The three-dimensional pore structures in waste activated sludge floc were identified using the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) images. The meshes of the three-dimensional porous configuration of a sludge floc were constructed from the CLSM series images. The intrafloc flow field was simulated for the constructed floc model when it was subjected to a uniform flow field, based on which the Darcy's permeability was estimated. The permeability (k(DL)) of original floe was estimated as 8 X 10(-12) m(2). Flocculated flocs had higher k(DL) due to their large pore size, while the corresponding values of k(DL) of the freeze/thawed flocs were lower. The calculated results indicate that a few large pores in the floc determine the permeability. The fractal dimension and compactness, however, are not correlated with the permeability of the flocs. (C) 2004 Elsevier Ltd. All rights reserved.