Gas Transfer Limitations in Environmental Respirometry
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.' 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 a~ t iv i ty .~ 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
To successfully interpret the data obtained using any instrument, its limitations must be known. Tabak et al.s identified various physical and biological factors which limit respirometric determinat i on~ . 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.