Improved performance of a biofilm membrane bioreactor (BF-MBR) process by addition of iron and alum based inorganic coagulants was studied with respect to reduction of surface area of submicron particles, enlargement of suspended flocs, and reduction of the submicron organic content. Iron based coagulant was the most effective in enlarging surfaces of flocs in the bulk phase. Improvement in membrane performances was found to correlate to reduction of measured particle surface area in the size range 0.040.3 mu m, enlargement of floc surface area for particles >8 mu m. Total phosphorus removal for both coagulants added was in the same range.