A Critical Review of UOD Analysis
Using Respirometer Techniques
EDWIN L. BARNHART, Vice President
GEORGE J. KEHRBERGER, Senior Engineer
Hydroscience, Inc.
Leonia, New Jersey
INTRODUCTION
The oxygen consumption equivalent of a waste is usually expressed in terms of
its Biochemical Oxygen Demand (BOD). This test is performed by mixing bacteria
with the waste in a dilute solution and observing the amount of oxygen consumed
during a specific time intervaL For convenience, a group of British scientists in the
late nineteenth century decided to adopt the five-day value of oxygen consumption
as their standard test. Their reasoning was influenced to a large degree by their belief
that all streams in England reached the ocean in five days. The acceptance of the
five-day value of BOD was also based on the acceptance of biological oxidation as a
one-phase, first-order reaction. This manner of thinking has, until just recently, remained prevalent in pollution control analysis, despite continuous evidence that the
actual reactions follow a significantly different path. Ideally, the first stage of the
BOD reaction is the result of the oxidation of organics to carbon dioxide and water,
while the second stage reaction involves the oxidation of ammonia to nitrate. It must
be noted that the rate of reaction observed in the BOD test is a function of the
measurement environment and will change with experimental conditions.
Two techniques are available for measuring oxygen utilization: the dilute
bottle technique, called the BOD test, and a respirometer type test, commonly called
a Warburg study (1).
In the bottle testj very dilute samples of the substrate are mixed with suitable
bacterial seed and are incubated under stagnant conditions for five-days, or another
period of interest. The DO in the bottle at the beginning and end of the experiment
is noted and the difference is the BOD of the system. The principle limitations of the
test result from the very dilute solution which must be studied (less than six mg/l of
BOD, since any greater value would deplete the available oxygen to an unacceptable
level), and the lack of mixing in the system.
The measurement of oxygen consumption in a respirometer is accomplished by
measuring pressure differences in the system. As oxygen from the atmosphere over
the liquid passes into solution to replace the oxygen used in oxidation, the carbon
dioxide produced during oxidation is adsorbed in a potassium hydroxide well,
causing a measurable pressure change. Mixing, seed concentration, sample concentration and sampling interval is left to the discretion of the experimenter. This method
of testing has rarely been applied outside of the research laboratory, because of the
complexity of performance and data interpretation.
Today, large involved computer models of the national river systems are being
constructed to provide information on the steps required to achieve the desired levels
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