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A layer of sand or gravel on the bottom of a reservoir or tank, used to filter water or sewage.
1. (Civil Engineering) a layer of sand or gravel in a tank or reservoir through which a liquid is passed so as to purify it. Compare bacteria bed
2. (Chemistry) any layer of material through which a liquid is passed so as to filter it
a pond or tank having a false bottom covered with sand and serving to filter river or pond waters.