Supercritical water that exhibits complete miscibility with oxygen is a homo­geneous reaction medium for the oxidation of organic molecules. This feature of SCW allows oxidation of harmful or toxic substances at low temperature in a process known as supercritical water oxidation (SCWO) or cold combustion. In a typical SCWO unit, the entire mixture (water, oxygen, and waste) remains as a single fluid phase with no interphase transport limitations. This allows very rapid and complete (>99.9%) oxidation of the organic wastes to harmless lower-molecular-weight compounds like H2O, N2, and CO2. Unlike thermal incineration, SCWO produces toxic by-products such as dioxin. This method of waste treatment is especially attractive for highly dilute toxic wastes in water.

One important shortcoming of this process is the production of highly cor­rosive liquid effluents because chlorine, sulfur, and phosphorous, if present in the waste, are converted into their corresponding acids (Serani et al., 2008). The destruction of polychlorinated biphenyls (PCBs) in supercritical water, producing carbon dioxide and hydrochloric acid, may be represented by the following simple reaction:

C12H10-mClm (PCB) + (19 + m)/2 O2 + (5 — m) H2O = 12CO2 + mHCl (7.4)

Conventional thermal incineration uses very high temperature to destroy by­products like dioxin, which results in the production of another pollutant, NOx. This is not the case with SCWO owing to its low-temperature operation (450­600 °C).