The horizontal flow grit chamber is the oldest type of grit removal system. Grit is removed by maintaining a constant upstream velocity of 0.3 m/s (1 ft/s). Velocity is controlled by proportional weirs or rectangular control sections, such as Parshall flumes. In this system, heavier grit particles settle to the bottom of the channel, while lighter organic particles remain suspended or are resuspended and transported out of the channel. Grit is removed by a conveyor with scrapers, buckets, or plows. Screw conveyors or bucket elevators are used to elevate the grit for washing or disposal. In smaller plants, grit chambers are often cleaned manually.