A sludge extractor with a receiving tank, on which a suction connection for a suction element, which draws in a sludge-containing liquid, and a discharge connection for discharge element, which discharges the sludge-containing liquid, are mounted, and with a motor for driving a suction device in such a way that a negative pressure is created in the receiving tank, which negative pressure keeps a vacuum valve on the discharge element closed, is characterized in that the negative pressure acting on the vacuum valve is lower than the pressure which acts on the vacuum valve as a result of the sludge-containing liquid when a predetermined fill level is reached by the sludge-containing liquid in the receiving tank.