Abstract: In a plant for the mechanical-biological treatment of sewage, an immersion element is rotatably mounted within a settling tank so that the element rotates about a horizontal axis. The immersion element includes a perforated outer tube and an inner tube spaced concentrically within the outer tube. The opposite ends of the tubes are connected by plates so that the immersion element forms an enclosed space filled with support elements for biological growth. The outer tube is formed of sheet metal and holes, elongated in the rotational direction, are punched out of the sheet metal with the punched material forming outwardly directed projections. The immersion element is rotated by a drive motor and during rotation at relatively high speeds the projections from the outer tube extend above the water level and draw air into the sewage and form a sewage-air mixture rotating at high speed.