Wastewater usually contains a relatively large amount of inorganic solids called grit (e.g. sand, cinders, and gravel). This material will damage pumps by abrasion and cause serious difficulties in sedimentation tanks and sludge digesters. Thus, it is common practice to remove this material in grit chambers.
After larger solids are screened out of the effluent, raw sewage rapidly flows into the grit chamber. The purpose is to settle out inorganic material, but not to allow organic material to settle. Removing grit is important to protect downstream pumps, centrifuges and heat exchangers and to reduce deposits from forming in the pipelines and channels.