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Alpha factor is the ratio of the oxygen transfer rate in wastewater to the oxygen transfer rate in clean water. Alpha factor is dependent on the aeration device used and also on the presence of chemicals such as surfactants in the wastewater.

Jet aerators produce a high alpha factor in pulp and paper mill wastes. The high alpha factor is due to the presence of surfactants in pulp and paper wastewater.

Oxygen transfer is enhanced due to the high shear at the gas/liquid interface. Typical alpha factors achieved with various aeration devices are shown below.

Alpha Factor for Various Devices
Jet Aerators 0.9
Coarse Bubble Diffusers 0.8
Fine Bubble Diffusers
(Membrane of ceramic)
0.4
Surface Aerators 0.85

High shear aeration devices such as jet aerators yield a higher alpha factor due to surface renewal at the gas/liquid interface. Low shear aeration devices such as fine bubble diffusers (membrane or ceramic) yield a lower alpha factor because of insufficient renewal (oxygen saturation) at the gas/liquid interface.

All aeration systems are standardized for performance at standard conditions. Therefore, the process oxygen (AOR) is converted to standard oxygen (SOR) and is inversely proportional to the alpha factor. For this reason aeration systems with low alpha factors must be designed for a higher SOR.

 

Relative Oxygen Required (SOR)
Jet Aerators 100%
Coarse Bubble Diffusers 112%
Fine Bubble Diffusers
(Membrane of ceramic)
225%
Surface Aerators 106%