The inherent issue with reverse osmosis is that you build up a "brine" on the input side. And the higher the gradient of these ions, the harder it is to do to the reverse move. The normal osmotic action is to pull pure water back through to dilute the brine.

I've heard of systems that attempt to just let the brine diffuse back through the source water system rather than dumping it. But, I think this is against code in any modern, thoughtful regulatory environment. You normally want explicit back-flow prevention to reduce the chance of contamination of the water system by end users.