filter bed

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun 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.

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