out of solution, meaning they can be treated using conventional methods for solid-waste residuals. Efficient oxidation reactions occur at low temperature (400-650 °C) with reduced
NOx production.