Atmospheric Oxygen Transfer During In Situ Oxygen Uptake Rate Measurements

In situ apparent oxygen uptake rate (OUR) measurements of laboratory bioreactors open to atmospheric air are practical and useful, but atmospheric oxygen transfer into the reactor may significantly underestimate the actual values of OUR. The effects of the atmospheric oxygen transfer were quantitatively assessed using an oxygen mass transfer model. A factor Ψ representing the degree of underestimation of actual OUR by the in situ apparent OUR (OURa) technique was introduced. The Ψ factor for the primary settling tank effluent of a local water pollution control plant was found to be unity in the beginning of reaction period when the microbial activity was high (OURa = 27 mg/L?h), but decreased to 76% at the end of reaction period when the microbial reaction rate was low (OURa = 2 mg/L?h).