Where Does RO Waste Water Go? Find the Answer Here!

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If you’re thinking about purchasing a reverse osmosis system, you naturally want to know all about it.

By now, you must have learned that RO systems waste water, but where does all the waste water go?

In this article, we’ll explain where RO waste water goes and suggest tips for minimizing the amount of waste water your system produces. We’ll also discuss how much water reverse osmosis systems waste on average and how you can reuse your RO waste water.

Key Takeaways

  • RO waste water is flushed out of a reverse osmosis system and down the drain.
  • Under sink RO systems have a direct drain connection. This means waste water goes directly down the drain pipe.
  • Countertop RO systems don’t usually have a direct drain connection. Instead, they often have a separate tank that stores waste water temporarily. The tank needs to be emptied manually from time to time.
  • Whole house reverse osmosis systems all have a direct drain connection.

Where Does RO Waste Water Go?

So, where does RO waste water go?

During reverse osmosis, water diffuses through a semipermeable membrane. But not all of the water makes it through. Some remains on the RO membrane’s feed water side together with all the contaminants.

This stream of leftover water and contamination has to go somewhere – also to stop the membrane from becoming overwhelmed by contaminants. So, the RO system flushes the waste water out and down the drain.

blue reverse osmosis membrane

How your RO waste water goes down the drain depends on the kind of reverse osmosis system you use:

  • Most under sink RO systems have a direct drain connection. This means a waste water tube leads from the RO membrane to the sink’s drain pipe. The tube is connected to the drain saddle, which needs to be installed on said pipe. The drain saddle is basically a hole in the drain pipe where you fit in the drain tube. Due to this connection, waste water from an under sink RO goes directly down the drain.
  • Countertop reverse osmosis systems operate differently. Most of them are not connected to the plumbing system, so they send waste water to a separate tank that needs to be emptied occasionally. Other countertop systems have a loose drain tube that you can direct into any drain or container for temporary storage.
  • Whole house RO systems always have a direct drain connection. Since they produce a lot of waste water, there is no way of storing all of that.

Why Do RO Systems Send Water Down the Drain?

As mentioned before, RO systems have a semipermeable membrane that rejects contaminants as water passes through it. To protect the membrane from getting clogged by those contaminants, the system flushes some of the feed water out and sends the resultant waste water down the drain.

How Much Water Does Reverse Osmosis Waste?

All reverse osmosis systems waste water, but the amount of waste water produced depends on several factors.

The first factor is feed water pressure. A system with a high feed water pressure will waste less water than one with a low feed water pressure. That’s also why RO systems with an inbuilt pump waste less water than those without one.

For example, a standard under sink reverse osmosis system without a pressure pump will produce approximately 3-5 gallons of waste water per gallon of filtered water. At the same time, a model with an inbuilt pump may only produce about 0.25-1.0 gallon waste per gallon of filtered water.

Other factors that determine how much water an RO system wastes include:

Tips to Reduce Waste Water

Knowing that your reverse osmosis filter system wastes water can be uncomfortable. Fortunately, there are ways to reduce the amount of waste your system produces. You can:

Use an Up-to-Date Reverse Osmosis System

Reverse osmosis systems have always been known to waste lots of water compared to the amount of filtered water they produce. However, technology is constantly improving, and most modern RO systems have been designed to waste as little water as possible. Some have waste water ratios as low as 1:4 (meaning they waste 1 gallon of water for every 4 gallons filtered).

So if you want to minimize the amount of water wasted by reverse osmosis in your home, your first step should be to get an up-to-date system.

Under Sink Reverse Osmosis System

Increase Feed Water Pressure

Reverse osmosis systems use pressure to push water through their membranes. RO systems require a pressure of around 60 psi to work optimally. If your feed water pressure is lower than required, your RO system will be a lot slower and waste a lot of water. A simple solution to this problem is to install a booster pump. Booster pumps can cut down waste water by about 90%.

Permeate Pump

Permeate pumps installed into a reverse osmosis unit use the reject water from the system to power a piston that pressurizes filtered water into the storage tank.

In other words, permeate pumps reduce waste water and increase the system’s efficiency by reducing the backpressure the storage tank puts on the RO membrane.

Replace Reverse Osmosis Filters According to the Schedule

The RO membrane and filter elements don’t last forever. After a while, you’ll need to replace them. As a rough guide, pre-filters and membranes need to be replaced every 6-12 months and 2-5 years, respectively.

Replacement periods vary with different brands, so check the product manual to know how long your membrane or filters are designed to last.

Replacing filters according to schedule ensures that your RO system performs optimally, and reduces the risk of producing too much waste water.

Brine Water Recycling

Brine water or waste water can be recycled to curtail waste. You could redirect the waste water from the drain tube into the RO system for it to get re-filtered. You could also put two or more membranes in a row so that the waste water drained from the first RO membrane could get filtered by the second one in the row. That way, you get to salvage most of your waste water.

Note that this is only practical with whole house RO systems, however, because of all the complicated connections you’ll have to make.

How to Reuse RO Waste Water

If recycling waste water doesn’t appeal to you, you can consider reusing it in your daily tasks. You can use your waste water for:


Your RO waste water can be used for washing dishes, cleaning floors, and scrubbing windows. It’s perfectly safe most of the time and doesn’t harm your home in any way.


You can also use the waste water to pre-rinse your laundry. It’s a great way to recycle it instead of letting it go down the drain. However, you must avoid pre-rinsing delicate clothing with waste water because it could ruin them.


Gardening doesn’t demand 100% pure water, so watering your plants is an excellent way to reuse the waste water from your RO system. Plus, some of the minerals in your waste water could serve as nutrients for your plants.

If you have any thoughts about the question, where does RO waste water go, 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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