Method of treating liquid by sedimentation in which reagents are injected into a flow of untreated liquid and there is created in the flow a colloid mixing and stabilization area; the flow is passed into an intermediate aggregation area and then into a sedimentation area equipped with separator plates from which clarified liquid is taken; characterized in that insoluble granular material which is more dense than the liquid is injected into the liquid in predetermined proportions in the mixing area where the flow is made turbulent, turbulence is induced in the intermediate aggregation area to maintain the granular material in suspension, virtually all of the granular material is brought into the sedimentation area, the sludge recovered in the sedimentation area is removed, the granular material is removed therefrom (D) and is recycled after cleaning.