Gas Transfer Kinetics in Oxygen Activated Sludge

Abstract

The validity of a dynamic model based on the two‐film theory is examined for predicting gas transfer in an oxygen system using surface mixers. Model predictions are compared to field measurements taken from a four stage, closed‐tank carbonaceous reactor at the City of Houston, Texas, 69th Street Complex. A liquid film limited model is found to be effective in predicting gas and liquid phase composition although direct interfacial transfer increased the overall oxygen transfer rate. DO predictions were best when using a linear relationship between the KLa for oxygen and the oxygen uptake rate. Carbon dioxide and nitrogen levels were adequately predicted using constant values for KLa, but the model was not sensitive to changes above 50% of the estimated KLa values. Leaks through cracks in the concrete tank cover had little effect on the gas phase composition in the first two stages, however, fourth stage leaks and intentional venting resulted in increased oxygen levels as nitrogen was displaced from the gas phase. Carbon dioxide levels were not appreciably affected by leaks or venting.