Grit is one of the most unpredictable and difficult materials a sewage treatment plant must handle. Grit can be defined as the heavy mineral matter present in sewage. It is principally made up of sand and soil, but frequently contains cinders, coffee grounds, seeds, corn, and other coarse sediment which reaches a sewage disposal plant. It is desirous to remove this material as it cannot be treated, reduced in size, or eliminated by treatment methods. It presents a problem to waste treatment as it is hard and abrasive. It wears pumps and other mechanical devices. It is heavy and accumulates in clarifiers, treatment basins, digesters, etc., where it must often be removed by hand.
Grit removal devices of various designs have been proposed to remove grit from a flowing stream of water. One such grit trap device, which has been very commercially successful, is disclosed in U.S. Pat. No. 3,941,698 to Frank G. Weis, which patent is assigned to the same assignee as the present invention. This grit trap device includes an upper settling chamber and a lower grit storage chamber. The settling chamber, being of large diameter, communicates with the storage chamber through a relatively small opening in a substantially flat transition surface therebetween. Rotating paddles positioned within the settling chamber, a short distance above the transition surface, cause the flow of liquid entering the settling chamber adjacent the outer periphery to rotate about the chamber as a forced vortex resulting in an upward spiral flow which urges the settled particles across the transition surface towards the opening. The heavier settled particles fall through the opening into the storage chamber and the lighter organic particles rise in the spiral flow. The contents in the storage chamber are lightly air scoured prior to removal to cause any organics therein to be lifted out of the storage chamber and returned to the settling chamber.
A similar type of grit removal device as the one discussed hereinabove is disclosed in U.S. Pat. No. 4,107,038 to Frank G. Weis, which patent is also assigned to the same assignee as the present invention. In this patent, a ramp is provided in communication with the flume portion of the inlet trough to cause grit to follow the ramp down towards the transition surface. A baffle is also positioned in the settling chamber against which the rotating liquid impinges to deflect the liquid downwardly into a generally toroidal flow pattern that spirals around the periphery of the settling chamber. The toroidal motion of the liquid moves the grit on the transition surface towards the center opening.
The above-described prior art devices operate on the forced vortex principle. In these devices the head at the periphery of the settling chamber is higher than at the center of the settling chamber. This causes liquid to flow down the wall of the settling chamber to the bottom thereof and across the bottom to the point of lower head at the center thereof. It is this transverse circulatory flow pattern which permits the device to work. The particulate matter in suspension must follow this path to reach the bottom of the settling chamber and be carried to the center of the transition surface to the storage chamber. This takes some time and some of the particulate matter may not travel the full circuit before it is caught in the flow passing out the effluent, which results in a lowering of grit removal efficiency. The influent flume and the effluent flume of the above-described devices are at right angles to each other and in most installations require a 90.degree. bend to get them back in line.