Floc characteristics play a major role in the removal of contaminants from water in physicochemical treatment processes. The efficiency of the main removal processes is a function of floc size, strength and density Changes in these parameters affect floc removal and hence the removal of adsorbed organic matter. Coagulation and flocculation efficiency and floc strength are often assessed using a jar tester. Here, CFD was used to model the flow field within a standard jar test apparatus and, using a Lagrangian particle trajectory model, to study the effects of turbulence on individual flocs. Combining numerical and experimental data, velocity gradient values at which floc breakage occurs are postulated for three different floc suspensions. Although the threshold values are determined using jar test and CFD data in combination, they are based on the flocs' resistance to induced velocity gradients. This is a significant result, as previous breakage thresholds have been expressed in terms of mixing speed and cannot be applied at full scale. The results shown here can be adopted for use in other situations and can be used to assess the performance of existing flocculators or to design new installations. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.