Design Example

Theory of Aeration
Adjustment for Field Conditions
Aeration Facilities

Theory of Aeration

Aeration is a gas-liquid mass transfer process in which the driving force in the liquid phase is the concentration gradient (Cs - C) for slightly soluble gases.

Mass transfer per unit time =KL.a (Cs - C)

where, KL = Liquid film coefficient
              =Diffusion coefficient of liquid (D)
                   Thickness of film (Y)
               a  = Interficial area per unit volume
          Cs =saturation concentration at the gas-liquid interface and           C  = some lower value in the body of the liquid.

The value of a increases as finer and finer droplets are formed, thus increasing the gas transfer. However, in practice, it is not possible to measure this area and hence the overall coefficient (KL.a) per unit time, is determined by experimentation.

Adjustment for Field Conditions

The oxygen transfer capacity under field conditions can be calculated from the standard oxygen transfer capacity by the formula:

N = [Ns(Cs- CL)x 1.024T-20a ]/9.2

N = oxygen transferred under field conditions, kg O2/h.
Ns= oxygen transfer capacity under standard conditions, kg O2/h.
Cs= DO saturation value for sewage at operating temperature.
CL= operating DO level in aeration tank usually 1 to 2 mg/L.
T = Temperature, degree C.
a = Correction factor for oxygen transfer for sewage, usually 0.8 to 0.85.

Aeration Facilities

  • Oxygen may be supplied either by surface aerators or diffused aerators employing fine or coarse diffusers.
  • The aeration devices apart from supplying the required oxygen shall also provide adequate mixing in order that the entire MLSS present in the aeration tank will be available for biological activity.
  • Aerators are rated based on the amount of oxygen they can transfer to tap water under standard conditions of 20°C, 760 mm Hg barometric pressure and zero DO.