mud, mire, or ooze; slush.
a deposit of ooze at the bottom of a body of water.
any of various more or less mudlike deposits or mixtures.
the sediment in a steam boiler or water tank.
broken ice, as on the sea.
a mixture of some finely powdered substance and water.
sediment deposited during the treatment of sewage.
Also called activated sludge. Bacteriology. sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.
a fine, mudlike powder produced by a mining drill.

1640–50; variant of dial. slutch, slitch, Middle English slich slime, wet mud (compare its derivative slucched muddy); apparently of expressive orig.

de-sludge, verb (used with object), de-sludged, de-sludging. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sludge (slʌdʒ)
1.  soft mud, snow, etc
2.  any deposit or sediment
3.  a surface layer of ice that has a slushy appearance
4.  (in sewage disposal) the solid constituents of sewage that precipitate during treatment and are removed for subsequent purification
[C17: probably related to slush]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1649, possibly from M.E. slutch "mud, mire," or a variant of slush.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Now the rains turn the roofs to sludge, which causes buildings to collapse.
Tiny bits of food get caught up in that calcified bacterial sludge, where they
  can remain for millennia without disintegrating.
From there, they can wash down drains, ultimately ending up in the sewage
  sludge of wastewater treatment plants.
The remaining sludge is processed to recover cobalt, which is used to make
  battery electrodes.
Image for sludge
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