Bubbling requirements for membrane bioreactors (MBRs) are typically split into fine bubbles for aeration and larger coarse bubbles for fouling control. This article examines the possibility of reducing air requirements by using the same air supply to achieve both tasks for a submerged flat-sheet MBR. A bubble-size transformer (BST) comprising converging channels and a tube-bank unit was used to coalesce fine bubbles into coarse bubbles. The BST was located under the flat-sheet membrane modules and generated coarse bubbles to scour the membrane surfaces. Approximately 65% of the fine bubbles produced from fine ceramic air diffusers were transformed into coarse bubbles, and 35% of the fine bubbles were used for biological activities and to prevent the settlement of mixed liquor suspended solid (MLSS) in the bioreactor. Both bubble diameters and bubbling velocities were found increased after the transformations. The BST increased bubble diameters from 0.5-2 mm to 9-18 mm. The coarse bubbles after transformation were bigger than the gaps between the flat-sheet modules at 5 nun so that slug flow probably occurred in the module gaps. Long-term MBR experiments at air flowrates of 3 and 5 L/min with permeate fluxes of 22.5 and 35 L/m(2).h were used to evaluate the bubble-size transformation technique,for the MBR. The benefit from the BST can be seen by comparing the "sustainable" flux period before a rapid rise in transmembrane pressure. (c) 2006 American Institute of Chemical Engineers.