In Situ characterization of local hydrodynamic parameters in 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. Bubble size, liquid level and oxygen mass transfer coefficient were measured in situ for a same air flow rate (1,555 Nm3 h-1) and for two different horizontal liquid velocities (0 and 0.42 m s-1). The increase in the global oxygen transfer coefficient is of 29% when the mean horizontal 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%) can not explain the increase in kLa20. This increase in kLa20 with the horizontal liquid velocity is mainly due to the attenuation of the vertical liquid circulation induced by the gas injection.