Most theoretical models assume constant permeability of wastewater sludge floe. This work shows that, at creeping flow limit with small intrafloc Reynolds number, the permeability of floc can not only be affected by floc structure, but also by the external flow condition. The three-dimensional structure of flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) was firstly probed. Then, the volumetric grid models for sludge floes were constructed. We noted that the floc permeability could keep unchanged, increased, or decreased at increased Reynolds number (Re). Flow redistribution among channels of various sizes contributes to the noted Re-dependent permeability of flocs. (c) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.