Now, other people may see a problem there, but I see it as an opportunity. Rather than spending lots of money changing all the wastewater treatment facilities, what's needed is just a different attitude--a different outlook on the situation. All they really need to do is convince the public that psychoactive drugs in the water supply are a feature, not a bug!
A good way to start, I think, would be with some creative pro-psychoactive slogans. Something like "I'm Xany for Xanax," perhaps. With enough positive spin, people might start expecting--hell, demanding--antidepressants in their water.
And why stop at water? Just think of the marketing possibilities here! "Hey kids, are you bummed out because Mom is pressuring you to clean your room? Stressed over homework? Disappointed about the prom? Try new Kellogg's Pop-Tarts Plus. The "Plus" is Prozac!"
Anyway, just thinkin'.