Mechanical aeration wastewater treatment

The function of aeration in a wastewater treatment system is to maintain an aerobic condition. Water, upon exposure to air, tends to estabUsh an equihbrium concentration of dissolved oxygen (DO). Oxygen absorption is controlled by gas solubiUty and diffusion at the gas—hquid interface. Mechanical or artificial aeration may be utilised to speed up this process. Agitating the water, creating drops or a thin layer, or bubbling air through water speeds up absorption because each increases the surface area at the interface. 

The activated sludge process for domestic wastewater treatment was introduced to the world in 1914.1 Since then, many studies have been conducted to improve the oxygen transfer efficiency. Among the aeration devices introduced have been a porous diffuser, a filter type diffuser, a mechanical aeration device, an orifice type diffuser and a fine-pore air diffuser. The aeration market is in a substantial state of flux in the USA today. Emphasis on high efficiency has led many intensive research programmes to aim at the evaluation of the design, operation and control processes to improve overall system performance. 

Some kraft mills use basins without mechanical aerators. Known as stabilization basins, this is the simplest form of aerobic treatment. This process uses shallow basins that cover very large areas and relies on natural diffusion of air into the wastewater and algae to create aerobic conditions. At depths greater than 1.2 m (4 ft), anaerobic microorganisms will become active in lower depths thus, stabilization basins are shallow. Typically, the basin is earthen although some are lined with compacted clay. Wastewater retention time may last up to 30 d to achieve up to 90% BOD5 removal. 

Aeration basins are wastewater ponds or lagoons that have air introduced by mechanical action. Aeration may be performed to assist aerobic bioremediation and/or to remove volatile organic compounds. In an aeration basin, oxygen is usually supplied by surface aerators or by diffused aeration units. The action of the aerators and that of the rising air bubbles from the diffuser is used to keep the contents of the basin in suspension. Aeration is widely used in wastewater treatment and can be adapted to treat groundwater. 

Mechanical aerators (4,6) can effectively remove VOCs and are commonly used in wastewater treatment, not water treatment. The aerators use surface or subsurface mechanical stirring mechanisms to create water-air mixing turbulence. Figure 14 illustrates a surface and subsurface aerator installations. 

Gas-to-liquid mass transfer is a transport phenomenon that involves the transfer of a component (or multiple components) between gas and liquid phases. Gas-liquid contactors, such as gas-liquid absorption/ stripping columns, gas-liquid-solid fluidized beds, airlift reactors, gas bubble reactors, and trickle-bed reactors (TBRs) are frequently encountered in chemical industry. Gas-to-liquid mass transfer is also applied in environmental control systems, e.g., aeration in wastewater treatment where oxygen is transferred from air to water, trickle-bed filters, and scrubbers for the removal of volatile organic compounds. In addition, gas-to-liquid mass transfer is an important factor in gas-liquid emulsion polymerization, and the rate of polymerization could, thus, be enhanced significantly by mechanical agitation. 

The previous sections dealt with gas-liquid contacting without mechanical agitation in such devices as bubble-columns and airlift towers. To obtain better gas-liquid contacting, mechanical agitation is often required. The discussion is confined to baffled sparged stirred-tanks with impellers. Aeration by surfaction impellers are sometimes used in one wastewater treatment facilities (Zlokarnik. 1978). 

Lee, J. C. and Buckley, P. S. In Biological Fluidised Bed Treatment of Water and Wastewater by Cooper, P. F. and Atkinson, B., (eds.), Ellis Horwood, Chichester (1981), 62. Fluid mechanics and aeration characteristics of fluidised beds. 

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