WET AIR OXIDATION UNITS

Description

A wet air oxidation unit is used to treat wastewater containing hazardous organic chemicals. The unit promotes a liquid phase reaction between oxygen and organic material in water. It operates at pressures up to 10,000 kPa and 250 C. Under these conditions, hydroxyl radicals (OH) are formed from water. These are highly reactive and are capable of oxidising complex organic compounds to simpler forms. The simpler molecules tend to be nonhazardous, such as carbon dioxide, water, salts and nitrogen, or easily biodegradable.

In operation, the wastewater is screened, pressurized and then heated by a heat exchanger. Air is compressed and injected, and the oxidation takes place in a pressure vessel. The outflowing liquid is cooled as it passes through the heat exchanger for the second time.

The oxidation in the pressure vessel is exothermic, and the heat generated can be sufficient to operate the wet air oxidation unit autothermally after start-up. Heat can be recovered from the steam produced by the unit. Although no smoke, ash or oxides of sulphur or nitrogen are created, the spent air produced by the unit may require some additional treatment before being emitted to the atmosphere.

Application

Wet air oxidation is used to treat hazardous industrial wastewaters and sludges. It can be used for pretreating concentrated waste prior to further treatment, for the virtually complete oxidation of organics in dilute wastewaters or for the destruction of organic sludges. Wet air oxidation can achieve COD reductions of 90 % and can convert intractable COD to BOD. Specific compounds can be destroyed with a 99% efficiency.

Wet air oxidation is used to treat municipal sewage sludge, pesticides, landfill liquors and certain spent scrubbing liquors. Wet air oxidation is ideal for wastewater too dilute to incinerate, yet too toxic to treat biologically. However, it is not recommended for the treatment of dioxins, PCBs or hexachlorobenzene.

Sizing

A typical wet air oxidation unit treating about 35 m3/hour of wastewater would have an electrical rating of about 100 kW. The electrical consumption is primarily due to the high-pressure wastewater pump, and the air compressor. The efficiency of a wet air oxidation unit is governed by the resistance of the contaminants to destruction, which influences the residence time, operating temperature and oxygen levels required.