Continuous, automated and simultaneous measurement of oxygen uptake and carbon dioxide evolution in biological systems

Commercial respirometers are capable of continuously and automatically measuring oxygen uptake in bioreactors. A method for continuously and automatically measuring carbon dioxide evolution can be retrofitted to commercial respirometers. Continuous and automatic measurements of carbon dioxide evolution are achieved by measuring the change in electrical conductivity of barium hydroxide solution. Continuous stirring of the barium hydroxide solution was necessary to prevent the carbon dioxide absorption from controlling the oxygen uptake rate. Three separate experiments (aqueous biodegradation of phenol, soil slurry biotreatment of soil contaminated with phenol, and aqueous biodegradation of crude oil) were conducted to test the oxygen uptake and carbon dioxide evolution measurement system. The respiratory quotient, defined as the ratio of carbon dioxide evolution to oxygen uptake, is close to one until a plateau of oxygen uptake is reached, after which it is greater than one due to degradation of biomass and metabolite products. Unsteady-state simulation of CO2 evolution in batch reactors, based on the experimental data, showed that carbon dioxide exists primarily as bicarbonate in the liquid phase. The concentration of carbon dioxide in the gas phase is small mainly due to fast reaction with the stirred barium hydroxide solution.