Oxygen transfer in the activated sludge system involves physical mass transport across a two-film layer that consists of a gas film and a liquid film. For gases with limited solubility such as oxygen, the gas diffuses so slowly through the liquid film that mass transfer through the gas film can be neglected. The transfer of oxygen into clean tap water is expressed as:
Rc = KLa (Cs - C)
where
Rc = oxygen transfer rate in the water, mg/L-h
KLa = oxygen mass transfer coefficient, h^-1
Cs = dissolved oxygen (DO) saturation concentration in liquid, mg/L
C = DO concentration in liquid, mg/L
However, in respiring systems, a phenomenon called gas depletion occurs in the gas phase during oxygen transference. Would this equation be changed to:
Rd = KLa (Cs - C) - Ru
where
Rd = oxygen transfer rate in dirty water, mg/L-h
Ru = oxygen uptake rate of the mixed liquor suspended solids, mg/L-h?
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