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Slow sand filtration
Slow sand Filters work by allowing gravity to draw water through a filter media that composed of sand and gravel. This type of filtration is differs form Gravity filtration in that Gravity filtration systems are generally designed to be back washed, when the filter becomes clogged. In slow sand filtration the top layer of sand acts to trap devolved solids and so when the filter becomes clogged instead of backwashing the whole system only the top layer of sand is removed and discarded. This saves the operators of the water treatment system the expense of having to design a backwash system.
More Information can be found about Slow Sand Filtration by contacting the Office of Water Programs at CSU Sacramento 
See Help:Footnotes for more. Water Treatment Plant Operation, A field Study Training Program. California State University Sacramento Office of Water Programs 2008
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