CE 428 Water and Wastewater Design
Aeration
Aeration in Biological Systems
- the low solubility of oxygen and the consequent low rate of oxygen transfer means that
sufficient air or oxygen and large exchange surfaces must be provided or generated for the
transfer
- can be provided in the form of bubbles or liquids in the form of droplets exposed to atmosphere
- in wastewater treatment plant aeration is accomplished by dispersing air bubbles in the liquid at
depths up to 10 m (in Europe depths of 30 m has been used).
Gas transfer from gas to liquid is dependent on the extent of saturation of the gas in the liquid
and is typically modeled using the two-film theory (see Section 4.8, page 285).
The mass transfer of a gas to a liquid phase is given by:
r
=
dC
 K (C s  C)
dt
where dC/dt = rate of change of the liquid phase concentration (mg/L/time)
K
= overall mass transfer coefficient (1/time) and is given by KLa where KL is the
mass transfer coefficient (mass/time) and a is the specific surface area
(area/volume)
Cs
= saturation concentration (mg/L) at a given temperature
C
= liquid concentration (mg/L)
Factors affecting the mass transfer are as follows:
(1) Effect of temperature - K is dependent on temperature.
typically used:
KT

T
=
Van't Hoff-Arrhenius equation is
K20  T-20
= constant (usually 1.024)
= temperature o C
(2) Effects of wastewater characteristics - the differences in oxygen solubility due to constituents
present such as salts and particulates require a correction for the wastewater characteristics (also
known as the salinity-surface tension correction factor, , typically 0.95 - 0.98):
Cs,ww =  Cs,tw
(3) If the application is at a different altitude, correction must be made for the saturation
concentration.
Cs,alt = Cs [1 - (altitude (m)/9450)]
(4) Most evaluation of oxygen transfer coefficients of aeration systems are conducted in clean
water. In this procedure, oxygen in tap water is depleted using excess sodium sulfite and the
temperature is kept at 20o C. Air is bubbled and the change in oxygen is measured.
The overall mass transfer coefficient in clean water may be different from in the wastewater.
Typically a correction factor is used:
Kww
=  KTW
Putting all of these together, we have
rww
=
Kww(Cs,ww - C)
=
 KT,TW ( Cs,TW - C)
=
 K20,TW  (T-20) ( Cs,TW - C)
(1)
Note that in industrial literature, the most convenient information is to relate the above with the
standard oxygen transfer rate (SOTR) in tap water at 20 o C and zero dissolved oxygen, kg O2/hr.
This is a piece of information that can be supplied by the manufacturer for their equipment. The
SOTR is related to the actual conditions by using the factors above.
r 20,TW = SOTR = K20, TW (Cs,20 - C)
With C = 0 mg/L then we have SOTR = K20, TW Cs,20
(2)
Divide (1) by (2)
rww
----------
For
r 20,TW
AOTR
--------SOTR
 K20,TW  (T-20) ( Cs,TW - C)
= -----------------------------------------------------------
See Table 5-25
= ------------------------------------------
K20, TW Cs,20
  (T-20) ( Cs,TW - C)
= -------------------------------------- F
Cs,20
Where F = fouling factor (0.65 - 0.9)
Aeration Systems
  (T-20) ( Cs,TW - C)
Cs,20