Fundamental Aspects of Surface Aerator
Performance and Design
J. R. McWHIRTER, Research Manager
Mixing Equipment Company, Inc.
Rochester, N. Y.
INTRODUCTION
Considerable effort during the past decade has been devoted to developing
methods and techniques for measurement of the oxygenation capacity of aeration
equipment. In spite of this effort, however, there seems to exist a significant degree of contradictory information and confusion in the literature.
This paper discusses several common methods for determining the oxygenation or mass transfer capacity of surface aerators. Comparison of the applicability
of these methods is made as well as specific problems relating to their use in
evaluation of surface aeration equipment.
Performance information concerning commercial aeration devices have been
largely obtained using four common methods of measurement (1). Of these techniques, the most widely used are the unsteady-state aeration of pure tap water and
the steady-state oxidation of sodium sulfite solution. In addition, however, both
steady-state and unsteady-state aeration of activated sludge systems are reported.
A new method which involves steady-state aeration of pure tap water will also be
discussed.
UNSTEADY-STATE AERATION OF PURE TAP WATER
This method involves reaeration of pure tap water from which the dissolved
oxygen has been previously removed. Removal of the dissolved oxygen may be
accomplished by stripping with inert gas or chemical reaction with a stoichiometric amount of sodium sulfite. The hasic theory governing the rate of diffusion of
oxygen from a gas to a liquid had been developed many times (1,2,3,4) and
therefore will not be repeated. Equation (1) describes the unsteady-state transfer
of oxygen which applies during the reaeration.
^ = ^KLa(c*-C) (1)
dt      W
in which Kj^a = combined overall liquid phase mass transfer coefficient and inter-
facial area, lbs, Og^Hr) (ppm) driving force
c = bulk average dissolved oxygen concentration in liquid, ppm.
c* = dissolved oxygen concentration in liquid in equilibrium with gas
phase.
W = mass of water, lbs
t = aeration time, hrs
The use of Equation (1) implies the following basic assumptions.
1. The primary resistance to mass transfer is in the liquid phase and the inter-
facial liquid concentration can be taken to be essentially in equilibrium
with the gas phase.
2. The entire liquid contents are instantaneously perfectly mixed and therefore of a uniform composition at any time, t.
- 75 -