Changes in biofilm architecture and membrane filterability were investigated in submerged membrane bioreactor (MBR) under various operating conditions. Using confocal laser scanning microscopy (CLSM) and image analysis techniques, the porosity and biovolume of a biofilm formed on a membrane surface was analyzed along the length of hollow fibers. The addition of a membrane fouling reducer (MFR), a type of cationic polymer. to a conventional MBR led to the flocculation of activated sludge, resulting in a more porous biofilm on the membrane surface, which substantially enhanced membrane filterability. Soluble foulants in the bulk phase of MBR, such as soluble COD and soluble extra-cellular polymeric substances (EPS) were also entrapped by the microbial flocs during the course of the flocculation, leading to an increase in the concentration of bound EPS. The porosity of the biofilm changed greatly along the length of the hollow fibers. The lowest porosity was observed at the potted ends of membrane fibers which can be easily compressed by suction pressure. The biovolume of the biofilm near the potted ends was greater than that near the free-moving ends. With the addition of MFR, porosities were increased whereas biovolumes were decreased along the. length of the fibers. The spatial distributions of both porosities and biovolumes, however, became more uniform along the length of fibers. (c) 2006 Elsevier Ltd. All rights reserved.