Can RO Waste Water Be Used for Cooking?

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On the menu today, we have soup de jour! Made with recycled RO waste water, this soup contains complex and concentrated flavors such as metal, rotten egg, chlorine, and dirt! Pair it with a glass of our in-house lemonade, brewed heat free with that same waste water, and you could have a free trip to the bathroom with some E. coli.

If the above does not sound appetizing to you, please do not consider using your reverse osmosis waste water in your cooking.

But wait a second, why exactly can RO waste water not be used for cooking?

Key Takeaways

  • RO waste water should not be used for cooking unless the only contaminant is microorganisms, in which case it needs to be boiled properly.
  • RO waste water contains all the contaminants taken from the feed water, but now in highly concentrated amounts, and it is not recommended for consumption.

Can RO Waste Water Be Used for Cooking?

Just in case the above did not make it clear enough, using RO wastewater in cooking is not recommended. So no, RO waste water cannot be used for cooking. Why?

It contains contaminants found in your source water but in even higher concentrated amounts. Not only could this potentially affect the flavor of your food and beverages, but it will also increase the number of harmful contaminants you are ingesting and basically undo all the good that the RO system has done for your health.

The only case in which it may be okay to use RO waste water in cooking is if your only contaminant is microorganisms and you would be boiling the cooking water for long enough to kill them.

cooking pasta in boiling water

Why Does Reverse Osmosis Waste Water?

Reverse osmosis filters the majority of contaminants from water. It does this by pushing water through a semipermeable membrane, which allows the H2O molecules through, but not all the dissolved solids, chemicals, etc.

These are then eliminated as RO waste water or “brine”. The brine contains all the contaminants and impurities rejected by the RO membrane. It is not usually safe for consumption or bathing but could be used for other purposes, meaning the water is not “wasted”.

By the way, there is no way to run reverse osmosis without water waste, but some systems with inbuilt pressure pumps do reduce it considerably.

RO Waste Water Ratio

It depends on the system and setup, but in most cases, the average RO unit will waste 3 to 5 gallons of water for every gallon of purified water produced. Permeate and booster pumps in some systems can improve this ratio by a whole lot.

How Can You Minimize Waste?

The best way to minimize waste from an RO system is to purchase one with a low waste water ratio. If this is not an option, here are some other things you can try.

RO System Maintenance

Staying on top of your maintenance is essential. Replacing filters and membranes and checking for wear and tear will prevent leaks or reduction in the RO system’s water pressure, which will keep the RO output optimal and maintain the waste water ratio.

Incoming Water Pressure

The water pressure of the feed water is the main thing that affects the waste water ratio of a reverse osmosis system. So if you can’t or don’t want to use an RO system with an internal pump, consider increases your home’s water pressure (but not above safe levels!).

Most reverse osmosis systems should be operated at around 60 psi.

Other Ways to Use RO Waste Water

Nevertheless, no matter how powerful your pump and how strong your water pressure is, there will be waste water. Repurposing this water in other ways can save you a bit of cash on your water bill. The waste water is not recommended for bathing or watering plants, but it can be used in other ways.


Providing the water does not have a very high level of TDS (total dissolved solids), RO brine can be used for cleaning, particularly your floors or your car. If the TDS level is high, this may stain the floor, so check first!

Washing Utensils

RO waste water is safe to use to wash utensils and kitchenware. You will need to store the water in a container, add dish soap, and ensure the items are thoroughly dried before use.

Cleaning/Flushing Toilets

RO reject water can be used to clean and flush toilets, depending on the level of TDS. If it discolors the toilet, it indicates that the TDS is too high, and the water needs to be diluted with unfiltered water.

Laundry Pre-Rinse

If the water does not have too many TDS, it may be suitable for a laundry pre-rinse, as it won’t stain or affect delicate fabrics.

If you have any thoughts about the question, can RO waste water be used for cooking, please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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