Click here to enlarge imageSince the introduction of the original Pista in the early 1970s, the prevailing grit removal system in the U.S. is the circular or generically dubbed “vortex” grit chamber. While some sloped-bottom units are considered “vortex” because of their circular configuration, they often rely on particle settling rather than forced hydraulic movement. Two key, patented features distinguish the circular (360-degree) Pista Grit Chamber: its flat basin floor and baffle elements, which work in concert to create a true vortex.
Before encountering the actual system, grit removal efficiency is greatly enhanced by having a long, straight inlet flume where the flow can become as laminar as possible. In contrast, a turbulent flow stirs up grit in the flow stream, making it difficult for any grit chamber to achieve high removal efficiencies. Laminar flow maintains a steady tangential direction as it enters the grit chamber and properly conditions the grit for entrapment.
An inlet baffle at the end of the flume causes the flow entering the Pista® Grit Chamber and the flow inside to impinge. The combined flow moves across the floor toward the center of the basin. At the center of the chamber, rotating paddles maintain upflow currents, forming a toroidal flow path. The velocity of the upflow currents lift the organic materials, which are lighter than the specific gravity of grit, and pass them through the grit chamber. The grit, meanwhile, is swept along the basin floor by the toroidal flow path produced by the combination of paddles, flow baffles and flat floor inside the chamber. This hydraulic activity directs the grit toward a small center opening and into a lower storage hopper.
The new Flow Control Baffle allows an increase in chamber velocity during low flow periods, thus maintaining high removal efficiencies by extending the grit extraction path within the chamber. Beyond this, it permits design flexibility so that water elevations can be controlled.
Controlling the water level is important because it upholds the proper velocities approaching the grit chamber. Previously, the most common way to accomplish this was with a submerged weir downstream of the grit chamber to back up the flow.
The new baffle, with its pre-set inlet and outlet openings, supplants the downstream submerged weir. By integrating the water elevation settings with the baffle, the overall outlet footprint requirements decrease as much as half the typical distance. This also affords the design engineer the flexibility to allow an outlet channel to make sharp turns immediately after leaving the circular portion of the grit chamber. The resulting smaller footprint provides significant construction cost savings.
Grit consists of a variety of particles including sand, gravel and other heavy inorganic materials. A large majority of grit found in domestic sewage - in upwards of 90% and more - are coarser particles of 50 mesh size grit and larger. The remainder composition of smaller grit particles mostly ranges between 50 and 100 mesh. In some coastal regions, there may be higher concentrations of finer grit, which can be smaller than 100 mesh. Grit particles that reach 200 mesh in size have buoyant characteristics like silt which prevent them from settling anywhere in the treatment scheme.
Smith & Loveless publishes actual data from numerous wastewater treatment plants, derived from standardized test procedures that are widely accepted and practiced by consultants and municipality throughout the industry. These results help demonstrate removal efficiencies offered by the company’s grit removal system.
The company will have an actual working model of the Pista® Grit removal System on display at WEFTEC ‘05.
