Grit and sand are very abrasive and if not removed from the sewage flow can foul equipment and rapidly wear pumps and other plant downstream. Sand and grit are washed into the sewers from roads and from the ground through cracks in sewers. Grit gets carried along with the flow but if the flow of the sewage is slowed down the grit and sand will settle out.
It is important that the speed is not too slow or organic material, which should pass forward for further treatment, will also be settled out.
Upon entry to the Detritor Chamber the sewage flow is evenly distributed through adjustable, vertical concrete baffles (by civil contractor). Adjustable mechanism supplied by Ham Baker Adams.
The flow slows down to about 0.3m/s and the grit and sand settle to the floor of the tank. Scrapers push the material to small sumps at the side of the tank, from where it is pumped to washing equipment which removes any organic matter. The width of the chamber is determined by the flow rate to ensure that settling velocity is achieved, i.e. when the grit falls out of suspension before the outlet weir. The outlet weir, except at minimum flow, will usually be submerged to form a barrier to ensure that the settled grit is contained in the flat bottomed chamber.
The relatively clean grit is then deposited in skips for transportation to landfill.