sludge vs sapropel

sludge and sapropel both are nouns.

Word	Noun	Adjective	Verb	Adverb
sludge	Yes	No	No	No
sapropel	Yes	No	No	No

As nouns, sapropel is a hyponym of sludge; that is, sapropel is a word with a more specific, narrower meaning than sludge:

sludge: any thick, viscous matter
sapropel: sludge (rich in organic matter) that accumulates at the bottom of lakes or oceans

sludge (noun)	sapropel (noun)
any thick, viscous matter	sludge (rich in organic matter) that accumulates at the bottom of lakes or oceans
the precipitate produced by sewage treatment