sludge

American  

[sluhj] / slʌdʒ /

noun

• sludges
  plural

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. the sediment in a steam boiler or water tank.

5. broken ice, as on the sea.

6. a mixture of some finely powdered substance and water.

7. sediment deposited during the treatment of sewage.

8. Also called activated sludge.  Bacteriology. sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.

9. a fine, mudlike powder produced by a mining drill.

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sludge British  

/ 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

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Other Word Forms

Derived Forms

• de-sludge
  verb (used with object)

Inflected Forms

Nouns

• sludges
  plural

Etymology

Origin of sludge

First recorded in 1640–50; variant of dialectal slutch, slitch, Middle English slich “slime, wet mud” (compare its derivative slucched “muddy”); apparently of expressive origin

Explanation

Sludge is thick, damp, almost sticky stuff. If your friend dares you to climb through an old sewer pipe, you might want to check first to see if it's full of sludge. Sludge is often used to talk about the byproduct of a process like sewage treatment or oil refining, but you can use it for any viscous goop. Sludge floating in your fish tank is a sign that you should clean the water, and sludge inside a car's engine can affect the way the car runs. Sludge is a bit of a mystery — it may come from the Middle English slutch, "mud," or might be related to slush.

Example Sentences

See Examples For:

sludge sludges

Looking out over the green sludge covered in cleaning equipment, speaking over the hum of the generators and the vacuums and the pumps, Hier smiled.

From Slate ● Jun. 19, 2026

That’s when a tank gets so low that drawing hits dregs — an all-but-unusable mix of oil sludge and debris — before a physical bottom is reached.

From MarketWatch ● Jun. 6, 2026

The disaster unleashed toxic sludge into the Kafue River, contaminating land and leaving residents without clean water.

From The Wall Street Journal ● May 1, 2026

"We can't show directly that that's happening, but we think it's a reasonable way that they could be winding up in the air. Sewage sludge fertilizers have been shown to release similar compounds."

From Science Daily ● Apr. 11, 2026

She spit out the oatmeal-like sludge and took a deep breath.

From "Beauty Queens" by Libba Bray

"When sewage sludges are spread across the fields, those toxic compounds could be released into the air," Katz said.

From Science Daily ● Apr. 11, 2026

These little guys live in basically every natural setting in the world, including deserts, soils, and, in particularly unlucky cases, the sludges of the human body.

From Slate ● Mar. 8, 2024

While working in a lab in the Netherlands in the late 2000s, Strous noticed that a type of methane-feeding bacteria often found in lake sediments and wastewater sludges had a strange way of life.

From Scientific American ● Aug. 22, 2023

For players and officials both as the NHL sludges through this pandemic with a human toll being taken nightly at arenas, including one the Kraken calls home.

From Seattle Times ● Dec. 28, 2021

With regard to the manurial value of the resulting sludges, much difference of opinion has existed.

From Manures and the principles of manuring by Aikman, Charles Morton

Related Words

• goo
• grease
• gunk
• muck
• sediment
• silt
• slime