Abstract: In an activated sludge process for the purification of wastewater, in a completely mixed tank in the presence of carrier particles for the microorganisms, the aerating gas feed creates a tubular spiral circulating flow of the wastewater about a horizontal axis extending from the inlet zone to the outlet zone of the tank. To avoid a buildup of the carrier particles in the outlet zone of the tank, the gas feed maintaining the circulating motion is reduced along the axis of flow with respect to the gas feed set in the initial zone of the flow path. Gas may also be sparged along an axis transverse to the flow near the outlet to create a tubular flow which also clears the carrier particles from the outlet.