Estrogen from birth control cannot easily be filtered out of the water.
https://www.sciencedaily.com/releases/2016/03/160304092230.htm
https://www.ehn.org/troubled-water-estrogen-and-its-doppelgangers-2641102160.html
https://www.cbc.ca/news/science/birth-control-pill-threatens-fish-populations-1.2796897
https://www.livescience.com/20532-birth-control-water-pollution.html