Sand and gravel are the most widely used of the rigid media simple contact. Most of the sand used this way is for the clarification of water for drinking or industrial uses. Washed, screened silica sand is sold in standard grades for this work and is used in depths ranging from a few inches to several feet, depending on the type of filter and clarification requirements. Heavy, irregular grains, such as magnetite, give high rates of flow and low penetration by the solid particles, and are easily cleaned. They are, however, considerably more expensive than silica sand, so their use is limited. Sand beds are often gravel-supported, but gravel alone is seldom used as the filter medium.
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