Chemical conditioners have been employed widely to improve sludge mechanical dewaterability, but there is still a large quantity of water in the flocculating constituent of activated sludge, which leads to the difficulty in further reducing the water content of dewatered sludge. The presence of extracellular polymers (ECP) is believed to be one of the unfavorable elements in activated sludge dewatering. This paper investigated the effect of removal of ECP from the solid surface by the use of surfactant and acid before commonly used conditioners utilized on activated sludge filtration dewatering. The moisture content of dewatered sludge dropped by 3-5% if activated sludge was pretreated with acid or surfactant ahead of the conventional conditioners used, which decreased by 7-11% in the case of combining the utilization of acid and surfactant. It was found that the use of acid and surfactant pretreatments before conditioning with polyacrylamide or calcium oxide and ferric chloride reduced the sludge water content from around 82 and 80% to about 75 and 69% when sludge MLSS was 12.5 and 9.9 g/l, respectively, which resulted in an almost 28 and 35% reduction in dewatered sludge volume compared to that without surfactant and acid pretreatments. (C) 2004 Elsevier B.V. All rights reserved.