Effect of Dissolved Oxygen Concentration
on the Metabolic Response of
Completely Mixed Activated Sludge
G. J. THABARAJ, Research Assistant
A. F. GAUDY, JR., Professor
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
The effect of dissolved oxygen tension on various metabolic activities of
microorganisms has been the subject of much study and controversy. Early work in
this field has been reviewed by Smith (1), by Okun and Lynn (2), and by Gaden (3).
It has been shown that below certain critical oxygen tensions the rate of microbial
activity may be limited. The values for critical dissolved oxygen tensions reported
for various microorganisms cover a wide range, from zero to 15 mm Hg (0-0.7 mg/1
DO at 30 C) (4,5,6,7). In the water pollution control field, minimum DO levels
recommended for operation of activated sludge plants have ranged from 0.5 mg/1 (8)
to two mg/1 (9). The work of Gaudy and Turner (10) supported the hypothesis that
the DO tension which would affect the metabolic rate of activated sludge lies below
0.5 mg/1. Changes in predominating species of organisms at different DO tensions
have been suggested by Wuhrmann (11), Heukelekian (12), and von der Emde (9).
In an earlier communication, Rickard and Gaudy (13) reported results of their
studies on the effects of dissolved oxygen tension on the growth of heterogeneous
populations in a completely mixed continuous flow reactor under conditions approximating a steady state. With constant agitation (i.e., constant velocity gradient),
they observed no change in O2 uptake rate, cell yeild, protein content, or RNA content of the sludge for a range of DO concentrations from 1.4 to 7.1 mg/1. Rickard
(14) made preliminary studies on the effect of velocity gradient during quantitative
shock loading. He observed that, at a low level of agitation, the introduction of a
shock load led to the depletion of DO and a reduction in efficiency of COD removal. On increasing the level of agitation, the oxygen limitation was prevented and
the system responded successfully. Since in these studies the dissolved oxygen concentration of the medium varied with the velocity gradient, it seemed possible that
DO could play a role in the ability of the activated sludge process to accommodate a
quantitative shock loading. The present investigation was therefore designed to study
the effect of dissolved oxygen tension on the shock load response under continuous
flow conditions with constant agitation.
MATERIALS AND METHODS
The general experimental plan included the establishment of a steady state at a
specific level of dissolved oxygen, an increase of the concentration of substrate in the
feed, observation of the transient and new steady state behavior and, in some experiments, reduction of substrate concentration to the original level and observation of
the transient and new steady state. The experimental plan for each experiment reported is given in Table I. Experiments I and II were run at high DO levels, experiment III at an intermediate level, and experiment IV at a low level of DO. The DO
concentration in the initial steady state was controlled by varying the composition of
the aerating gas. Double and triple organic loadings were applied.
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