This work estimates the interior permeability of waste activated sludge flocs by observing the motion of individual flocs moving vertically toward an impermeable flat plate. Through a comparison of experimental data with theoretical predictions in the literature, floc permeabilities of original sludge and freeze-thaw-conditioned sludges are found to be 0.7 × $10^{-7}$ to 5 × $10^{-7}$ ${\rm m}^{2}$ and 0.9 × $10^{-9}$ to 6 × $10^{-9}$ ${\rm m}^{2}$ , respectively. Moreover, incorporating inertial effect delays the influence of the flat plate, thereby causing the creeping-flow-based correlation to fail.