I have a zero-waste reverse osmosis system under my kitchen sink (a Watts ZRO-4 specifically, though there are other systems to which this applies) and my house is also pre-plumbed for a softener in my garage. From what I've read, the "zero waste" portion of this system means that it still has waste water (like any other RO), but just that it pushes it into the hot water line rather than down the drain as other systems do.

I'd like to install a softener in my garage, but since the RO has hookups into both the hot and cold under my sink (the hot is softened and the cold is non-softened), I just want to make sure I won't end up with salty water into my RO. If the system consumes hot water at all (or even if it doesn't maintain proper pressure into the hot water line when it's not discarding waste), it seems like this is a risk.

It sounds like the system only pushes waste into the hot water line and doesn't actually consume hot water during operation, but can anybody confirm that these two systems (zero-waste RO and softener) can operate together without any issues?

1 Answer 1


I own an RO system and it is fed solely with soft water. The mineral content in soft water is less fouling to the RO membrane than my hard utility water, and the membrane lasts several times longer than it otherwise would. I predict you will have no compatibility issues with your zero waste RO system circulating a small amount of water through your hot water loop.

That said, I am not a fan of the zero waste RO systems. RO systems require regular maintenance and if the filters are not sanitized and changed regularly, the system can become fouled with bacterial slime. With a zero waste system, that bacterial slime is recirculated into the whole house water supply instead of being flushed down the drain.

I prefer an electric booster pump or a permeate pump to reduce RO waste water:

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These provide a more positive shut-off when the RO pressure tank is full, and although they are not zero waste, they do significantly save water over a basic system.

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