Wet air oxidation (WAO) is a technology used to treat the waste streams which are too dilute to incinerate and too concentrated for biological treatment. The WAO process was originally developed by Zimmermann and its first industrial applications appeared in the late 1950s. It can be defined as the oxidation of organic and inorganic substances in an aqueous solution or suspension by means of oxygen or air at elevated temperatures and pressures either in the presence or absence of catalysts. According to this method, the dissolved or suspended organic matter is oxidized in the liquid phase by some gaseous source of oxygen, that may be either pure oxygen, or air. The usual temperature range, 150–320°C, requires high pressure to maintain a liquid phase.