Fluid dynamics is used for fouling mitigation in membrane bioreactors (MBRs), whereby a proper understanding of the fluid dynamics is of great interest. The influence of fluid dynamics has led to the use of computational fluid dynamics for optimizing MBR systems. In this work, a model has been validated for flat sheet membranes, with use of the Eulerian multiphase method. The model is validated against a comparable setup where the liquid velocities are measured with a laser Doppler anemometer (LDA). Furthermore, the Eulerian multiphase approach is validated against the more numerical direct volume of fluid (VOF) approach with sludge properties for the liquid, resulting in an error between the models of less than 2% for the wall shear stresses. The VOF model further showed that the horizontal components contribute significantly to the total wall shear stresses. The model has been applied to a full-scale setup for studying the effect of deflecting membranes as deflections have been seen in production. Minimizing the deflection of the membrane sheets was crucial to achieve a good operating condition as a deflection of 2 mm in a setup with a gap of 7 mm decreased the wall shear stresses with as much as 40% on average on the specific membrane surface.