Biological Factors in Aerator Performance
A. W. BUSCH, Professor
Environmental Science and Engineering
Rice University
Houston, Texas
INTRODUCTION
In general, a minimum of approximately 0.4 lb of oxygen is required to
convert one lb of soluble biodegradable organics (measured in oxygen equivalents) to
bacterial cells (1) as shown in Figure 1. Dependent upon process design and system
operation, actual oxygen requirements may approach the theoretical one lb
oxygen/lb oxygen equivalents (T^OD) and exceed one lb of oxygen/lb BOD5
required. The amount in excess of the minimum is required in auto-oxidation of
cells. Where other than soluble organics are removed, absorbed and entrapped
particles must be oxidized in situ. Thus, the 1934 Kessner and Ribbius (2)
recommended provision of not less than 1.5 lb 02/lb BOD5 applied, for full
treatment of municipal waste water.
As shown in Figure 2, Hiser's (3) concept of measuring change in mixed liquor
COD is a quick, convenient, and accurate method of determining actual total oxygen
requirements.
The rate of biological reaction fixes oxygen transfer rate and biological
reaction rate is logically expressed by oxygen uptake rate. However, oxygen transfer
is often assessed by BOD5 removal. Thus aerator "efficiency" depends on the factor
applied as "oxygen required/lb of BOD5 removed." Even recent work (4,5,6) has
shown some confusion regarding these points. Some workers (4) use a figure of one
lb of oxygen required/lb of BOD5 removed as a broadly basic relationship. In other
cases, the role of reaction rate (in turn dependent on reactor conditions) in setting
oxygen transfer rate and "efficiency" seems to have been overlooked. Oxygen cannot
be transferred faster than the biological reaction utilizes it. Thus the "transfer
potential" of an aerator assessed at standard conditions of zero DO has little to do
with "efficiency" under field conditions unless the reaction uses the full capability of
the aeration device. BOD5 is not an adequate or valid parameter of process
performance and BOD5 removal is not an adequate or valid indication of oxygen
transfer or aerator "efficiency."
In 1930, Metcalf and Eddy (7) cited earlier work by Clark which showed the
average oxygen requirement to "stabilize municipal waste water of average strength"
to be 42.6 mg/1 or 350 lb 02/mil gal. If 130 mg/1 BOD5 remains after primary
sedimentation (using common figures of 200 mg/1 BOD5 less 35 per cent removal by
primary settling.), this is 0.33 lb 02/lb BOD5 removed. Metcalf and Eddy noted that
this oxygen requirement represents some two per cent of that contained in air
supplied at the common rate of one cu ft of air/gal of municipal waste water. These
authors further recognized the role of returned solids in determining reaction time
and overall oxygen requirements.
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