Submerged membrane bioreactors (SMBR) are widely used in wastewater treatment.The permeability of a membrane declines rapidly because of the formation of a cake layer on the membrane surface.In this paper,a multiple staining protocol was conducted to probe the four major foulants in the cake layer formed on a filtration membrane.Fluorescent images of the foulants were obtained using a confocal laser scanning microscope (CLSM).The three dimensional structure of the cake layer was reconstructed,and the intal flow was calculated using computational fluid dynamics (CFD).Simulation results agreed well with the experimental data on the permeability of the cake layer during filtration and showed better accuracy than the calculation by Kozeny-Carman method.β-D-Glucopyranose polysaccharides and proteins are the two main foulants with relatively large volume fractions,while α-D-glucopyranose polysaccharides and nucleic acids have relatively large specific surface areas.The fast growth of β-D-glucopyranose polysaccharides in the volume fraction is mainly responsible for the increase in cake volume fraction and the decrease in permeability.The specific area,or the aggregation/dispersion of foulants,is less important to its permeability compared to its volume fraction.