Grit Chambers
If a sewage disposal plant is operated in connection with a combined sewerage system grit chambers are usually necessary for the removal of sand, gravel and dirt before the sewage passes on for further treatment. Where a city has a separate system of sewerage grit chambers are held by some authorities to be unnecessary unless the first wash of the street after a storm is intercepted and the waste is treated. Gregory has expressed the belief that the safest plan under ordinary conditions seems to be to provide a grit chamber. It is generally agreed that the chambers should be so constructed that the sewage will flow through slowly enough for the grit to settle out, but fast enough to carry the organic matter in suspension. To insure proper operation the chamber must be cleaned out frequently. At the Cleveland Sewage Testing Station it has been found that velocities ranging from 30 to 60 feet per minute produce a grit of proper character. The California State Board of Health has advocated chambers with a capacity such that a net period of storage of at least three minutes be allowed and a velocity of not less than five feet per minute.