Figure 28. Process scheme for Wet Air Oxidation (WAO).
Note that RCOOH denotes short chain organic acids such as acetic acid which make up the major fraction of residual oxidation intermediates in a typical wet oxidation effluent. Properties of wet oxidation liquid effluent include: negligible NOx and S02, negligible particulate matter, and some VOCs, depending on the waste. Wet oxidation is a mature technology with a long history of development and commercialization. Wet oxidation is applicable to numerous types of waste and is used commercially for the treatment of high strength industrial wastewater, ethylene and refinery spent caustic sludge. There are two other processes that we should mention that are used in conjunction with the WAO process. The first of these is thermal sludge conditioning/low pressure oxidation (LPO). Thermal sludge conditioning is used for the conditioning of biological sludge for dewatering. Thermal conditioning is accomplished using temperatues of 175 to 200° C (350 to 400° F). The low temperature allows for low operating pressures. Thermal conditioning is most commonly used for municipal wastewater treatment sludge. It has also been applied to industrial sludge processing. The technology is applicable to any organic sludge which is difficult to dewater or that contains pathogenic components. The LPO process heats sludge to a point where the biosolids break apart, releasing much of the water trapped within the cell structures, allowing filter presses, vacuum filters , belt presses and other dewatering technologies to perform their jobs more effectively. This process along with dewatering achieves a 90 to 95 % sludge volume reduction, while at the same time destroys any pathogens in the sludge. A schematic of the process is illustrated in Figure 29. The second process used in conjunction with WAO is wet air regeneration. This is a liquid phase reaction in water using dissolved oxygen to oxidize sorbed contaminants and biosolids in a spent carbon slurry, while simultaneously regenerating the powdered activated carbon. The regeneration is conducted at moderate temperatures of 400 to 500° F and at pressures from 700 to 1000 psig. The process converts organic contaminants to C02, water and biodegradable short chain organic acids; sorbed inorganic constituents such as heavy metals can be converted to stable, non-leaching forms that can be separated from the regenerated carbon. The technology can be more cost and energy efficient than incineration, and the regeneration is accomplished in a slurry without NO,, SO„ or particulate air emissions.
Here Are Some Important Terms for You to Remember
BOD: Biological Oxygen Demand COD: Chemical Oxygen Demand
Hydrothermal Oxidation Process: Processes which involves oxidation/reduction in an aqueous matrix at elevated temperatures. Examples of hydrothermal oxidation processes include:
Wet Oxidation (WO): The oxidation of oxidizable substances in water using the oxygen in air, pure or enriched oxygen, hydrogen peroxide, nitric acid or some other oxidizing agent as the source of the oxidant. The oxidation process is conducted at subcritical temperatures (<374°C).
Wet Air Oxidation (WAO): Wet oxidation using air as the oxygen source.
Supercritical Water Oxidation (SCWO): Wet oxidation occurring in supercritical water at temperatures greater than 374°C (705°F) and pressures greater than 221 bar (3204 psig).
Hydrothermal Treatment Process: Processes which involve the use of hydrolysis or that use oxidation/reduction in an aqueous matrix at elevated temperatures.
SCWO: Supercritical Water Oxidation TKN: Total Kjeldahl Nitrogen TOC: Total Organic Carbon TSS: Total Suspended Solids WAO: Wet Air Oxidation
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