What is not Removed by Reverse Osmosis?

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Reverse osmosis can help you address a great number of potential issues in your tap water. From possible water contaminants to aesthetics like taste and smell helps to get rid of them all.

But do you know that reverse osmosis alone isn’t sufficient for handling water problems? This is because some contaminants are molecularly smaller than water, meaning that they can pass through the RO membrane with ease.

In this post, we’ll go through some water contaminants that reverse osmosis can’t remove from your water completely.

You’ll also learn how to remove these contaminants from your water now that RO seems not to work on them.

What Does Reverse Osmosis not Remove from Water

What is not Removed by Reverse Osmosis


Although reverse osmosis can remove almost every other contaminant from your water, it can’t remove all bacteria.

Well, it isn’t all that surprising since it isn’t only RO that can’t. In fact, no other filter can remove bacteria from your drinking water.

Ultraviolet water treatment systems and distillation are the only water purification systems that can completely kill bacteria and remove them from your water.

You should know that distillation treats water by boiling, and the bacteria present in the water are killed during the process. (check price on amazon best countertop water distiller)

If you choose to go the ultraviolet way, get a UV system that comes with an NSF/ANSI55 certification. It’s the only one that can help you get rid of bacteria from your drinking water.(check price on Amazon)

Carbon Dioxide

Reverse osmosis can’t remove carbon dioxide from water since it’s a gaseous component. You need to know that although the RO membrane stops so many other contaminants, gases pass through with ease.

Should you be worried about this? No. Any carbon dioxide present in the water is completely harmless and won’t affect the smell or taste of your water.


Radon is a radioactive element, just like radium and uranium. And yes, reverse osmosis can remove radioactive elements from water.

But since radon is a gas (colorless, odorless, tasteless, and radioactive gas), reverse osmosis cannot remove it from water.

Radon can only be removed by aeration treatment or granular activated carbon.


You definitely don’t want to drink water that is rich in pesticides. However, their particles pass easily through the RO membrane since they are smaller than water molecules.

To ensure that your water is free of pesticide residue, add carbon-based filters to the system.


Just like pesticides, these hazardous chemicals have particles that are molecularly smaller than water. They, therefore, pass freely through the RO membranes.

To prevent these carcinogenic chemicals from invading your water stream, you need to use a carbon filter as well.


Although reverse osmosis systems remove 85 to 92% chlorine from water, a small amount may pass through damaged or old membranes. The good news, however, is that the quantity is usually too small to harm your health.

If the chlorine level is high, activated carbon can be a great solution to removing it.

Things to note

Different reverse osmosis systems remove different contaminants from water. So, don’t be surprised if you come across a unit that can actually remove some of the above-mentioned water contaminants.

For instance, after a lab test on different RO systems, EPA found that some systems can actually remove 99% of herbicides and pesticides. It also found that some systems only removed 23%.

One important thing to know is that if a system’s RO membrane is made from cellulose acetate or polyamide, it can’t remove pesticides or herbicides. Avoid it at all costs!

Even with an activated carbon filter, some RO systems may not remove chlorine effectively from your drinking water. Only those with the National Sanitation Foundation Certificate 42(NSF-42) remove chlorine.

Also, for a system to remove amoeba from your water, it must have the NSF-58 or NSF-53 certification. Even without the certification, an RO system can remove amoeba if has an absolute pore size of one micron or smaller.


Although reverse osmosis has situations where it shines in water treatment, there are times when it doesn’t work at all.

To get the best results for your home water, it’s important to understand what is in your water and how much of the contaminants might be present in your water supply. This way, you’ll be able to determine the right water treatment method for you.