Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidation reactions occur in superheated water at a temperature above the normal boiling point of water (100° C), but below the critical point (374° C).
The system must be maintained under pressure to avoid excessive evaporation of water. This is done to control energy consumption due to the latent heat of vaporization. It is also done because liquid water is necessary for most of the oxidation reactions to occur. Compounds oxidize under wet oxidation conditions that would not oxidize under dry conditions at the same temperature and pressure.
Wet oxidation has been used commercially for around 60 years. It is used predominantly for treating wastewater. It is often referred to as the Zimpro process, after Fred T. Zimmermann who commercialized it in the mid 20th century.
Commercial systems typically use a bubble column reactor, where air is bubbled through a vertical column that is liquid full of the hot and pressurized wastewater. Fresh wastewater enters the bottom of the column and oxidized wastewater exits the top. The heat released during the oxidation is used to maintain the operating temperature.
The majority of commercial wet oxidation systems are used to treat industrial wastewaters, such as sulfide laden spent caustic streams. Almost as many systems are also used for treating biosolids, in order to pasteurize and to decrease volume of material to dispose of.
A special type of process was the so-called "VerTech process". Here was the required pressure supplied by installing the system in de below ground pressure vessel (Gravity Pressure Vessel GPV). The pressure was supplied by feeding the material to a reactor with a depth of 1200 meters. The deep shaft reactor also served as a heat exchanger, so no pre heating was required. The operating temperature was about 270 degrees Celsius, with a pressure of about 100 bar. The installation was operational in Apeldoorn (the Netherlands) between 1994-2004, but was eventually shut down due to operational problems.
- Zimmermann, F. Waste Disposal, US Patent 2665249, 1950.
- Mishra, V.; Mahajani, V.; Joshi, J. "Wet Air Oxidation", Ind. Eng. Chem. Res.", 34, 2-48, 1995.
- Maugans C.; Ellis, C. "Wet Air Oxidation: A Review of Commercial Sub-Critical Hydrothermal Treatment", Twenty First Annual International Conference on Incineration and Thermal Treatment Technologies, New Orleans, May 13-17, 2002. WAO History Paper
- Patria, L.; Maugans, C.; Ellis, C.; Belkhodja, M.; Cretenot, D.; Luck, F.; Copa, B. "Wet Air Oxidation Processes", Advanced Oxidation Processes for Water and Wastewater Treatment, S. Parsons Editor, pp 247–274. 2004, IWA Publishing.
- Giudici, D.; Maugans, C. "Improvement of Industrial Synthesis of Methyl Methacrylate Application of a Wet Air Oxidation Process (WAO)", MMA WAO Paper
- "Zimpro History". Retrieved February 2010.