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.
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

sludge
    n 1: the precipitate produced by sewage treatment
    2: any thick, viscous matter [syn: sludge, slime, goo,
       goop, gook, guck, gunk, muck, ooze]