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The liquid velocity inside alum and activated sludge flocs was calculated using the size, settling velocity and permeability of activated sludge flocs. The permeability of activated sludge flocs has been determined experimentally. The permeability of alum coagulation flocs was assumed to be half of the permeability of activated sludge flocs based on the size of the pores in these flocs. The average flow velocity inside an activated sludge floc was calculated to be 1575 µm/s, which is in the range of the flow experimentally measured inside biofilms at a distance of about 100 µm from the substratum by Beer et al. (1995). The flow inside an alum coagulation floc was calculated to be 318 µm/s. The flow velocity inside the same flocs estimated with Davies permeability model were 0.7 µm/s for activated sludge flocs and 20 µm/s for alum coagulation flocs. Therefore, the flow velocities estimated on the basis of experimentally determined permeability were much higher than the velocities calculated with Davies permeability model. Davies permeability model assumes homogeneous distribution of porosity inside an aggregate. Direct observations made during the analysis of floc sections have proven this assumption to be wrong. Flocs have fractal structure and the models predicting their permeability should be based on this feature. Flow rates through alum and activated sludge flocs predicted on the basis of the fractal model of a floc compared well with experimental results.

Flow Rates Through Alum Coagulation and Activated Sludge Flocs