The document discusses a drum filter, which is a surface filtration device used to filter slurries. It consists of a horizontally mounted, perforated metal cylinder that is 3 meters in diameter and 3.5 meters long, providing 20 square meters of surface area. As the drum rotates, slurry is deposited on the perforated surface and a vacuum pulls liquid through the filter cloth, leaving a solid cake that is continuously removed. Drum filters can filter concentrated slurries continuously at low labor cost due to their large surface area and ability to control cake thickness by varying rotation speed.