In membrane bioreactors (MBRs), aeration is widely used for fouling mitigation. To characterize the bubbly flow in MBRs, employing a solution that properly mimics the sludge behavior is essential. Therefore, this work investigates (i) the influence of the aerator pore size on the bubbly flow in xanthan gum solution, and (ii) the effect on the bubbly flow of using water as the continuous fluid and of the absence of the membrane module, conditions sometimes employed as model systems for MBRs. A MBR with a hollow fiber membrane module and a sintered glass plate for the aeration system was studied. Image analysis of the flow was made to assess key bubble parameters: area, circularity, Feret diameter, quantity, and velocity. The bubbly flow formed in the xanthan gum solution presented a lower turbulence level, differing greatly from the one formed in water. Nevertheless, for both fluids, the aerator with smaller pores generated a higher quantity of small bubbles, indicating a larger area of contact with the membrane surface. Such bubbles presented a higher level of size reduction with the presence of the membrane module and higher stability (higher circularity under similar velocities). This suggests that smaller bubbles should present higher antifouling activity. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.