What Is Reverse Osmosis (RO) Rejection Rate?
Written by: Alexandra Uta // Last Updated: Aug 25, 2023
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When you read up about reverse osmosis online, you might stumble upon the term reverse osmosis rejection rate.
Let’s find out what it is!
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Key Takeaways
- Reverse osmosis (RO) rejection rate is the rate of total dissolved solids in water removed by a reverse osmosis filtration process.
- A brand new reverse osmosis membrane should reject at least 90% TDS.
What Is Reverse Osmosis (RO) Rejection Rate?
So, what is reverse osmosis rejection rate?
The rejection rate of a reverse osmosis system indicates the percentage of TDS (Total Dissolved Solids) removed from the water supply by the RO filtration process.
This can be pretty close to 100% for many dissolved solids, but it’s still not quite there.
Also, the older and less effective a reverse osmosis membrane gets, the lower its salt rejection rate becomes. As such, the rejection rate of a reverse osmosis system is perhaps the best indicator you have available for its current performance.
This makes it important to measure your RO rejection rate on a regular basis. Ideally, you should have some schedule which allows you to keep track of it as time goes by. By taking measurements at regular intervals, you can spot any potential deviations more easily. Plus, it’s a good idea to keep your reverse osmosis system in check in the long run in any case. We’ve explained the appropriate levels to look out for further in this article.
We’ve developed this calculator if you want to get an idea of the rejection rate of your own reverse osmosis system/membrane. The calculation is based on a simple formula which you can technically calculate yourself, but with the help of this tool, it’s as simple as inputting the TDS of your unfiltered water and that of your filtered water, and that’s all there is to it!
The most important thing here is to ensure that you’re measuring your TDS correctly. We’ve prepared a brief guide for that below, so please go over it carefully before attempting to measure your TDS levels. Otherwise, you might contaminate the test results and get the completely wrong impression of how well your filter system is working.
- You are going to need a digital TDS meter.
- Remove the meter’s protective cap and turn it on.
- Submerge the TDS meter in the water sample. Do not immerse more than indicated.
- Stir the TDS meter to remove air bubbles.
- Wait for the TDS level to be recorded.
Further Reading:
Typical TFC RO Membrane Rejection Rates
Reverse osmosis membranes work very well against most types of salts and other dissolved solids, removing many of them with an effectiveness of more than 95%.
There are some minor exceptions, though, where a reverse osmosis membrane might not work that well, such as boron which is often only removed with an effectiveness of around 50%-70%. But still, a reverse osmosis system performs much better than most other types of water filtration, all things considered…
| Aluminum | 97-98% |
|---|---|
| Ammonium | 85-95% |
| Arsenic | 94-96% |
| Bicarbonate | 95-96% |
| Boron | 50-70% |
| Bromide | 93-96% |
| Cadmium | 96-98% |
| Calcium | 96-98% |
| Chloride | 94-95% |
| Chromate | 90-98% |
| Chromium | 96-98% |
| Copper | 97-99% |
| Cyanide | 90-95% |
| Ferrocyanide | 98-99% |
| Fluoride | 94-96% |
| Iron | 98-99% |
| Lead | 96-98% |
| Magnesium | 96-98% |
| Manganese | 96-98% |
| Mercury | 96-98% |
| Nickel | 97-99% |
| Nitrate | 93-96% |
| Phosphate | 99+% |
| Polyphosphate | 98-99% |
| Potassium | 92% |
| Radium | 97% |
| Selenium | 97% |
| Silica | 85-90% |
| Silicate | 95-97% |
| Silver | 95-97% |
| Sodium | 92-98% |
| Sulphate | 99+% |
| Sulphite | 96-98% |
| Zinc | 98-99% |
| TDS (Total Dissolved Solids) | 95-99% |
At What TDS Rejection Rate Should You Change Your Reverse Osmosis Membrane?
Regularly measuring your RO TDS rejection rate gives you an idea of when it’s time to replace the membrane of your reverse osmosis system. It’s a good idea to record your results for future reference, so that you can notice if there are any alarming trends like a sudden drop.
In general, you should aim to maintain a rejection rate of 80% at the very least. If it drops below that, this is a good indicator that it’s time to replace the membrane of your reverse osmosis system.
Even 80% is pushing it a bit though, especially if your water is not that heavily contaminated in the first place. As a general rule of thumb, you should replace the membrane of your reverse osmosis system once every 2 to 5 years. This will vary depending on your water consumption habits and the contamination level of your water.
If you have any thoughts about the question, what is reverse osmosis rejection rate, please don’t hesitate to leave a comment below!
About the Author Alexandra Uta
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