Basic Variables of Oxygenation from
Diffused Air in Aeration Tanks
JATINDER K. BEWTRA and WILLIAM R. NICHOLAS
University of Iowa
Iowa City,   Iowa
One of the important functions of bubble aeration in the activated sludge
waste treatment process is to provide the oxygen required for the metabolic
activity of the biological life present. Because the microorganisms use oxygen in the dissolved form, the phenomenon of oxygen transfer from air bubbles to liquid has been studied and reported by many investigators. Most of
these studies were conducted in column tanks of various sizes or small-scale
aeration tanks. These studies provide valuable information concerning the
variables of oxygen transfer that are independent of the tank geometry and
aeration devices. During the past four years, oxygen transfer studies have
been conducted in a four-ft long section of a full-scale aeration tank located
at the Philip F. Morgan Sanitary Engineering Laboratory. During this period
the important variables of oxygen transfer from air to water were studied. In
this paper these variables are presented mathematically and their relative importance is discussed.
i
OXYGEN TRANSFER
When air is introduced in an aeration tank, it is dispersed into small bubbles. These bubbles rise in the liquid with nearly a constant terminal velocity, which is attained within a distance of three to four cm from the point
of bubble formation. In addition, the liquid surrounding the air bubbles may
be moving with a velocity depending upon the conditions of aeration. Thus,
each air bubble remains in the liquid for a period of time before it escapes at
the surface. During the time the air bubbles are in contact with the liquid,
there is a continuous transfer of oxygen from air to liquid. The oxygen transfer occurs by molecular diffusion through the interfacial surfaces 01 the bubbles at a relatively slow rate. The rate of this mass transfer, R, is given by
(1).
dm
K, A(C.- C, )
Kl = overall oxygen transfer coefficient,
A = aera of interfacial contact between bubbles and liquid
Cl and Cj are the dissolved oxygen concentrations in the body of liquid
and at the interface respectively.
In bubble aeration, the total rate of oxygen transfer takes place in three
phases. The first phase of oxygen transfer is during bubble formation at the
interstitial openings of the aeration devices. At any instant, the bubbles in
formation have a mean surface area of contact with the liquid and a continuous oxygen transfer takes place through this surface. For a given diffuser the
duration of bubble formation will vary with the airflow rate through the diffuser.    With porous diffusers,  having small interstitial openings and a large
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