Wet Air Oxidation
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. Click here to read more about this patented process from Siemens Water.
Commercial systems typically use a bubble column reactor, where air is bubbled through a vertical column that is 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.
Catalytic Wet Air Oxidation is a variation of WAO that uses a catalyst. CWAO process is capable of converting all organic contaminants ultimately to carbon dioxide and water, and can also remove oxidizable inorganic components such as cyanides and ammonia. The process uses air as the oxidant, which is mixed with the effluent and passed over a catalyst at elevated temperatures and pressures. If complete COD removal is not required, the air rate, temperature and pressure can be reduced, therefore reducing the operating cost.CWAO is particularly cost-effective for effluents that are highly concentrated (chemical oxygen demands of 10,000 to over 100,000 mg/l) or which contain components that are not readily biodegradable or are toxic to biological treatment systems. CWAO process plants also offer the advantage that they can be highly automated for unattended operation, have relatively small plant footprints, and are able to deal with variable effluent flow rates and compositions.
The process is not cost-effective compared with other advanced oxidation processes or biological processes for lightly contaminated effluents (COD less than about 5,000 mg/l).
Organic and some inorganic contaminants are oxidised in the liquid phase by contacting the liquid with high pressure air at temperatures which are typically between 120°C and 310°C. In the CWAO process the liquid phase and high pressure air are passed co-currently over a stationary bed catalyst. The operating pressure is maintained well above the saturation pressure of water at the reaction temperatures (usually about 15-60 bar) so that the reaction takes place in the liquid phase. This enables the oxidation processes to proceed at lower temperatures than those required for incineration. Residence times are from 30 minutes to 90 minutes, and the chemical oxygen demand removal may typically be about 75% to 99%. The effect of the catalyst is to provide a higher degree of COD removal than is obtained by WAO at comparable conditions (over 99% removal can be achieved), or to reduce the residence time.
Organic compounds may be converted to carbon dioxide and water at the higher temperatures; nitrogen and sulphur heteroatoms are converted to to molecular nitrogen and sulphates. The process becomes autogenic at COD levels of about 10,000 mg/l, at which the system will require external energy only at start-up.
Siemens Water website