This paper concerns the clarification step in water treatment and in particular the flocculation and settling processes. The objective of this study is to analyze the impact of local hydrodynamics onto the overall treatment efficacy. A first set of flocculation experiments is performed to investigate the relation between the floc characteristics (size, shape, fractal dimension) and the hydrodynamics in a mixing vessel (jar-test). The floc particles thus formed are then settled and their settling velocity is measured. Particle Image Velocimetry (PIV) technique is used in this first experimental stage. Different types of water have been used to assess also the influence of water quality parameters on the floc characteristics and behavior in settling. In a second stage, on the experimental result basis, the settling phenomena is modeled using Computational Fluid Dynamics (CFD) on both pilot and full-scale tanks to reproduce the major local mechanisms controlling the process. Includes 10 references, figures.