The effect of adding activated carbon on the fouling of activated sludge filtration was investigated using a complete-mix cell with a flat-sheet cellulosic membrane at a constant pressure gradient of 70 kN/m(2). Four sludge samples were tested in parallel: a sludge without additive served as control, plus three sludge samples dosed with individual additives, including inert diatomaceous earth, activated carbon pre-sorbed with 112 mg-C/g of EPS (extracellular polymeric substance), and virgin activated carbon. Results showed that the EPS of activated sludge was a major contributor of membrane fouling. Additives may reduce the filtration resistance of the film deposited on the membrane surface, depending upon their EPS-sorbing capacity. As compared with the control, virgin activated carbon reduced the film filtration resistance by 22%, from 6.4 +/- 0.5 x 10(12) m(-1) to 5.0 +/- 0.1 X 10(12) m(-1), whereas the reduction was only 14% by the activated carbon pre-sorbed with EPS and negligible by the inert diatomaceous earth. Of the three main constituents of EPS, polysaccharide and humic substance were of significance to the fouling; protein had little effect.