I'm not sure but I'm guessing gray water (or treated waste water) is not suitable for cooling purposes? Particle charge in small pipes and scaling may be a problem. Also, collecting gray water or channeling treated waste water - depending on the location that might be a problem.

Not that I'm in favor of using drinking water for cooling slop factories, but I guess the reason we don't see waste water being used for cooling is cost (unless governments start mandating that...)