main vs sewer line

main and sewer line both are nouns.

Word	Noun	Adjective	Verb	Adverb
main	Yes	Yes	No	No
sewer line	Yes	No	No	No

As nouns, sewer line is a hyponym of main; that is, sewer line is a word with a more specific, narrower meaning than main:

main: a principal pipe in a system that distributes water or gas or electricity or that collects sewage
sewer line: a main in a sewage system

Other hyponyms of main include electric main, gas main, sewer main, water main.

main (noun)	sewer line (noun)
a principal pipe in a system that distributes water or gas or electricity or that collects sewage	a main in a sewage system
any very large body of (salt) water

main (adjective)	sewer line (adjective)
(of a clause) capable of standing syntactically alone as a complete sentence
most important element
of force; of the greatest possible intensity