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Sand filters are used for water purification. There are three main types; # rapid (gravity) sand filters # upflow sand filters # slow sand filters All three methods are used extensively in the water industry throughout the world. The first two require the use of flocculant chemicals to work effectively while slow sand filters can produce very high quality water free from pathogens, taste and odour without the need for chemical aids. == Sand bed filtration in context == A sand bed filter is a kind of depth filter. Broadly, there are two types of filter for separating particulate solids from fluids: * Surface filters, where particulates are captured on a permeable surface * Depth filters, where particulates are captured within a porous body of material.〔Rushton, A, Ward, A S and Holdich, R G (1996). ''Introduction to Solid-Liquid Filtration and Separation Technology.'' Wiley VCH. ISBN 978-3-527-28613-3〕 In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning screen filters, hydrocyclones and centrifuges.〔 There are several kinds of depth filter, some employing fibrous material and others employing granular materials. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small amounts (<10 parts per million or <10 g per cubic metre) of fine solids (<100 micrometres) from aqueous solutions.〔Coulson, J M; Richardson, J F; Backhurst, J R and Harker, J H (1991). ''Chemical Engineering.'' Vol.2, 4th Ed. ISBN 0-7506-2942-8.〕 In addition, they are usually used to purify the fluid rather than capture the solids as a valuable material. Therefore they find most of their uses in liquid effluent (wastewater) treatment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sand filter」の詳細全文を読む スポンサード リンク
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