The present research work introduces a recirculated airlift bioreactor (biocontactor), which combines advantages of packed bed and external loop airlift bioreactors. This bioreactor is characterized by an internal gas redistributor placed between two beds of packing in the riser while working fluid recirculates through an external downcomer. The main objective of this research work was to characterize the mixing hydrodynamics of this reactor through the non-intrusive flow visualization technique of electrical resistance tomography (ERT). The tomography images were employed to examine the effects of packing, gas redistributor, sparger configuration, gas flow rate, and liquid height on the mixing time and superficial liquid velocity. Results showed that the mixing time in the bioreactor was decreased with an increase in the superficial gas velocity in the riser. It was found that at a constant riser superficial gas velocity, the mixing time was increased and the superficial liquid velocity decreased when a cross shaped sparger was replaced by a circular sparger, which had the same diameter, and the same number and size of holes. Results also showed that mixing time increased and the superficial liquid velocity decreased by reducing the liquid height in the bioreactor, in addition to installing the internal gas redistributor and packing in the riser. The presence of the internal gas redistributor and packing reduced the cross-sectional area available for flow and increased resistance in the liquid flow path. Results, which were obtained from this study, can be utilized to improve mixing in external loop airlift bioreactors for wider range of applications. (C) 2012 Elsevier B.V. All rights reserved.