filter bed

filter bed
    n 1: filter consisting of a layer of sand or gravel for
         filtering water
    

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.
   [1913 Webster]

   {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}.
      [1913 Webster]