Sludge (noun)

1. A thick, viscous mixture, especially of mud, waste matter, or sewage.
2. A sediment of impurities that accumulate in a liquid, especially in an industrial process.

Origin:

Middle english, from old english slǣw, from slǣwan to smear.

Examples:

1. The sludge in the bottom of the oil tank had to be cleaned out.
2. The waste water was full of sludge, making it difficult to pump.
3. The sludge was thick and difficult to remove from the pipes.
4. The sewage treatment plant had to process the sludge before it could be released into the river.
5. The sludge at the bottom of the pond was a breeding ground for mosquitoes.