From here:

Google financed the development of a "sidestream facility" about five miles away from its data center. The system intercepts water from the Douglas County Water and Sewer Authority’s treatment plant that would otherwise be discharged into the Chattahoochee River — which holds status as a National Water Trail. That water is then sent to Google’s facility for use in the cooling process. Any water that isn’t evaporated is treated using effluent equipment at Google’s site, and then returned to the river.

The article notes that some of that water is ground water that's drinkable, too. But there's no proportion of drinkable to waste water provided.