sewage farm
noun /ˈsuːɪdʒ fɑːm/
  /ˈsuːɪdʒ fɑːrm/
(also sewage works)
 (both British English) (also sewage plant especially in North American English)
- a place where chemicals are used to clean sewage so that it can then be allowed to go into rivers, etc. or used to make manure