In physical-chemical treatment processes for waste water, the separation of the solids formed as hydroxides or carbonates can take place by sedimentation following precipitation. This is the basis for the elimination of metal compounds, which may also result in a reduction of the concentrations of organic material and phosphorus compounds in the waste waters. The settling rate of the solids depends on the solids concentration and the size and density of the aggregates formed. These properties can be obtained from batch sedimentation tests. When a batch test is carried out with flocculated suspensions containing a low concentration of solids, the upper supernatant-suspension interface is not always sharp. Nevertheless, the rate of rise of the sediment formed at the bottom can be measured. Taking into account the Kynch theory and other relationships proposed in the literature, the mean size and the ratio of water/solids in the aggregates can be deduced. The method explained in this paper has been applied to different suspensions with various flocculating agents. The aggregate diameter deduced is close to that observed directly for all the runs.