Dick wrote:
But what I don't understand is where does all that water go? If it
takes 4 gallons of waste for every 1 gallon of filtered water, I don't
see how you can stuff 4-gallons of water into a water heater that is
already full. There may be a little room, but 4-gallons? Maybe it
just puts a little at a time into the hot water tank when there is
room.

The waste water flows in a loop. It is injected into the hot water line
under pressure and migrates back towards the water heater. Displaced
water flows out of the cold water inlet of the water heater through the
cold water line and back to the RO system. Of course, if you never use
any of the hot water (i.e. in a shower, to wash dishes, etc.) then
eventually the RO system will start to receive previously processed
waste water at it's inlet, and that will cause the RO membranes to have
a shortened life. But for most practical purposes the 'zero waste'
system should work fine.