filter bed

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A layer of sand or gravel on the bottom of a reservoir or tank, used to filter water or sewage.

from the GNU version of the Collaborative International Dictionary of English

• n. a pond, the bottom of which is a filter composed of sand gravel.

from The Century Dictionary and Cyclopedia

• n. A pond or tank having a false bottom covered with sand, and serving to filter river- or pond-waters.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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