46    GAS TRANSFER LIMITATIONS IN ENVIRONMENTAL
RESPIROMETRY
Ricardo B. Jacquez, Professor
Fernando Cadena, Associate Professor
Somavarapu Prabhakar, Graduate Student
New Mexico State University
Las Cruces, New Mexico 88003
Martha I. Beach, President
N-CON Corporation
Larchmont, New York 10538
INTRODUCTION
Biochemical oxygen demand (BOD) and oxygen uptake rate (OUR) are commonly used to characterize wastewater. BOD is used in monitoring the purification efficiencies and oxygen requirements in
wastewater treatment plant operation. OUR is used in defining microbial activity and biokinetics of
treatment processes.1 Despite the application of OUR to determine biodegradation rates and substrate
inhibition kinetics, its use in wastewater treatment control represents a controversial application.2,3
Due to the importance placed on BOD and OUR by the wastewater industry, considerable effort
has been made to standardize measurement methods which maximize accuracy and precision. Several
instruments, such as the Warburg, electrolytic BOD (EBOD), and pneumatic computerized BOD
(PCBOD) respirometers, can be used as alternates to the standard bottle tests to provide a more
realistic representation of microbial activity.4 Most of these devices operate as constant pressure or
constant volume manometers. The Warburg, and open cell EBOD respirometers are constant volume
manometers in which pressure changes are measured in a constant volume reactor. Constant pressure
respirometers (for example the Gilson, Wilson, closed cell EBOD, and PCBOD) are unaffected by
barometric pressure changes as opposed to the constant volume respirometers. A schematic representation of a constant pressure respirometer is shown in Figure 1. The operational principles as well as
advantages and disadvantages of the respirometers listed above have been discussed by Cadena et al.4
To successfully interpret the data obtained using any instrument, its limitations must be known.
Tabak et air identified various physical and biological factors which limit respirometric determinations. They concluded that selection of microbiota, population dynamics, substrate concentration,
nutrient deficiency, acclimation of biomass, gas transfer limitations, temperature, atmospheric pressure compensation, and nitrification play an important role in respirometric experimentation.
In respirometric devices, dissolved oxygen (DO) concentration in a biologically active wastewater
sample is a function of the oxygen transfer rate through the gas-liquid interface. The oxygen transfer
OXYGEN SUPPLY SYSTEM
CARBON DIOXIDE REMOVAL SYSTEM
CO, * 2KOH "'MCCCV Hp
OTHER GASES
GAS PHASE /\
..CO]    CO, * up * BACTERIA <* |   BACTERIA . FOOO * Oj
LIQUID PHASE
cq + h,o   ; hjCO,               isamplfi
I '  (SAMPLE) | pirometer.
Figure 1.    Schematic of a constant pressure res-
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc.. Chelsea, Michigan 48118.
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