What is "filter bed"

Filter \Fil"ter\, n. [F. filtre, the same word as feutre felt, LL. filtrum, feltrum, felt, fulled wool, this being used for straining liquors. See Feuter.] Any porous substance, as cloth, paper, sand, or charcoal, through which water or other liquid may passed to cleanse it from the solid or impure matter held in suspension; a chamber or device containing such substance; a strainer; also, a similar device for purifying air.

Filter bed, a pond, the bottom of which is a filter composed of sand gravel.

Filter gallery, an underground gallery or tunnel, alongside of a stream, to collect the water that filters through the intervening sand and gravel; -- called also infiltration gallery.

n. filter consisting of a layer of sand or gravel for filtering water