A grit chamber is used to sift solids, such as sand or other particulates or foreign materials, from liquids. Such a chamber is primarily found in waste treatment facilities and sometimes in other industrial applications. A typical grit chamber is designed to reduce the speed of flowing liquids and to allow particles to settle out through the action of gravity. This serves a purpose beyond the simple task of cleaning the liquids of pollutants. It keeps material from clogging other machinery, such as valves and pumps, reduces buildup of sediments and particulates in pipes, and reduces wear on other elements of treatment systems.
Many different designs of grit chamber are used around the world, but all have the same basic purpose. Most grit chambers are carefully designed for the specific gravities, or densities, of the particles to be removed. A typical design will remove only particles up to a certain density, while allowing lighter particles to continue, to be removed during later steps in the treatment process. Some grit chambers are long, narrow, enclosed metal tanks, while others may be nothing more than a concrete lined pond or tank set in the ground.
Most grit chambers are one of two main types. A simple, or passive, grit chamber relies on control of liquid flow speeds and gravity to remove particles. The dimensions and shape of the chamber are laid out in such a way as to maximize the removal of particles up to a certain size and density, while allowing smaller, lighter particles to pass. Ideally, mostly inorganic particles such as sand, gravel and bits of glass are removed this way, while allowing organic particles and materials to continue.
Aerated tanks or chambers use infusions of air to aid in the separation process. By introducing jets of air at the surface, perpendicular to the direction of the main flow, a series of small spiraling eddy currents are created. These serve two purposes. Small bits of organics are rinsed from the larger, inorganic particles, improving the differentiation between the two types and allowing the smaller organics to pass. The spiraling currents also have the effect of improving the removal of particles over a shorter overall distance of travel for the main flow when compared to an identical tank without aeration.
Mechanically cleaned grit chambers are designed with machinery or other devices for cleaning the accumulated sediments. These can be very simple or rather elaborate, depending on the particular design. Some very simple grit chambers have no such systems and must be manually cleaned. Most facilities that employ grit chambers have more than one, sometimes several, so that individual tanks can be cleaned without hindering the overall operation.