I was reading a book[1] on bacteria phages aka viruses that attack bacteria and they mentioned that in countries that used phages (e.g. Soviet Russia due to a lack of antibiotics), the sewage plant is the first place they would go to look for phages to combat intestinal diseases. There were large amounts of the offending bacteria so it was a prime place to find viruses that had evolved to fight them.

[1] Viruses vs Superbugs https://amzn.to/2IZXZiJ