To optimize membrane bioreactor (MBR) equipment specifications and operating conditions in order to reduce the aeration rate, we investigated effects of the flat sheet membrane clearance and the bubble diameter on the shear stress on the membrane surface. The results were obtained using an apparatus consisting of a visible-channel, simulated-flat sheet MBR, in which the membrane clearance and bubble diameter could be varied. The shear stress on the simulated membrane surface was measured directly. We found that large bubbles with two-dimensional amorphous shapes between the membranes could make the shear stress large in comparison to the case of bubbles smaller than the membrane clearance. Moreover, we determined how to get the local maximum shear stress in the case of large bubbles. We developed a dimensionless index, consisting essentially of the membrane clearance and the bubble diameter that can be applied to describe the condition for maximizing the shear stress. Because the parameters of this index are easily obtainable, it is of significant benefit for effectively designing the operating conditions and equipment specifications of MBRs to promote scouring. (C) 2010 Elsevier B.V. All rights reserved.