Treated waste water can still impact the environment

The goal of waste water treatment is improving the quality of waste water so that it is clean enough to go back into nature.

But once it's cleaned, it doesn't mean all substances are removed. That is why it still has an effect on the environment.

For example, remaining nutrients can cause fish to go big:

https://medium.com/@ErlijnG/how-waste-water-can-make-fish-go-big-c6c3cb632e19

A blue gill fish, which can go big in waste water (credit: RLS Photo on Shutterstock)