Abstract
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. Typical conditions for WAO are 150–320°C for temperature, 2–15 MPa for pressure, and 15–120 min for residence time; the preferred chemical oxidation demand (COD) load ranges from 10 to 80 kg/m3 (1,2). WAO destroys toxics in industrial wastewater by breaking down complex molecular structures into simpler components such as water and carbon dioxide, without emissions of NOx SO2, HCl, dioxins, furans, and fly ash. It is reported that the WAO process is capable of a high degree of conversion of toxic organics with more than 99% destruction rate; however, some materials are not oxidized completely to carbon dioxide and water, instead, some intermediate compounds are formed, which represent a quarter of the original mass of organic matter. For example, small carboxylic acids: acetic acid and propionic acids, methanol, ethanol, and acetaldehyde. Removal of acetic is usually negligible at temperature less than 300°C. On the other hands, organic nitrogen compounds are easily transformed into ammonia, which is also very stable in WAO process. Therefore, WAO is pretreated of liquid wastes which requires additional treatment processes, for example, a bio-treatment is usually provided for final clean-up (3).
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Zou, L.Y., Li, Y., Hung, YT. (2007). Wet Air Oxidation for Waste Treatment. In: Wang, L.K., Hung, YT., Shammas, N.K. (eds) Advanced Physicochemical Treatment Technologies. Handbook of Environmental Engineering, vol 5. Humana Press. https://doi.org/10.1007/978-1-59745-173-4_13
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