The interior of sludge floe is highly heterogeneous, while the large pores in the floe control the advective flow. This work for the first time numerically details fluid flow and mass transfer processes in pores of activated sludge floe. The dimensionless permeabilities and mass dispersion coefficients were contoured against pore size ratio and the floe Reynolds number. With a pore size less than 20% of the floe size, the commonly adopted homogeneous model overestimates the floe permeability, and pore velocity is less than 2% of the bulk velocity. This is particularly true for floes with low porosity. Although the convective flux is low, the dispersive mass transfer rate can be much higher than the diffusional rate, attributable to the strong Taylor dispersion effect. The three-dimensional pore structures in waste activated-sludge floe were identified using confocal laser scanning microscope (CLSM) images. Large pores were used to numerically estimate the permeability and dispersion coefficient for these pores. The permeability and the dispersion coefficient of the tortuous pores can be one order of magnitude lower than those for the equivalent straight pores. Besides the dispersion effect, the pore tortuosity appeared as the most important geometrical factor retarding the advective flow in the sludge pores. In addition, the small side pores connected to the large pore had only a mild effect on the flow process, and can be neglected in analysis. (C) 2004 Elsevier Inc. All rights reserved.