How to Check RO Membrane Performance | Guide

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A reverse osmosis (RO) membrane filters contaminants from water using its tiny pores.

But the membrane could become clogged or faulty and fail to work as intended.

Checking your RO membrane performance is the only way to determine if it still works, or if something is wrong.

This guide will show you how to check your RO membrane performance using factors like TDS rejection rate and pressure drop.

Key Takeaways

  • The simplest and most reliable way to check RO membrane performance is to calculate its salt rejection rate using a TDS meter (learn steps below) – replace membrane if it drops below 80%.
  • You can also check the permeate flow rate, pressure difference, or water recovery rate.
  • Unpleasant smell coming from an RO membrane as well as discoloration are aesthetics indicators.

How to Check RO Membrane Performance

There are many parameters you could use to check your RO membrane performance. But as someone using a home RO system, the easiest and most reliable option is to check your TDS/salt rejection rate (our first option here).

Although it’s not always easy, you could also check for the other parameters mentioned further below.

TDS Rejection

TDS rejection rate measures the total dissolved solids your RO membrane/system removes from your water. It is the easiest and most reliable way to check RO membrane performance, especially if you use a home RO system.

If your TDS rejection drops below 80%, your RO membrane is performing poorly and should be replaced.

We’ll explain how to measure TDS rejection shortly.

Permeate Flow Rate

The treated water that flows from your RO membrane is the “permeate.” By measuring its flow rate, you can tell whether your RO membrane is doing great or poorly. To get the permeate flow rate accurately, disconnect your RO tank and measure the flow rate of the water from the RO membrane tubing.

Compare the flow rate now to when you just got the membrane. If the permeate flow rate has dropped, the membrane has started to clog.

blue reverse osmosis membrane

Pressure Drop

You may want to skip this if you use a home RO system. It involves measuring your water pressure before and after the RO membrane. The pressure drop will remain constant if there are no obstructions within the membrane.

It’s not really easy to get accurate readings here, so this option is not practical for home systems.

Rate of Recovery

That is the amount of water that gets purified from the feed water. Ideally, the rate of recovery should always be at least 20%. So if you have 5 gallons of feed water, at least 1 gallon should come out as purified water (the other 4 gallons become wastewater flushing out contaminants).

If you find yourself having more and more wastewater and less pure water, it’s a sign that your RO membrane is clogged.


Ideally, you should visually inspect your RO membrane at least every 6 months (when you maintain your reverse osmosis system). If you notice heavy discoloration while checking, it’s a sign that your RO membrane has some unresolved issues.

Foul Smell

Nothing good smells bad. If you notice some weird, unusual smell coming from your RO membrane, you can’t trust the water it produces. You can try sanitizing, but it’s probably better to replace said membrane.

Factors Affecting RO Membrane Performance

If you check your RO membrane performance and find it’s not working effectively, it’s natural to wonder why. Here are some factors that affect how your RO membrane performs.

Feed Water Pressure

The water flowing through an RO membrane should be quite high. The membrane simply cannot block enough contaminants if your water pressure is too low. Higher pressure at the feed water equals better RO performance – also in terms of pure-to-wastewater ratio.

Feed Water Temperature

If your feed water has a high temperature (like during summer), your water flow rate will increase. That’s because the water diffuses through the membrane faster when the temperature is high.

However, a high temperature also allows slightly more contaminants to make it through. So you’ll have more TDS in your water when the feed water is warm.

Water pH

RO membranes made from cellulose triacetate (CTA) only work effectively when the water pH is between 3 and 9. The membrane will remove fewer contaminants if your water pH exceeds this range. RO membranes made from thin-film composite (TFC) work well for most water pH.

Feed Water TDS

If your feed water contains lots of TDS, you’ll have less purified water and more wastewater flushing out contaminants.

How Long Does an RO Membrane Last? When Do I Change My RO Membrane?

RO membranes usually last 2-5 years, depending on your feed water quality and how well you maintain the entire filtration system.

You should change your RO membrane especially if it produces water that smells or tastes terrible, if the system drain runs continuously, or if the TDS rejection rate drops significantly.

How to Measure Water TDS Using a TDS Meter

A TDS meter is the best way to measure your water TDS. To use it, add your water sample into a cup, then follow these steps:

  1. Remove the cover from the TDS meter.
  2. Press the on/off button. The meter should show read “000”.
  3. Dip the meter about ½ inch deep into the water.
  4. The meter will display a number. The reading indicates the total dissolved solids (TDS) in your water.

You can compare input and output water TDS to calculate your salt rejection rate. If that rate drops below 80%, it’s time for a new membrane!

If you have any questions about how to test reverse osmosis membrane performance 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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