Effect of Oxygen Tension on O2
Uptake and Sludge Yield in Completely
Mixed Heterogeneous Populations
M. DAVID RICKARD, Environmental Engineer
Merck, Sharp and Dohme Research Laboratories
Rahway, New lersey
A. F.   GAUDY, Professor and Director
Bioengineering and Water Resources
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
The oxygen requirements of aerobic bacterial systems have been a subject of
continuing research interest in the field of microbiology, and an excellent understanding of the role of oxygen tension in the growth and biochemical behavior of
pure culture systems has been obtained. In general, it may be stated that the
metabolism of a given microorganism has been found to be independent of the
oxygen tension if the DO concentration is maintained above a minimum value,
0.2 - 0.8 mg/l (1,2), termed the critical DO. Although the critical DO originally referred only to that oxygen tension at which respiration rate became independent of oxygen tension, additional studies (1, 3) have demonstrated that viable
count and cell mass yield also are not affected by changes in DO above this level.
Unfortunately, in the field of biological waste treatment the relationship between DO concentration and cell growth has not been so well defined because
these microbial systems employ much higher biological solids concentrations than
are normally used in microbiological studies and because naturally occurring heterogeneous populations are subject to as yet unpredictable variations at the ecological level. Therefore, the application to biological waste treatment systems
of relationships derived from studies using pure cultures at relatively low solids
concentrations must be viewed with caution. In addition, short term "batch" investigations, even when conducted on heterogeneous populations, may not allow
the full expression of selective pressures and hence may not provide a full assessment of all factors involved.
The investigation reported in this paper was conducted in an attempt to determine the effect of DO concentration on the behavior of a natural population
under steady-state, continuous flow conditions. It was hoped that an investigation of this type would provide more complete understanding of the response of
the response of waste treatment systems to oxygen tension at both the biochemical and ecological levels.
A number of investigators (3,4, 5) have demonstrated the existence of a critical DO for activated sludge in terms of both respiration rate and substrate removal. However, Heukelekian (6) observed that the viable count developed in a
BOD bottle varied over wide limits as the initial DO was changed. He attributed
this variation to the development of different populations at the various DO levels.
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