A liquid surface decanter devised particularly for decanting clear liquid from settled effluent contained in a tank and comprising a rotatable suport pipe protruding through a side wall of a tank and a header pipe arranged parallel to the support pipe and connected thereto by a number of downcomer pipes. The header pipe is provided with longitudinal weir slots and is supported on the surface of the liquid by a buoyancy float. The level of the liquid relative to the weir slots is adjusted by the buoyancy float until liquid flows into the weir slots down the downcomer pipes and out through the support pipe at a predetermined rate causing the header pipe to fall within the tank and rotate about the axis of the support pipe.
