Bioleaching by sulfur oxidizing bacteria has been regarded as a novel dewatering process for the sludge treatment. Since the bioleaching process is a comprehensive biological and chemical process for sludge treatment, it is necessary to explore the dewatering mechanism of sewage sludge improved by bioleaching. The bioleached sludge showed a significant difference with the control sludge, mostly through a considerable reduction of pH (to 3.92) and an improved specific resistance to filtration (SRF), which reduced to 5.31 x 10(10) mikg after 72 h treatment. Separate sulfuric acid addition and Fe2+ addition did not result in significant decrease of sludge resistance filtration, indicating that the chemical acidification and Fe2+ addition are not the deciding factors for the improvement by bioleaching. The extracellular polymeric substances in bioleached sludge decreased considerably, with the protein and polysaccharide reduced by 97.42% and 76.00%, respectively. During the bioleaching process, the number of microbial genuses in the bioleached sludge gradually decreased and the dominant bacterial genus (Acidimicrobium ferrooxidans) shifted from 7.48% to 26.49% at the end of bioleaching. While many factors influence the dewaterability of the sludge, the bio-substitution which led to the decrease of EPS is the deciding factor for the dewaterability improvement of the bioleached sludge. (C) 2016 Elsevier B.V. All rights reserved.