| http://www.w3.org/ns/prov#value | - If the primary purpose of the upflow filter is to provide mechanical and biological filtration, the manifold is covered with a layer of 3/4 to 1-1/2 gravel at least twice as thick as the pipe diameter, and the large gravel is covered with 6 to 12 of pea gravel which is planted with water iris, cattails, and rushes, and other vegetation that thrives in saturated soil.
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