Gas phase characteristics have a significant impact on hydrodynamic conditions and oxygen mass transfer rate in activated sludge bioreactors. In the present study, the dynamic gas disengagement (DGD) technique was utilized to determine major gas phase properties (namely the number of bubble size classes, the contribution of each class to the overall gas holdup, bubble rise velocity, bubble size, and specific interfacial area for oxygen mass transfer) in an activated sludge bubble column bioreactor containing a wide range of MLSS concentration (0.712-15.86 g/L) for superficial gas velocities in the range 0.081-1.303 cm/s. Mixing performance of the activated sludge bubble column was also studied. All measurements were conducted by electrical resistance tomography (ERT) method because of its unique advantages in hydrodynamic analysis. Variations of bubble rise velocity, bubble size, specific interfacial area for oxygen mass transfer, and mixing time with MLSS concentration revealed the complex role of rheology in hydrodynamics and indicated that it is impossible to make a general conclusion about the impact of rheology without considering a wide range for its variations. A correlation was also presented for relating bubble size to the aeration rate and sludge rheology. (C) 2015 Elsevier B.V. All rights reserved.