Short Term BOD
Using the Automatic Respirometer
R. M. ARTHUR, Professor
W. N. HURSTA, Student
Biological Engineering Department
Rose Polytechnic Institute
Terre Haute, Indiana
INTRODUCTION
The automatic respirometer, invented in 1963, was first used to measure the
oxygen uptake of pure cultures of bacteria. It proved useful because it provided
a continuous recording of oxygen uptake eliminating tedious data taking and curve
plotting. Shortly thereafter the device was used to determine the oxygen uptake,
i.e., BOD, of non-homogeneous cultures such as found in waste water (1). It was
soon recognized that the results from the respirometer may be useful in predicting
five-day BOD of waste water, in addition to providing a continuous record of
oxygen uptake over any time period. In 1966 the Federal Water Pollution Control
Administration provided funds to demonstrate the machine's capability to predict
five-day BOD.   This paper reports on the results of the first stage of the project.
The automatic respirometer has been fully described in the literature (1,3,4).
The decrease in concentration of dissolved oxygen caused by respiration decreases
the partial pressure of oxygen of the gaseous phase in the closed system. The
change in pressure in sensed by a transducer which converts the information to an
electrical signal for readout on a continuous recorder. Both air and liquid are
recycled counter-current in the closed system creating an ideal environment for
rapid oxygen uptake. Carbon dioxide is absorbed by sodium hydroxide. The device is calibrated to record milliliters of oxygen at the test temperature.
To determine the BOD of waste water, the sample is placed in the aeration
chamber, recirculation started, and the system closed to the atmosphere. The
recorder indicates the oxygen demand at any time during the test run and also
provides evidence of changes in rate of oxygen demand. The present aeration
chamber has a volume of 1200 milliliters. All tests in this six-month series were
made on one liter samples. Examples of the curves obtained are shown in Figures 1 and 2.
IK
Figure 1 - Typical recorder printout of automatic respirometer.
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