Foulants with low concentration in effluent are generally considered minor for determining resistance to membrane filtration. This study probed spatial distribution of proteins, nucleic acids and alpha- and beta-D-glucopyranose polysaccharides in fouling layer using multiple staining technique and confocal laser scanning microscope (CLSM) imaging and clarify their individual contributions to the overall permeate flow resistance. In the present case, the beta-D-glucopyranose polysaccharides occupied over 80% of the volume of the fouling layer and were the major foulant. The other three components (proteins, nucleic acids and alpha-D-glucopyranose polysaccharides) were minor foulants. The computational fluid dynamics calculations revealed that the beta-D-glucopyranose polysaccharides contributed much greater to cake resistance than did the other three foulants; however, its contribution could only count 12-15% of the overall resistance of filter cake. Marked reduction in cake permeability is yielded by blockage of large pore in fouling layer by minor pollutants. (c) 2012 Elsevier B.V. All rights reserved.