This paper provides an estimate of the waste-activated sludge hoc interior permeability on the basis of observing the motion of individual floc moving vertically towards an impermeable flat plate. The fluid flow fields surrounding and inside the floc were modeled numerically, from which the hydrodynamic drag force was calculated. The experimental data correlate with the numerical solutions regardless of the floc Reynolds number. Over the floc size range investigated in this paper, the permeability is noted to be approximately proportional to the square of floc size, ranging from 2.5 x 10(-9) to 9 x 10(-6) m(2) for floc size ranging from 150 to 10 000 mum The structure of waste-activated sludge floc is proposed as a multi-fractal. Permeability measured in this work is thereby mainly attributed to the global structure level of the flocs.