The aeration process is widely used in wastewater treatment plants to remove microorganisms from sludge. Among aeration reactors, bubble columns have been extensively used due to their simple geometry. In this study, a computational fluid dynamics (CFD) technique was applied to investigate the hydrodynamics involved in an activated sludge bubble column. The CFD modelling integrated an incorporation of the continuity, momentum, and Eulerian-Eulerian multiphase equations along with the species transport equation. The validated CFD model was then applied to explore the effect of superficial gas velocity, the location of the tracer's injection, and the sparger type on mixing time. The design of experiment (DOE) and full factorial design method were carried out to investigate the interaction among the independent variables. A significant relationship between superficial gas velocity and tracer injection location on the mixing time have been noticed. The performance of a novel gas sparger, the modified star-shape gas sparger, was also evaluated. The modified gas sparger showed an enhanced hydrodynamic performance compared to the typical sparger.