Grit Removal - Considerable amount of grit is carried along with the flow of sewage and these pose problems in most sewage systems in particularly with combined sewers. If not removed these grit particles may damage the mechanical parts of the treatment system. Grit particles are of large size and have a high density when compared with the organic particles in sewage. 

Thus they can be removed using the principle of differential settling. Grit particles, with a diameter of 0.20mm and a specific gravity of 2.65, settle at a velocity of about 1.2m/min whereas most of the suspended solids in sewage have considerably lower settling velocities. 

With a retention time of 30-60 seconds, the flow of sewage through a channel of sufficient length can allow the settling of grit particles at the bottom while the remaining suspended solids continue to flow through the channel. The settled grit is removed at intervals, washed and then disposed off. Other types of grit removal employ a short retention settling tank.

http://www.microbiologyprocedure.com/waste-water-microbiology/grit-removal.htm