The goal of this study was to evaluate the role of the membrane within a membrane bioreactor(MBR) and to model the flux. Two models developed for external membrane bioreactor enabled the evaluation of the importance of mixed liquor suspended solids. The effect of backwash was also quantified. A performance comparison of two membrane bioreactors using membranes with different pores demonstrated the predominant importance of the sludge layer (deposited at their surface and inside of the membranes) versus the pore size.