50   USE OF PURE OXYGEN TO OVERCOME OXYGEN
TRANSFER LIMITATIONS IN ENVIRONMENTAL
RESPIROMETRY
Fernando Cadena, Professor
Martin O. de la Garza, Graduate Student
Civil Engineering Department
New Mexico State University
Las Cruces, New Mexico 88003
INTRODUCTION
Environmental respirometers can provide valuable information regarding biodegradability of waste
samples. However, the capacity of these instruments to simulate actual biological activity can be
masked by physical and chemical constraints. To obtain an accurate representation of the biological
system it is necessary for the oxygen to be transferred from the headspace to the microorganisms.
Carbon dioxide generated during respiration must be removed from the gas phase to maintain
constant partial pressure of oxygen in the reactor. Misleading results are common in systems where the
oxygen transfer rate is lower than the theoretical biochemical oxygen uptake rate (OUR). Such
conditions are typically found in very active microbial samples, such as activated sludge and contaminated soils. Oxygen transfer rate can be improved by increasing the degree of agitation in the system
and by increasing the partial pressure of oxygen in the headspace. Increasing agitation is impractical
in solid samples and thick slurries, while oxygen enrichment is a relatively simple alternative to
implement. This option is particularly attractive since it requires minimal procedure changes and can
be used with both solid and liquid samples.
Experiments using oxygen enrichment show linear OUR response over a wider range of microbial
concentration than those found using a natural atmosphere for endogenous activated sludge samples.
Deviations from linear behavior are more marked when an inadequate or insufficient alkali trap is
used to strip carbon dioxide from the headspace. Recommendations are made on improvements to
respirometric analysis. Such improvements should reduce the probability of obtaining erroneous
results from environmental respirometers.
Manometric devices have been used extensively in the medical and biological sciences since the turn
of the century.' Some of these analytical tools were used to measure oxygen uptake (respirometers).
The 1960's saw great progress in waste management. In particular adaptation of respirometric instruments to the environmental sciences opened new research avenues in areas related to biodegradation
of organic wastes.2 Perhaps the greatest contribution at this time was the introduction of the electrolytic cell for continuous monitoring of oxygen uptake.3 Use of respirometers in environmental sciences declined significantly after the protocol was eliminated without explanation from the twelfth
edition of Standard Methods.* Even as recently as 1988 Cadena et al.4 report limited use of respirome-
try in environmental applications.
Renewed interest in respirometry in the last five years is likely due to the need to evaluate biodegradation of hazardous organic wastes. Improvements to environmental respirometry have resulted in
elimination of several insidious problems. Simultaneously, introduction of computerized instrumentation has increased the amount of data collected and has greatly simplified instrument operation.
These changes have catalyzed the use of respirometry as a tool to assess biodegradability of hazardous
wastes, determination of biodegradation kinetics and many other environmental applications. However, use of improved respirometers and the voluminous data generated by computer aided analyses
does not necessarily translate into accurate information. Environmental analysts should be aware of
instrument limitations and potential error sources. Results should be interpreted considering these
factors and corrective measures should be conducted to insure data validation.
Environmental respirometers, though simple to operate, are subject to a variety of potential physical, chemical and biological interferences. Several interferences and possible solutions are described
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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