Abstract: A rotary sedimentation apparatus (10), such as a clarifier or thickener, includes a conventional rake arm structure (26, 28, 30) adapted to urge sedimented sludge toward the center of the bottom wall of the apparatus and a centrally mounted drive and lift unit (24) adapted to rotate the rake arm structure. The lifting unit includes an inner cage (84) that is adapted to rotate with the rake arms but to remain at the same elevation when the rake arms are lifted. An improved scum skimming mechanism (12) includes a flat elongate inner blade 40 that is connected by two cables (66 and 68) to the inner cage (84) so that the inner blade remains partially submerged at the liquid level, even though the rake arm structure may be raised. The inner blade is vertically slideably mounted to the upper end of the support frame (50) that extends upwardly from the outer end of a rake arm (30).