The breakage of activated sludge flocs under turbulent shear conditions was investigated as a function of floc size. Municipal activated sludge flocs were fractionated by sieving to narrow size fractions. Shear stress distribution functions for the breakage of activated sludge floc samples were obtained. It was found that by increasing the floc size, this distribution was skewed towards smaller shear stress values and became broader. Results of experiments showed that the median shear stress, 750, required for floc breakage reduced by about 23% from 3.9 Pa for 45-63 mu m sieve fraction to 3 Pa for the 150-180 mu m sieve fraction. Under steady conditions, the median shear stress for the breakage of fragments that formed due to the breakage of larger flocs was as much as three times larger than that of the original flocs. (C) 2009 Elsevier B.V. All rights reserved.