A type of filtration device that consists of a horizontal drum covered by a filter cloth. The drum rotates in a bath that is continuously fed with the process solution to be filtered. A vacuum is applied to the inside of the drum and the solid suspension collects on the surface of the rotating drum as a cake as the filtrate is drawn through. The filter cake is removed from the surface by a scraper or knife. Alternatively, a string arrangement is used in which a series of endless belts run around the drum and a separate cylinder. The strings continuously lift off the cake. Drum filters are used in the preparation of certain catalysts.