Submicron particles represent one of the major foulants in the biofilm membrane reactor BF-MBR. Reduction of the amount of submicron particles (colloids) adjacent to the membrane is one measure in order to provide better fouling control in BF-MBR systems. A submerged hollow fiber (Zenon Zeeweed) membrane reactor was redesigned by introducing a flocculation zone below the aeration device of the membrane module. This resulted in reduction of submicron particles around the membrane from 8.2% to 6.9%, expressed in differential number percentage. The size of the most abundant particle fraction consequently increased from 0.70 to 0.84 mu m. Furthermore, the modified membrane reactor design provided longer operational cycles, 40% reduction of suspended solids around the membrane, and improved retentate/concentrate characteristics, i.e., dewaterability (CST), settleability (SVI/SSV) and filterability (TTF).