In situ characterization of local hydrodynamic parameters in closed-loop aeration tanks

The objective of this experimental study was to collect and to interpret data in order to better understand the oxygen mass transfer phenomena occurring in full-scale aeration tanks equipped with fine bubble diffusers and slow speed mixers (inducing horizontal liquid flows). Bubble size, local depth and oxygen mass transfer coefficient were measured in situ for a given air flow rate (1555 N m3 h−1) and for two different axial liquid velocities. The increase in the global oxygen transfer coefficient is of 29% when the mean axial liquid velocity varies from 0 to 0.42 m s−1. The small influence of the liquid velocity on the local bubble Sauter diameter (about −4%) cannot explain the increase in kLa20. This increase in kLa20 with the axial liquid velocity is mainly due to the attenuation of the vertical liquid circulation induced by the gas injection.