Some Thoughts on Reclaiming Reverse-Osmosis Waste Water

Drip, drip, drip...there are a number of options to reclaim reverse-osmosis waste water

Drip, drip, drip…there are a number of options to reclaim reverse-osmosis waste water

While reverse-osmosis units are among the more practical, affordable means for producing purified tap water suitable for sensitive reef systems, there’s no question that they tend to generate a lot of waste water relative to the volume of purified product. In fact, they often produce upwards of four gallons of waste water for every one gallon of purified water. It’s only natural for conservation-oriented hobbyists to look for ways to reclaim this waste water and put it to meaningful use rather than send it swirling down the drain.

But finding realistic ways to use all that water isn’t as simple as it seems, especially if you have a relatively large aquarium system and, therefore, need to produce a hefty amount of RO water on a fairly regular basis.

The usual advice is to use the waste water to give your plants or lawn a drink. These are certainly valid options, but let’s face it, you’d have to have an awful lot of plants to keep pace with all the waste water produced. And as far as watering the lawn is concerned, I’m sure someone more inventive than I (which isn’t saying much) could find ways to do this efficiently using RO waste water, but I haven’t figured out a method that would work for my yard yet.

I do have a decent-sized back yard with terraced landscaping, so I’ve got more than enough thirsty outdoor plants to water through summer and possibly into fall depending on the weather. However, Ohio tends to be quite wet in spring, so the plants don’t need any help from me at that time of year, and winter around these parts is bone-chillingly cold and usually snowy, putting all the plants into dormancy, so I’m not apt to be out watering then either. I also have houseplants to water year round, but again, they can only soak up so much before root rot spirits them away.

So where does that leave me or anyone else who has more RO waste water on their hands than they know what to do with? Looking at it from a practical standpoint, here are my thoughts on the quandary:

It’s not an all-or-nothing proposition

By this, I mean that reclaiming some RO waste water is better than none. Don’t beat yourself up if you can’t find a good use for every drop and have to send some down the drain. Think of your RO system as another water-using utility like your dishwasher or clothes washer (but instead of using water to produce clean dishes or clothes, you’re using it to make pure water for your reef system) and do your best to reduce the waste as much as possible.

You’ll need to get creative

As I noted with the plant watering example above, no single use is likely to consume all the waste water your RO system generates, but figuring out multiple uses around the house can certainly make a dent. In addition to watering, here are a few other waste-water uses you may or may not have considered:

  • Soaking/pre-rinsing dishes and silverware bound for the dishwasher
  • Washing the car
  • Mopping floors
  • Cleaning countertops
  • Topping off the swimming pool
  • Flushing toilets
  • Partially filling the bathtub

With a little creativity, you can expand this list considerably. Or, if you can’t come up with any good uses right away, you could always just run the waste-water line into a good-sized storage container to collect it and use the water as ideas occur to you.

You can reduce your water consumption elsewhere

If you’re still not satisfied with the amount of RO waste water diverted down the drain, you could always compensate by cutting your household water consumption in other areas, for example taking a shower instead of a bath, taking shorter showers, postponing washing clothes until you have a full load, etc. You’ve heard of “carbon credits”? Think of these as ways to accumulate “water credits.”

RO systems vary in their waste output

Shop around and you should be able to find RO systems—or upgrades to systems—that create more gallons of product water per day (e.g., 150 GPD) and less waste. There are even zero-waste RO units on the market nowadays. As I understand it, rather than sending the waste water down the drain, these units pump it to the home’s water-heating system.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. My water is metered so every drop of RO waste that goes down the drain feels like I’m throwing away money. So I need something that regularly utilises large quantities of clean water hard water. My answer is a Malawi cichlid tank, the main maintenance requirement is to carry out large water changes and I have that available in abundance.

    In the summer it still finally ends up on the garden. My plants are thriving on the “nutrient” rich water that comes out of the Malawi tank!

    • That’s an excellent idea, Richard! Should work for Tanganyika cichlids as well. Might be time to set up that shelly tank I’ve been pondering for some time now.

  2. I just today found a use for the waste water from RO/DI. It was looking me in the face for years and all of a sudden as I was looking at my worm tank I said. “Like Duh”. I am wasting all this water and my blackworm tank is right under my RO unit. So now the waste water goes into the worm keeper, then to a drain. My tank evaporates about a gallon a day so that makes about 4 gallons of waste water. My worm tank only holds about 2 gallons so it is a win win situation.

  3. I have hooked up a 2nd ro bladder using the waste water from the 1st. This cuts the waste about another 25%. Every little bit helps!

  4. In Florida we have holding tanks aerators . So when I put my system together I ran a line back to the tank. It mixes with new water and go’s through the house. The water coming out of my well is 0.04 so there is zero waste.

    • Jeff Kurtz says

      That’s an excellent idea, Dennis! Thanks for sharing it. I imagine many other readers out there might be in a position to do the same.

  5. Thank you all for sharing your views.

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