> The additives used by datacenters in some cases can't be filtered or processed out practically or in economically viable ways, so the water returns back as unusable, esp. if they get water from a body of water and return it with added chemicals the process gets very damaging.

Why would any water be returned? Datacenters (and literally every industrial process that requires cooling) use water for evaporative cooling. The water evaporates, then returns as rain elsewhere.

The only water being "returned" I can think of would be from the intermittent or continuous blowdown systems to maintain TDS levels; but blowdown is a small fraction of the water consumed and is typically returned back via the city sewer system.