:Is it possible to make hot tubs green?
I think this depends on what aspect you are addressing.
Hot tubs are fairly "green" already in that they are reused. The concentration of chlorine has to be relatively low, since chemical burns from the chlorine concentration haven't occured, as far as I know.
The only other aspect I can think would be any extra chemicals used to treat standing water, and they could be eliminated by draining the tub after each use, but then that would waste water
Which aspect of hot tubs did you have in mind?