Refiltering RO Waste Water – How It’s Done

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Even though reverse osmosis is one of the most thorough methods for purifying water available on the consumer market, it’s not without its downsides.

Most notably, the process can lead to a lot of wasted water.

You might not care much about this, but given the ratio of waste water to purified water of some reverse osmosis systems, it’s a good idea to consider options for refiltering RO waste water…

Key Takeaways

  • Refiltering RO waste water means recirculating some of it for another round of purification, or using a second reverse osmosis membrane which is fed by the waste water stream of the first.
  • Both puts a higher burden on the RO membrane.
  • Refiltering RO waste water is mainly found in industrial applications.

How to Refilter RO Waste Water

How can you refilter RO waste water?

Generally, consumers don’t tend to refilter their reverse osmosis waste water. This is mostly reserved for industrial applications. Still, it’s not impossible to do. Your two main options are to either feed the waste water to a secondary reverse osmosis membrane or to reroute some of it back to the original one.

Both have the same disadvantages. First and foremost, you will have to spend more money on your reverse osmosis setup. Why? Because refiltering RO waste water will put a lot more strain on any reverse osmosis membrane, meaning it will get clogged rather fast by the extra contaminant load. This will require more frequent maintenance, making your RO system more expensive in the long term.

Bottom line: Refiltering RO waste water can work, but reusing it might be a better solution…

blue reverse osmosis membrane

How to Reuse RO Waste Water

A much better option for a regular household is to reuse RO waste water instead. You have multiple options available for this. You can use it for various cleaning purposes where the purity of water is not that critical, or you can even use it for gardening. However, in the case of gardening, you have to take some special precautions to avoid harming your plants.

The main challenge here is finding an efficient way to store your RO waste water. You can use buckets as a convenient option, although they are quite limited and will fill up fast. Alternatively, you can set up a dedicated storage tank.

Cleaning Your Home

You can use reverse osmosis waste water to clean your home. You can mop the floors with it, clean the countertops, and generally clean anything that doesn’t require pure water. Just make sure to be careful with surfaces that come in contact with food.

Washing Your Clothes

There is generally no downside to using RO waste water for your laundry. The only negative aspect of this is that it can be a bit inconvenient. Your laundry machine needs a lot of water for a full load. This can make it unfeasible to store enough reverse osmosis waste water.

Washing Your Car

The same goes for washing your car. Reverse osmosis waste water will not be worse than regular water in this regard. The only possible exception is if your water is heavily contaminated with certain elements like iron, which could leave a residue on your car when they are in a higher concentration. Give your car a test wash, let it dry off fully and see if it does the job as a test.

Flushing Your Toilet

One of the simplest ways to reuse RO reject is by flushing your toilet. There are no downsides to this, other than the fact that you would have to manually pour buckets of water into the toilet every time you want to flush it. This at least gives you the opportunity to make full use of all that waste water which would normally go down the drain.


Using reverse osmosis waste water for gardening is another option. However, you have to be careful here. Many plants are sensitive to sodium and other elements which are commonly found in tap water. Normally it’s not a problem to use that water for watering your garden because the concentration would not be high enough to cause any harm.

Reverse osmosis waste water, however, has a higher concentration of all original contaminants, which could push it over the safe limits. We recommend that you do a small test by watering a part of your garden for a limited time to see if it has any negative effects.

How Much Water Does Reverse Osmosis Waste and Why?

By default, reverse osmosis tends to waste quite a lot of water. The ratio can be as high as five gallons of waste for each gallon of purified water. This might seem quite excessive, but you should know that many modern systems have extra features that improve their efficiency. This includes increasing water pressure first and foremost. You can expect a more advanced, modern reverse osmosis system to create as little as one gallon of waste water for each 3-4 gallons of purified water.

If you’re wondering why exactly reverse osmosis leads to wasted water, it’s actually a feature of the process. Not all water can make it through the membrane. At the same time, measures must be taken to prevent the filter from clogging. This inevitably leads to some wasted water.

There is no way to make a reverse osmosis system waste no water at all. You have to accept the fact that some water is going to go to waste when you decide to invest in a reverse osmosis system. However, even when taking into account all that wasted water, you would still get a cheaper solution for filtering your tap water compared to many other options available on the market.

How to Reduce Waste Water

If you want to reduce the amount of waste water produced by your reverse osmosis system, there are a few things you can do, such as:

  • Invest in a more expensive, modern system. Many high-end reverse osmosis systems come with advanced features that help reduce the amount of waste water they generate.
  • Increase the feed water pressure. There is a direct correlation between the level of pressure in a reverse osmosis system and the amount of waste water produced. If the pressure is too low, this will lead to more wasted water.
  • Use a permeate pump. Permeate pumps are an alternative option to booster pumps. One major advantage to them is that they don’t use any electricity.

If you have any questions about refiltering RO waste water please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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