sewage system vs sewer system

sewage system and sewer system both are nouns.

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

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

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

Other synonyms of sewage system include sewage works.

sewage system (noun)	sewer 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