The rapid sand filter or rapid gravity filter is a type of filter used in

water purification Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for hu ...

and is commonly used in municipal

drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...

facilities as part of a multiple-stage treatment system. Rapid sand filters were first developed in the 1890s, and improved designs were developed by the 1920s. The first modern rapid sand filtration plant was designed and built by

George W. Fuller George Warren Fuller (December 21, 1868 – June 15, 1934) was an American sanitary engineer who was also trained in bacteriology and chemistry. His career extended from 1890 to 1934 and he was responsible for important innovations in water and ...

Little Falls, New Jersey Little Falls is a township in Passaic County, New Jersey, United States. The township was named for a waterfall on the Passaic River at a dam near Beattie Mill. As of the 2020 census, the township's population was 13,360 reflecting a decreas ...

. Rapid sand filters were widely used in large municipal water systems by the 1920s, because they required smaller land areas compared to

slow sand filter Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and bre ...

Design and operation

Rapid sand filters are typically designed as part of multi-stage treatment systems used by large municipalities. These systems are complex and expensive to operate and maintain, and therefore less suitable for small communities and

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. The filtration system requires a relatively small land area in proportion to the population served, and the design is less sensitive to changes in raw water quality, e.g.

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

, than slow sand filters. Rapid sand filters use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of

flocculation Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from ...

chemicals—typically alum. Since media other than silica sand can be used in such filters, a more modern term is "rapid filtration" instead of "rapid sand filtration." The unfiltered water flows through the filter medium under gravity or under pumped pressure and the floc material is trapped in the sand matrix. Mixing, flocculation and

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system. The two types of rapid sand filter are the gravity type (e.g. Paterson's filter) and pressure type (e.g. Candy's filter). A

disinfection A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...

system (typically using

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

) is commonly used following filtration. Rapid sand filtration has very little effect on taste and smell and dissolved impurities of drinking water, unless activated carbon is included in the filter medium. Rapid sand filters must be cleaned frequently, often several times a day, by

backwashing Swash, or forewash in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange. T ...

, which involves reversing the direction of the water and adding compressed air. During backwashing, the bed is fluidized and care must be taken not to wash away the media. The byproduct of backwashing is sludge which is either tankered away or is run to waste if the composition is within the tolerable limits. These tanks are known as "Dirty washwater tanks".

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* {{Authority control Water filters Water technology

Design and operation

See also

Notes

References