Sludge /(?)/

Sludge

n.
  1. Mud; mire; soft mud; slush.
  2. Small floating pieces of ice, or masses of saturated snow.
  3. See Slime, 4. (Mining)
  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.

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