filter bed

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

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

GNU Webster's 1913

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

WordNet 3.0

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