The performance of submerged membrane bioreactors (sMBRs) treating municipal or industrial wastewater is significantly affected by hydrodynamic conditions in the membrane unit. These hydraulics are induced by air bubbles injected into the stagnant suspension to remove the cake layer. The aeration sequence consumes the biggest proportion of the overall energy input of sMBRs. Therefore, the objective of this study was to investigate the impact of irregular fiber arrangement on the aeration efficiency. For this purpose, a geometry model based on X-ray computer tomography (CT) scans was developed to map the instantaneous displacement of fibers. The scanned images were processed and implemented into the CFD code as porosity and friction factor matrices. Single-phase simulations show the impact of local fiber arrangement, superficial inlet velocity and solids concentration on the distribution of liquid velocity and turbulent viscosity. Bypass streams were found to reduce the expected cake removal averaged over the corresponding cross-section of the hollow-fiber bundle. Initial multi-phase simulations suggest higher mixture velocities in cross-sectional regions without fibers compared to fiber segments. These results serve as a proof of principle and indicate the potential of this novel CFD approach in module design optimization to reduce aeration requirements. (C) 2011 Elsevier B.V. All rights reserved.