sewer main vs sewer line

sewer main and sewer line both are nouns.

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

As nouns, sewer main and sewer line are synonyms defined as:

sewer main (noun)	sewer line (noun)
a main in a sewage system	a main in a sewage system