From The Collaborative International Dictionary of English v.0.48:
Sludge \Sludge\, n. [CF. Slush.]
1. Mud; mire; soft mud; slush. --Mortimer. Tennyson.
[1913 Webster]
2. Small floating pieces of ice, or masses of saturated snow.
--Kane.
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3. (Mining) See Slime, 4.
[1913 Webster]
4. 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.
[Webster 1913 Suppl.]
Sludge hole, the hand-hole, or manhole, in a steam boiler,
by means of which sediment can be removed.
[1913 Webster]