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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "sludge":
   Dry Ice, berg, calf, clay, cryosphere, dregs, firn, floe, frazil,
   frozen water, glaciation, glacier, glacieret, glaze, glazed frost,
   glop, goo, granular snow, ground ice, growler, gumbo, gunk, ice,
   ice banner, ice barrier, ice belt, ice cave, ice cubes, ice dike,
   ice field, ice floe, ice foot, ice front, ice island, ice needle,
   ice pack, ice pinnacle, ice raft, ice sheet, iceberg, icefall,
   icequake, icicle, jokul, lolly, mire, muck, mud, neve,
   nieve penitente, ooze, pack ice, precipitate, residue, scum, serac,
   shelf ice, silt, slab, sleet, slime, slip, slob, slop, slosh,
   slush, snow ice, snowberg, splosh, squash, swill