A separator including a rotatable drum having a drum top part, a vertical axis of rotation and a disk stack arranged therein. Also included is a piston slide for opening and closing solids discharge openings in the drum. A radial gap is formed between the drum top part and the piston slide in an open condition of the piston slide. Radially successive annular chambers are located in the piston slide and in the drum top part and are mutually connected by a bottleneck.