In this study, a combined flocculation and sedimentation model is developed. The model predicts the time needed to reach a desired concentration of microalgal suspension in a sedimentation tank. The concentration of the particles as function of the time and the position in the tank is described. The model was validated with experimental data for Ettlia texensis. The concentration changes measured in time at different heights in the sedimentation vessel corresponded well with model predictions. The model predicts that it takes 25 h to reach a final concentration of 5.2 gDW L-1, when the initial concentration is 0.26 gDW L-1 and the tank height is 1 m. This example illustrates the use of this model for the design of the settling tank needed for pre-concentration of microalgal biomass before further dewatering. (C) 2013 Elsevier Ltd. All rights reserved.