sludge

UK: /ˈslʌdʒ/ US: /slʌdʒ/ ,(sluj)

WordReference Random House Learner's Dictionary of American English © 2025

sludge /slʌdʒ/   n. [uncountable]

1. mud, mire, or ooze;
   slush.
2. a deposit of ooze at the bottom of a body of water.
3. Civil Engineering the substance left over after treating sewage.

WordReference Random House Unabridged Dictionary of American English © 2025

sludge  (sluj), n. 

1. mud, mire, or ooze;
   slush.
2. a deposit of ooze at the bottom of a body of water.
3. any of various more or less mudlike deposits or mixtures.
4. Energy the sediment in a steam boiler or water tank.
5. Oceanography broken ice, as on the sea.
6. a mixture of some finely powdered substance and water.
7. Civil Engineering sediment deposited during the treatment of sewage.
8. Microbiology Also called activated sludge. [Bacteriol.] sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.
9. Mining a fine, mudlike powder produced by a mining drill.

• 1640–50; variant of dialect, dialectal slutch, slitch, Middle English slich slime, wet mud (compare its derivative slucched muddy); apparently of expressive origin, originally

Collins Concise English Dictionary © HarperCollins Publishers::

sludge / slʌdʒ/ noun

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

Etymology: 17th Century: probably related to slush