In BF-MBR, as in all sMBR systems, sufficient aeration is necessary to maintain local cross-flow conditions for sustainable membrane operation. Aeration rate is normally based on previous experiences and manufactures' recommendations. BF-MBR with an external submerged membrane unit, designed only for particle separation, could be relatively easily optimized in terms of membrane aeration. Combining observed fouling rates with measured particle size distribution and minimizing both parameters indicates a desirable sustainable aeration rate for given wastewater characteristic, operating parameters (i.e. production/backwash flux, recovery) and membrane module geometry.