In this decanter design the square shaped floating unit moves up and down along the frame mounted on the tank edge following the changing water level. The frame and the floating body are made of welded steel. Decanted treated water is removed through a plastic hose connected to the floating body. The floating body has two positions in relation to the water surface. “park” position is achieved by delivering and keeping pressurised air under the decanter. The other, decanting position is achieved by releasing the trapped pressurised air. The upper end position of the floating body is determined by detent pins while the lower is determined by the length of two link chains. When reaching the upper position the decanter acts as an emergency overflow.