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filter bed

noun
  1. a pond or tank having a false bottom covered with sand and serving to filter river or pond waters.

Origin of filter bed

First recorded in 1870–75
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for filter bed

filter bed

noun
  1. a layer of sand or gravel in a tank or reservoir through which a liquid is passed so as to purify itCompare bacteria bed
  2. any layer of material through which a liquid is passed so as to filter it
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012