Mechanical aeration

Fertilization of ponds to increase productivity is the next level of intensity with respect to fish culture, followed by provision of supplemental feeds. Supplemental feeds are those that provide some additional nutrition but caimot be depended upon to supply all the required nutrients. Provision of complete feeds, those that do provide all of the nutrients required by the fish, translates to another increase in intensity. Associated with one or more of the stages described might be the appHcation of techniques that lead to the maintenance of good water quaUty. Examples are continuous water exchange, mechanical aeration, and the use of various chemicals used to adjust such factors as pH, alkalinity, and hardness. 

Fig. 3. Commercial aerating systems. Diffused aerators utilizing (a), (b) pipes, (c) a sparger, and (d) underwater air-supply tubes (e) a mechanical aerator (f)...

Mechanical Aerators. Mechanical aerators are modular ia design and built usiag electric motors. This type of aerator is also known as a surface splasher because it pumps water vertically iato the air. During this process the water is broken up iato small droplets allowiag exchange of oxygen from the air to the water. One type of mechanical aerator is shown ia Figure 3e. 

Jet Aerators. Jet aerators are a cross between the diffused and mechanical aerators. Air and water are pumped separately under the water surface into a mixing chamber and ejected as a jet at the bottom of the tank or pond (Fig. 3f). Jet aerators are suited for deep tanks and have only moderate cost. Disadvantages include high operational costs, limitations caused by tank geometries, and nozzles that can clog. Additionally, they require blowers. 

The SORT value when divided by the standard conditions oxygen transfer rate (OTR) given for a mechanical aerator yields the oxygen concentration required for treatment. 

For aeration, diffused-air or mechanical units are used. Air diffusers are commonly used ia North America, but some mechanical aeration units are iastalled ia plants with capacities >3, 800 /d (10 gal/d). Aeration transfers oxygen to the sewage and maintains aerobic conditions, mixes sewage and floe, and keeps the floe ia suspension. 

Eoint-source pump of limited capacity. Experience has indicated that earings are a serious problem with mechanical-aeration equipment. Wave action generated within the aeration tank tends to produce lateral stresses on the bearings and has resulted in failures and increased... 

Dykman, M. and B. J. Michel, Comparing Mechanical Aerator Designs, Chem. Engineering March 10, 1960, p. 117. 

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. 

For effective volatilization using an enclosed mechanical aeration system, contaminated soil is mixed in a pug mill or rotary drum. The gasoline components are released from the soil matrix by the churning action of the air/soil contact. The induced airflow within the chamber captures the gasoline emissions and passes them through an air pollution control device (e.g., a water scrubber or vapor-phase carbon adsorption system) before they are discharged through a properly sized stack. 

About 75% of U.S. kraft mills use aerated stabilization basins.36 These basins are equipped with continuous mechanical aerators or diffusers to introduce air into the wastewater. By aerating the... 

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. 

Mechanical aeration by prolonged mixing alone is not an effective... 

A stabilization pond is a simple pond in which aeration is not mechanically enhanced. Its shallow depth allows the pond to function aerobically without mechanical aerators. Algae in the pond produce oxygen through photosynthesis, which is then used by the bacteria to oxidize the wastes. Because of the low loadings, little biological sludge is produced and the pond is fairly resistant to upsets due to shock loadings. 

Short-circuiting is one of the major problems associated with mechanical aeration equipment. Combined mechanical- and diffused-aeration systems have enjoyed some popularity for industrial-waste systems that treat variable organic loads. The mechanical mixers provide the fluid mixing with the diffused aeration varied for different oxygen-transfer rates. 

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