Abstract: A scum skimming device is disclosed for use in a waste treatment system having a circular sedimentation tank. Such tanks typically have a central waste liquid inlet pipe having inlet ports and an outlet weir around the periphery to maintain the waste liquid level in the tank at a predetermined desired level. The scum skimming device consists of at least one scum skimming blade which is biased into a position partly above and partly below the liquid surface and which sweeps the surface of the tank, the blade being supported from a bottom sludge rake by articulated support posts. The device also consists of a generally helical blade extending to the tank periphery and being positioned with a portion of the helical blade below the waste liquid level when the tank is in use, and a drive means for rotating the generally helical blade.