filter-bed

filter bed

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

Origin:
1870–75

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
filter bed
 
n
1.  Compare bacteria bed a layer of sand or gravel in a tank or reservoir through which a liquid is passed so as to purify it
2.  any layer of material through which a liquid is passed so as to filter it

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