Electroosmotic flow in sludge flocs is one of important transport processes playing a critical role in electroosmotic dewatering of sludge. In the present work both fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) are employed to obtain the interior structural information of flocs and the structural properties are incorporated into the computation of electroosmotic flow inside flocs. Some important parameters, including flowrate distribution, pressure drop and permeability, were specifically investigated to explore the flow characteristics. An investigation was also conducted to compare the electroosmotic flow with the corresponding pressure driven flow inside flocs. (C) 2009 Elsevier Ltd. All rights reserved.