Sludge \Sludge\, n. Anything resembling mud or slush; as:
   (a) A muddy or slimy deposit from sweage.
   (b) Mud from a drill hole in boring.
   (c) Muddy sediment in a steam boiler.
   (d) Settling of cottonseed oil, used in making soap, etc.
   (e) A residuum of crude paraffin-oil distillation.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Sludge \Sludge\, n. [CF. Slush.]

1. Mud; mire; soft mud; slush. --Mortimer. Tennyson.

2. Small floating pieces of ice, or masses of saturated snow. --Kane.

3. (Mining) See Slime, 4.

Sludge hole, the hand-hole, or manhole, in a steam boiler, by means of which sediment can be removed.

Source: Webster's Revised Unabridged Dictionary (1913)