sewer system vs sewage system

sewer system and sewage system both are nouns.

Word	Noun	Adjective	Verb	Adverb
sewer system	Yes	No	No	No
sewage system	Yes	No	No	No

As nouns, sewer system and sewage system are synonyms defined as:

sewer system and sewage system: facility consisting of a system of sewers for carrying off liquid and solid sewage

Other synonyms of sewer system include sewage works.

sewer system (noun)	sewage system (noun)
facility consisting of a system of sewers for carrying off liquid and solid sewage	facility consisting of a system of sewers for carrying off liquid and solid sewage