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What is the difference between rapid sand filtration and slow sand filtration (biological sand filtration)?

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Rapid sand filtration does not intentionally use a biological process, (some beneficial bacteria may grow between backwashing events 33 (p 18)) its design uses strictly physical filtration. Very few disease causing organisms are inactivated. Viruses are not removed. Water, that has had coagulant added, is forced through coarse sand under high pressure resulting from depth of water and gravity or added external pressure. These filters require cleaning every several days, or more often, by "backwashing" - forcing purified water and / or air through them from the bottom drain pipes in a reverse direction of normal flow. Chemicals such as chlorine or ozone must be added to the water to kill harmful bacteria and viruses. Large volumes of purified water are required for backwashing, and large volumes of toxic sludge are produced as a result of daily cleaning events called "backwashing". Rapid sand filters are not practical as a water purification method for individual use. Rapid sand ... more
slowsandfilter.org
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Rapid sand filtration does not intentionally use a biological process, (some beneficial bacteria may grow between backwashing events 33 (p 18)) its design uses strictly physical filtration. Very few disease causing organisms are inactivated. Viruses are not removed. Water, that has had coagulant added, is forced through coarse sand under high pressure resulting from depth of water and gravity or added external pressure. These filters require cleaning every several days, or more often, by "backwashing" - forcing purified water or air through them from the bottom drain pipes in a reverse direction of normal flow. Chemicals such as chlorine or ozone must be added to the water to kill harmful bacteria and viruses. Large volumes of purified water are required for backwashing, and large volumes of toxic sludge are produced as a result of daily cleaning events called "backwashing". Rapid sand filters are not practical as a water purification method for individual use. Rapid sand filtration ... more
slowsandfilter.org
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