liquid to be treated flows into a combined sedimentation and thickening tank . the upper part of this tank is a sedimentation zone in which the liquid passes through parallel flow paths between downwardly inclined separator plates below which the solids fall to the bottom of the tank into the thickening zone whereat the solids are stirred and removed by a suitable stirring and scraping device . the liquids flowing with the solids through the parallel paths are removed at the end of said paths and delivered directly to the top of the sedimentation zone thus leaving the thickening zone essentially flow - free . flocculants may be added to the water to be treated in a separate flocculation compartment arranged upstream from the sedimentation compartment . a separate destabilization compartment may be provided upstream from the flocculation compartment . a square sedimentation zone may merge smoothly into a circular thickening zone . the thickening zone may extend beneath the flocculation compartment for compactness . the scraping and stirring device may be rotatable or movable along a straight path .