Aeration serves two important purposes: supplying the required oxygen to the organisms to grow and providing optimum contact between the dissolved and suspended organic matter and the microorganisms. The aeration system consumes approximately 50 to 65 percent of the net power demand for a typical activated sludge wastewater treatment plant, therefore the efficiency of different aeration systems is an important consideration. The time that the mixed liquor is aerated varies from as little as 30 minutes to as much as 36 hours depending upon the treatment process used. Aeration can be performed mechanically or by using a diffused system.
Mechanical aerators physically splash the wastewater into the atmosphere above the tank and create turbulence assuring effective wastewater mixing. Mechanical aerators include brushes, blades or propellers that introduce air from the atmosphere. Surface aerators float at the surface or are mounted on supports in or above the basin. Mechanical aerators tend to incur lower installation and maintenance costs.
A diffused air system introduces compressed air through a perforated membrane into the wastewater. Diffusers are classified by the physical characteristics of the equipment, or by the size of the air bubble. The choice of bubble size, diffuser type, and diffuser placement can have a great effect on the efficiency of the aeration process. Porous (fine bubble) diffusers are attached to the bottom of the tank or positioned just below the surface. They are available in various shapes and sizes, such as discs, tubes, domes, and plates. Fine pore diffusers introduce air in the form of very small bubbles, maximizing the contact time the air bubbles have with the mixed liquor and encouraging mixing while at the same time, discouraging deposits on the tank bottom. These fine pore diffusers produce a high oxygen transfer efficiency, but they are susceptible to chemical or biological fouling and as a result, require routine cleaning.
Nonporous (course bubble) diffusers usually have fixed or valved orifices. Due to the larger bubble size, nonporous diffusers produce lower oxygen transfer efficiencies.
Other diffusion devices include jet aerators, which discharge a mix of
air and liquid through a nozzle, and aspirator aerators that use a propeller on the end of a hollow shaft, creating a vacuum as the propeller draws air from the atmosphere and disperses it into the wastewater.
Aeration
