system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are we still flushing toilets with clean, drinking water?

Why are we treating and cleaning water which will essentially be used to flush down the toilet? If costs to implement weren't a factor, why can't we use grey or even salt water?
gpt
We certainly _can_ use grey or salt water and many places do.

The reason that many places don't is cost of implementation.  You have to install and maintain two sets of pipes - potable and non-potable water - to every bathroom.  In areas where fresh water isn't scarce, there is little reason to do this.