Rapid sand filters use sand or other media to remove suspended solids from water. They are used by municipal water treatment plants and for treating waste and industrial waters. The filter’s effectiveness depends on the influent and granular medium properties, and it is cleaned through backwashing.
A quick sand filter is a water treatment device that uses sand or other grainy media to filter out suspended solid particles in the water. These suspended solids include dirt and other organic matter that make the water appear cloudy or cloudy. Fast sand filtration is commonly used by municipal water treatment plants to clean surface and groundwater intended for drinking water. It is also used to treat waste and industrial waters containing biological contaminants before these waters are discharged into the environment.
Flash sand filtration occurs in a tank containing one or more layers of grain media, such as sand, anthracite or hard coal, garnet, magnetite, or other minerals. The total depth of the mount typically ranges from 2 to 6 feet (about 0.6 to 1.8 meters). In many systems the incoming, or influent, water enters near the top of the tank and flows down through the media. The outgoing water, or effluent, exits through a drainage system called an underdrain that sits under the filter media bed.
As the influent passes through the rapid sand filter, many of the suspended solids in the water become trapped on or within the pores between the grains. This straining process is the primary mechanism by which solids are removed from the water. Furthermore, suspended solids are adsorbed on the surface of the grains due to intermolecular attractive forces. Other physical, chemical and biological processes also improve the removal of solids during filtration.
The effectiveness of a rapid sand filter depends heavily on the characteristics of the influent and the granular medium. Important influencing properties include the concentrations, types, and particle charges of the suspended solids in water. The depth of the media bed and particle charges, grain sizes and shapes are also crucial.
The smooth round grains crowd tightly together and create very small pores. More jagged grains can produce larger pores, but are better at catching solids as they pass. Typically, layers of different grains are used to take advantage of multiple removal mechanisms.
Over time, the media bed of a quick sand filter becomes clogged with accumulated solids. At this point, the filter is cleaned through a process called backwashing. The wash water is pumped up through the bed to remove solids and flush them out of the tank. When the wash water is drained, the granular media settle back into their original layers and the filter is ready to be used again.
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