/ slʌdʒ; NAmE / noun [ U ]
1.
thick, soft, wet mud or a substance that looks like it
SYN slime :
There was some sludge at the bottom of the tank.
2.
industrial or human waste that has been treated :
industrial sludge
the use of sewage sludge as a fertilizer on farm land
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WORD ORIGIN
early 17th cent.: of uncertain origin; compare with slush .