A rotary filter has a rotary drum with a perforated circumferential wall, the drum extending partly into a tank containing a slurry to be filtered. A sheet of a filter medium is wrapped around the circumferential wall. A first standpipe extends into the drum through a hollow bearing shaft from a filtrate pump, for withdrawing filtrate from the drum, and a second standpipe extending into the upper portion of the drum by way of the hollow bearing shaft is connected to a vacuum pump for separately withdrawing air. A weighing device is positioned to provide an output corresponding to the level of the filtrate in the drum, for maintaining this level within a given range by controlling the operation of the filtrate pump for the purpose of separately withdrawing the air and filtrate and prevent them from mixing.
