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Membranebioreactors (MBRs) are becoming more suitable alternativesfor conventional wastewater treatment devices. The performance ofa pressure-driven MBR is dominantly affected by the hydrodynamic conditionsof the system. This study was conducted to investigate various hydrodynamiccharacteristics including shear stress, cross-flow velocity, and membranefouling resistance, using computational fluid dynamics (CFD). Simulationof two- and three-phase flow for a flat-sheet submerged membrane modulewas carried out, and the results were compared with the experimentaldata. The CFD simulation was implemented to analyze the fluid-flowpattern, shear stress on membrane surfaces, and cross-flow velocitybetween membranes at various mixed liquor suspended solid concentrationsin the bioreactor. It was shown that the cross-flow velocity playsan important role in the membrane fouling and determination of thecritical particle diameter. To achieve an optimal operating condition,the critical particle diameter was calculated at different air flowrates and permeate fluxes. The CFD results showed that the outermostmembranes are more prone to fouling because of the lower shear stresson their surface as well as the lower cross-flow velocity betweenthem and the module wall. Moreover, the effect of the air bubble diameteron the air and liquid shear stress was investigated to determine anoptimal bubble size. [ABSTRACT FROM AUTHOR]