Originally Posted by Sly
It's not a lot of money. RO units do generate waste water but they don't run all the time. A RO unit only runs as long as it takes to fill up its reserve tank. Then it shuts off. Yes you will waste gallons of water, but you use much more taking a shower or flushing a toilet.
The waste water simply goes down the drain. It is concentrated with dissolved metals and chemicals and so I would not try to reuse it for anything. Usually you attach an adapter to your sink's drain and then you run the RO unit's waste hose down the drain through that adapter.
That only applies if the particular RO unit has a reserve tank, which many don't. What you can do is install a float valve in your good water tank that will shut off the flow when it is full.