Reverse Osmosis Waste Water – What Is It?

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Anyone who uses a reverse osmosis system knows that reverse osmosis waste water is inevitable.

But if you’re hearing about RO waste water for the first time, you might be wondering what it means and why it’s important.

In this article, we’ll explain what reverse osmosis waste water is, why it exists, and what to about it.

Key Takeaways

  • Reverse osmosis separates water into 2 streams: The pure water and the waste water.
  • Reverse osmosis waste water, also called RO reject water or brine water, is the water produced by a reverse osmosis system that carries all the contaminants.

What Is Reverse Osmosis Waste Water?

So, what is reverse osmosis waste water?

To understand reverse osmosis waste water, you first need to know how reverse osmosis works:

Reverse osmosis purifies water by forcing it through a semipermeable membrane with very small pores. The pores are small enough to hold back contaminants and only allow pure water molecules to diffuse through them.

But not all water will make it through the RO membrane. The remaining part plus the contaminants that settle on the membrane are washed out of the system and taken down the drain. This flushed mixture of contaminants and water is referred to as waste water.

In summary, reverse osmosis waste water is the concentrated “dirty” water flushed from the RO membrane and into the drain.

blue reverse osmosis membrane

What Is RO Reject Water? What Is RO Brine Water?

RO reject water is another name for RO waste water. The same goes for RO brine water.

The waste water is concentrated with rejected contaminants, including chemicals like salts. Brine refers to water “impregnated with salts”, so both reject water and brine water are fitting descriptions for RO waste water.

Why Does the Reverse Osmosis Process Waste Water?

As mentioned before, reverse osmosis systems purify water by passing it through a semipermeable membrane under high pressure. The membrane pores are very tiny and only allow water molecules to pass through them.

Which means during filtration a lot of contaminants accumulate on the RO membrane. These contaminants can build up fast and foul or ruin the membrane if left unchecked. To prevent that, a cross-flow of water is used for flushing. After cleaning the membrane, the water (now waste water) is flushed out of the system and down the drain.

So RO systems waste water to keep the membrane clean and prevent it from aging prematurely.

How Much Waste Water Is Being Produced?

On average, a standard reverse osmosis system produces 3-5 gallons of waste water per gallon of filtered water. However, several factors can influence how much water your RO system produces.

For instance, a system with high feed water pressure will produce much less waste water than one with low pressure. Likewise, an RO system with a worn-out membrane will produce more waste water than one with a new and well-maintained membrane. Other factors that can influence how much RO systems waste include:

  • Amount of contaminants in the water
  • Water temperature
  • Membrane recovery ratio

How Can You Reduce RO Waste Water?

There are different ways to reduce RO waste water. We’ve discussed some of them below:

Increasing Water Pressure

Water pressure is most important. On average, an RO system needs a pressure of 40-60 psi to work effectively. If your home water pressure is too low, your system produces more waste water.

You’d need to increase your water pressure with a booster pump to fix this.

Adding a booster pump to your reverse osmosis system is a great idea anyway. They can help reduce wastage by as much as 90%.

Waste Water Recycling

Recycling your waste water is the perfect way to make it useful. Fortunately, there are many ways to do this. You could:

  • Recirculate your waste water: Instead of letting your waste water go down the drain, you could recirculate it into the RO system or another round of filtration. Or you could use two RO membranes in a row.
  • Repurpose your waste water: Waste water is unsuitable for cooking and drinking, but you could use it for other purposes that do not require pure water, like washing cars and flushing toilets.

System Maintenance

Maintaining your RO system is important if you want to keep your waste water as low as possible. Regular maintenance prevents your membrane from fouling or clogging up, reducing the amount of waste water produced.

For most brands, the reverse osmosis membrane should be replaced every 2-5 years, while other accompanying filters must be replaced every 6-12 months.

Under Sink Reverse Osmosis System

Ways to Use Reverse Osmosis Waste Water

Cleaning Your Home

You can do tasks like mopping floors, cleaning windows, and washing utensils with RO reject water. It’s a great way to repurpose your water and curtail waste.


RO waste water is excellent for irrigation, too, since it’s rich in minerals and nutrients. It has also been observed to speed up the growth of plants and improve soil quality. Just make sure that there isn’t too much of sodium and chlorine in the water.

Cleaning Your Car

Washing cars consumes a lot of water. Since there’s no need to wash your car with perfectly clean water, RO waste water can be your alternative.


You can also use RO waste water for your laundry. For example, you could soak clothes with difficult stains or pre-rinse them with RO water before tossing them into the washing machine.

Flushing Toilets

Flushing toilets with RO waste water is one of the best ways to reuse it.

Sewage Pipes

You can use RO waste water to flush your kitchen or sewage pipes instead of water from your tap. This arrangement is perfect because sewage pipes don’t need to be flushed with pure water.

Do Zero-Waste Reverse Osmosis Systems Really Work?

Zero-waste reverse osmosis water filter systems are designed to eliminate waste water. Technically they work, but whether they’re worth it is another story.

There are two types of zero-waste RO systems:

  1. The first type recycles the waste water by sending it to the hot water line. This means that water isn’t sent down the drain but is passed out to your regular kitchen faucet for washing and other tasks. However, the waste water could be concentrated with different contaminants, and using it may expose you to these impurities that could be harsh on your skin. So while this zero-waste method works, the downside is your hot water line permanently runs waste water.
  2. The second type of zero-waste RO system recycles waste water by diverting it back into the RO system to be re-filtered. This sounds great, but there’s a catch: Sending waste water back into the system eventually takes a toll on the membrane. There’s an overload of contaminants with nowhere to go, and they begin to clog the membrane. This causes degradation a lot faster, disrupting the RO process.

Bottom line: While zero-waste reverse osmosis systems may work, they’re not exactly hitch-free.

If you have any questions about RO waste water 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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