Flocculation processes and its influencing factors (e.g. tank hydrodynamics) are generally not studied in detail. Classical models do not account for the interaction between activated sludge particles but simply use a lumped variable such as the sludge concentration. However, next to concentration also specific properties of the flocs such as size, density and porosity will have an important influence on the settling performance. These properties were included using PBM. This thorough understanding of flocculation processes in clarifiers could improve their design and operation, which can significantly improve the settling performance and control (both with respect to recycle flow and effluent quality).