Effect of Air Flow Rate on
Response of Activated Sludge
to Qualitative Shock Loading
A. F. GAUDY, JR., Associate Professor of Sanitary Engineering
Oklahoma State University
Stillwater, Oklahoma
B. G. TURNER, Public Health Engineer
Tennessee Valley Authority
Chattanooga, Tennessee
A considerable research effort in both the basic and applied fields has
been expended in studying the effect of oxygen tension on the activities of
microorganisms. It is not necessary here to review completely the status of
the controversy on the effects of extension or effective rate of aeration on the
various parameters of microbial activity. Most of this work has been thoroughly reviewed in other research reports (1, 2, 3). From the results of many
workers it would seem that fairly low values of oxygen tension, i.e., less
than one mg/l are permissible.
Smith (4) found that there was no effect on the rate of oxygen uptake at
dissolved oxygen concentrations of 0.2 to 6 mg/l. He also observed no effect upon the degree of BOD removal due to variation in oxygen concentration, provided that a residual existed in the mixed liquor at all times (2).
Recently Orford, Heukelekian and Isenberg have observed a small but significant increase in effluent BOD with decreasing concentrations of dissolved
oxygen at levels below 0.5 mg/l (5). Porges, Jasewicz and Hoover noted that
oxygen utilization dropped sharply in the region of 0. 35 to 0.5 mg/l of dissolved oxygen (6). Using pure cultures, Longmuir (7) has reported experimentally determined values of dissolved oxygen at which cells respire at one
half their maximum rate, (Km values). Tvpical examples for various organisms   are:    Escherichia coli,   2.22x10"°   Molar;      Bacillus  megatherium,
3.12 x 10"6 Molar; Micrococcus candicans, l.lOx 10"8 Molar. It is interesting to note that a value of 2 x 10"° molar is an oxygen concentration of
only 0. 00064 mg/l. These results lend support to work in the sanitary field
from which a low allowable dissolved oxygen concentration was concluded.
Differences in the very low values cited above and those found by workers in the sanitary field might be largely attributed to a difference in requirements for oxygen transport to individual cells in a suspension and to cells existing in flocculent masses. Recent results of van der Emde (8) similarly
showed that the rate of oxygen utilization was linear at all concentrations of
dissolved oxygen down to very low values for sludge-fed dilute concentrations
of substrate, indicating that oxygen utilization is largely independent of oxygen tension. However, he concluded that "the oxygen content in the aeration tank should be at least 2 ppm, since below this value, the purification
effect will decrease." Wuhrman (9) has reported that at low or medium
mixed liquor concentrations an increase of oxidation occurs with increasing
oxygen tension, while at high sludge concentrations this effect disappears.
It is seen from the work cited above that there is some doubt as to the effect and  the allowable limits  of oxygen tension in biological treatment
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