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 m 3 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 k L a 20. This increase in k L a 20 with the axial liquid velocity is mainly due to the attenuation of the vertical liquid circulation induced by the gas injection.