Abstract: The changes of sludge dewaterability and bound water content at various pH values were investigated in this study. The protein and polysaccharide contents in each layer of extracellular polymeric substances(S-EPS、LB-EPS and TB-EPS) and the organic acid and organic functional group contents in S-EPS were determined in an attempt to explain the influence of EPS on the sludge dewaterability and bound water content. The results indicate that the sludge dewaterability under acidic conditions is obviously better than that under neutral conditions,and the water content of dewatered sludge(WC) and the capillary suction time(CST) can be decreased to 60.8% and 25.4 s at pH 3.03,respectively. Under alkaline conditions,the bound water content in dewatered sludge,the WC and CST significantly increase,and the sludge dewaterability deteriorates. The addition of acid or alkali reduces TB-EPS,on the contrary,increases LB-EPS and S-EPS. Each layer of extracellular polymeric substances has significant correlation with the WC,CST and the bound water content,and S-EPS has most significant correlation with the sludge dewaterability and bound water content. In the process of acid-alkali conditioning,the amount and type of organic-functional groups improve obviously and hydrolysis of partly EPS generates organic acids and other small molecules.