Maybe. PlanetEarth online has an article discussing lab research addressing this possibility.
The idea is interesting. Certain types of epidemics might result in a lot of people excreting unmetabolized tamiflu (as well as dumping left-over pills) into the sewerage system. Sewage plants depend upon bacteria to treat wastewater, so if tamiflu kills or inhibits these microorganisms, treatment would suffer.
It’s not made clear in the study cited how the researchers decided what concentration of tamiflu to use in their study. In addition, such lab studies don’t fully simulate conditions at actual wastewater treatment plants. So, I’d put this possibility low on the scale of things to be worried about. Interesting research, though.
If there’s an epidemic, there might not be anybody to work at local sewage treatment plants, anyway, so they might go haywire because of that alone.