A Different Approach for Predicting Reaeration Rates in Gravity Sewers and Completely Mixed Tanks

Abstract

A new semiempirical approach is presented for predicting air‐to‐water oxygen transfer rates in mixed tanks and gravity sewers, using methods adopted from mixing theory. First, a flocculation unit was used to impart selected mean velocity gradients ( G ) into a completely mixed tank, from which oxygen was first removed, and dissolved oxygen concentrations were measured with time. Regression analysis was used to fit the rate of oxygen transfer equation against G . The reaeration rate in completely mixed reactors was found to be proportional to G 2 ( R 2 = 0.987). Subsequently, G was linked to headloss in sewers, and the equation was calibrated using a slope‐adjustable, 27‐m‐long, gravity‐flow, experimental sewer (internal diameter, D = 0.16 m). Here, the reaeration rate was proportional to G 1 ( R 2 = 0.981). The equation was compared with existing oxygen transfer models and validated against experimental data from the literature, to which the overall mass transfer coefficient for oxygen, K L a , derived by the new approach, conformed well.