How to Check If Your RO Membrane Is Working or Not

This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no extra cost to you. Learn more.

Reverse osmosis membranes are designed to block contaminates and provide purified drinking water.

But while they’re built to stay efficient for years, at some point, they start deteriorating.

In some cases, you can spot that something is off with your RO system easily. But even if everything seems normal, it’s important to know how to check if the membrane is still doing its job properly, to maintain water quality.

Key Takeaways

  • The most accurate way to check if your RO membrane is working properly is to calculate the salt rejection rate with a TDS meter. It is also wise to keep an eye on that rate and make sure it doesn’t drop below 80%.
  • Inspect the overall quality of the water and look for visible impurities or changes in odor and taste, as that can be a result of a fouled RO membrane.
  • If the output water flow has been lowered or there is constant draining, that might be a sign that the membrane is not functioning well.
  • Open the membrane housing and look for signs of discoloration. If the white parts of the membrane have taken on a yellowish color due to dirt, it should be replaced.
  • Check if the RO membrane has been replaced on time. Old membranes filter with impaired quality, so make sure it is up to date.

How to Check If Your RO Membrane Is Working or Not

The RO membrane is the heart of your reverse osmosis water filtration system. If it’s clogged, dirty, or no longer efficient to filtrate, it is important to catch these signs on time to continue drinking clean water.

Here are the best ways to check whether an RO membrane is working in tip-top shape or not.

Measure Permeate Water TDS (Salt Rejection)

If you’re looking for the most accurate way, then look no further. Checking whether the membrane is still efficient in rejecting the flow of dissolved substances is crucial, as it gives you the clearest picture of how pure your filtered water is.

How to Monitor Salt Rejection Rate

To check your RO membrane’s the salt rejection rate, you will need a TDS meter. Don’t worry, these aren’t overpriced gadgets, but they can be of real help when it comes to keeping your water quality high.

The TDS meter can test the conductivity of your filtered and raw water respectively. Meaning, it can check how well the passage of dissolved solids is obstructed by the membrane. Once you have the rejection rates from your filtered and unfiltered water, you can calculate the overall efficiency of the membrane and see if it needs changing.

The formula is simple, and you don’t need to be a math whizz to determine the salt rejection rate.

  1. Start by subtracting the TDS rate of the filtered water from the TDS rate of the unfiltered water.
  2. Then, divide this by the TDS rate of the unfiltered water.
  3. And finally, multiply this number by 100.

And there you have it! If your rate is below 80%, that is a sign of concern and a good indicator that it’s time for a new reverse osmosis membrane.

tds meter in glass of water

Output Water Flow Rate

Checking how well the output water flows can also be of great help. The RO membrane works by blocking debris and contaminants from passing through. And over time, especially if the feed water is rich in floating chunks, these impurities can get the membrane clogged.

If you have been dealing with obstructed water flow (and you’re sure you’re not dealing with other issues like pressure, temperature, etc.) then the culprit may be hiding in the RO membrane. In tank-dependent systems, this can cause the tank to take too long to fill.

RO Water Aesthetics

Sometimes you can spot that there is something off with your filtration system after only a sip. Or a glance. Or even a whiff. That’s right, your senses can also be good inspectors when it comes to the quality of your drinking water.

Fill up a glass of RO water and check the aesthetics. Does it look clean? Or is it murky? Does it have a certain smell? How does it taste? If you notice that something’s wrong with how the water looks/tastes/smells, the membrane might be to blame.

Water Quality Overall

If you have been enjoying the purified perks of an RO system, then you might be used to drinking your water at a certain quality. If the water quality is (even slightly!) lower than what it used to be, that may be a sign that the membrane has started losing its efficiency.

Continuous Draining

Over time, gunk can pile up on top of the RO membrane, which can result in blocking the flow of the water, redirecting it towards the drain line.

If you have an RO tank, a clogged membrane will never fill it up. That means that, unless the membrane gets replaced, the system will go on a never-ending run, rejecting water, and pushing it down the drain.

Membrane Discoloration

A clear sign of concern is if the membrane changes its color as a result of being coated with layers of debris. To check this, you need to open the housing of the RO membrane and see if it’s still white on its sides. If you notice red, yellowish, or brown discoloration, then it is time to replace the membrane.

Product Specifications

The manufacturer of the reverse osmosis membrane you’re using has specified the ideal time to have it changed.

Think of this as a “best before” date on a product. Enjoying it past that date may not necessarily make you sick, but it surely won’t provide the same results.

So, if the manufacturer states that it is best to change the membrane after 3 years, it probably really is. The membrane may still be able to prevent some impurities, but it surely won’t be as efficient as before.

Reasons Why an RO Membrane Is Not Working + Fixes

There are many reasons why the RO membrane may not be filtering properly. Sometimes, the impure feed water may not be even to blame. Knowing what affects the performance of the membrane is crucial. Getting a new membrane will not always fix the issue.

Old or Fouled Membrane

This is probably the most straightforward issue. The membrane doesn’t pass enough water through, causing a low flow rate, draining, and impaired quality. This happens as a result of either dealing with incredibly impure water or using the same membrane for too long. Layers of debris can block the pores of the membrane, affecting clean water production.

How to Replace an RO Membrane

If your RO membrane is past its prime date or blocked by impurities, it is time to replace it. And here is how to do it:

  1. First, close the cold water supply and close the RO system’s tank valve.
  2. Open the RO faucet to let all of the water out, and release pressure.
  3. Discard the old membrane and make sure to clean the housing thoroughly.
  4. Secure the new membrane tightly.
  5. Then, turn on the cold water supply and open the tank’s valve. Let the water run for a couple of minutes, making sure that the system’s working properly.
  6. Finish by flushing the system to get rid of any remaining preservatives

blue reverse osmosis membrane

Low Feed Water Pressure

A reverse osmosis filter system cannot function properly if the feed water pressure is low.

The pressure level has to be at least 40 psi for the system to be able to work, but at 60 psi, the results are much better. Generally speaking, you will need a pressure level of 40-85 psi for RO filtration.

If your feedwater pressure is lower than 40 psi, filter system will not be able to push enough water through its membrane. That will cause most of it to flow down the drain wasted.

For low water pressure, your best bet is to install a booster pump. The pump will then push the water through the system at a rate that’s fast enough for it to function properly.

Low Feed Water Temperature

The productivity of the RO membrane is sensitive to changes in the temperature of the feed water. As the water gets warmer, it also gets thinner, and more water permeates the membrane. As it gets colder, the water gets thicker, and so the flow rate of filtered water decreases.

The normal working temperature of RO membranes is 77 °F (or 25 °C), but anywhere from 40-100 °F (5-35 °C) will work.

At a higher temperature, the water has a higher diffusion rate and lower viscosity. This helps it pass through the membrane pores.

If you find that your water flow rate is affected by cold water temperature in winter, you might want to try boosting the water pressure a bit to improve it.

Feed Water pH and TDS

Another thing that can prevent a reverse osmosis membrane from working properly is if your feed water has an abnormal pH. Of course, this is incredibly rare as most water supplies have a pH that is within the recommended range, still, it is good to know that your water’s pH should be at least 2.0 and no more than 11.0.

The same goes for the TDS level in the water. The more dissolved solids, the harder the membrane works to let water through the pores and leave these impurities behind.

Clogged Pre/Post-Filters

Maybe your RO membrane isn’t the problem at all. Perhaps the RO system doesn’t seem to be working because of a pre or post-filter issue. Check your filter additions and see if they’re functioning properly.

How Long Do Reverse Osmosis Membranes Last?

Depending on the manufacturer, your feed water, as well as how often you utilize the filtered water, you can expect your RO membrane to be providing you with purified water for 2 to 5 years.

If you have any questions about how to check if an RO membrane is working or not 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.
Learn more about .


Information provided on BOS is for educational purposes only. The products and services we review may not be right for your individual circumstances.
We adhere to strict editorial guidelines. Rest assured, the opinions expressed have not been provided, reviewed, or otherwise endorsed by our partners – they are unbiased, independent, and the author’s alone. Our licensed experts fact-check all content for accuracy. It is accurate as of the date posted and to the best of our knowledge.

Leave a Comment: