Abstract: A vacuum toilet system comprises a waste-receiving bowl having an outlet, a sewer pipe that can be placed at a pressure that is lower than that in the interior of the waste-receiving bowl, and a discharge valve connected between the sewer pipe and the outlet of the waste-receiving bowl for controlling passage of material from the waste-receiving bowl to the sewer pipe. The discharge valve comprises a valve housing and a closure member. The closure member is movable relative to the valve housing between an pen position and a closed position. A drive mechanism is connected to the closure member for bringing about movement thereof relative to the valve housing. A sensing device provides a control signal in the event that the drive mechanism is not able to cause the closure member to attain its closed position.