What Does Reverse Osmosis Not Remove from Water?

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Reverse osmosis is probably the most effective filtration method you can use to provide safe and clean water for your family.

But that doesn’t mean that you’re getting a one-membrane-rejects-it-all deal!

Although the reduction rate for most water contaminants is way above that 90% mark, there are certain compounds that an RO membrane has difficulty rejecting.

Let’s find out what these are!

Key Takeaways

  • Reverse osmosis does not remove dissolved gasses because of their non-ionized nature, small size, and low weight.
  • An RO membrane can allow tiny pesticides like atrazine to pass through.
  • Reverse osmosis is also unable to reject certain organic solvents, such as benzene.
  • Good news is, all these contaminants are easily removed by the pre and post-filters found in all reverse osmosis systems.

What Does Reverse Osmosis Not Remove from Water?

So, what does reverse osmosis not remove from water?

Anything that is as tiny as the pores of a reverse osmosis membrane could theoretically pass through them.

As such, reverse osmosis water purification cannot possibly offer complete protection from water pollutants. Although at a very low rate, tiny contaminants can get to the filtered side of the RO membrane and end up in your drinking water.

Dissolved gasses

An RO membrane rejects water contaminants based on their size and charge. The higher the contaminant’s ionic charge, the less likely it will pass through the membrane. For instance, calcium has two charges, and so it is rejected at a higher rate than, say, one-charge sodium.

Dissolved gasses, on the other hand, are not ionized. Plus, they are tiny and have a very low molecular weight. This is why a reverse osmosis membrane is not that effective when rejecting gasses, which can result in carbon dioxide, radon, and/or chlorine coming out of your RO faucet.

When it comes to chlorine, an RO membrane can somewhat remove chlorine compounds. However, it is essential to know that chlorine can easily hydrolyze the membrane and shorten its lifespan. For that reason, it is recommended to use activated carbon to pre-filter out any chlorine and keep the membrane in tip-top shape.


Reverse osmosis is known to remove most pesticides and toxins from the feed water – most but not all of them.

It is not uncommon for tiny particles of pesticides, such as atrazine, to pass along with the filtered water.

Farmer Spraying Pesticides on Wheat Field

Organic Compounds

Organic compounds are easy to get absorbed by any filtering media because they are, well, organic in nature. An RO membrane is no exception, and it provides up to 99.9% protection against organic contaminants.

However, that protection also comes with a few exceptions, primarily for organic solvents. Some organic compounds, like benzene, are organic solvents and can easily dissolve in the RO membrane and then migrate to the filtered water.

While RO cannot do the job successfully on its own, with the help of additional activated carbon filters, a reverse osmosis system can be significantly more effective in rejecting these particular organic contaminants.

How Does Reverse Osmosis Water Purification Work and What Does It Remove?

To fully grasp what is not filtered during reverse osmosis water purification (and why), you need to understand what is actually being removed and how RO purification generally works.

Reverse Osmosis Principle

The basic principle behind reverse osmosis involves nothing but simple mechanics.

The RO membrane is semipermeable and allows the passage of water molecules. Almost anything larger than 0.0001 micron in size will be prevented from passing through. That creates two different streams of water – one for those tiny water molecules that have penetrated the membrane and one for the impure rejects stuck on the other side.

To fully grasp reverse osmosis, let’s take a look at osmosis first.

When two streams of water with different salt concentrations get separated by a semipermeable membrane, the water with fewer salts will naturally start flowing toward the side with the higher salt concentration. This is a natural occurrence triggered by the unbalanced salt concentrations.

Reverse osmosis, on the other hand, uses the same principle but in the opposite direction. Here, the process isn’t natural but requires energy to work. With the help of external water pressure, the feed water is pushed from the polluted side to the clean side.

What Is Removed

What does reverse osmosis remove from water?

Most biological contaminants can be removed with a success rate of 99.9%, in case the RO membrane has been properly installed and the whole unit has been maintained well.

Other things the RO membrane is successful at rejecting include dirt, sediment, asbestos, proteins, and more. Plus:

  • 98% of aluminum and ammonium
  • 97% of copper, mercury, nickel, sulfate, sulfite, and zinc
  • 96% of arsenic, barium, cadmium, calcium, iron, lead, manganese, and magnesium
  • 95% of selenium
  • 94% of chromium 6
  • 90% of potassium and sodium
  • 88% of chloride, cyanide, and fluoride
  • 80% of radium
  • 68% of nitrates


When it comes to removing waterborne bacteria, reverse osmosis provides great protection. That’s because bacteria cells are way too large to make it through an RO membrane.

But, RO filters have not been designed with bacteria or other pathogens in mind.

What does that mean? Well, even if the membrane has been installed and maintained correctly and has an excellent rejection rate, it can still eventually allow bacteria through. The bacteria will proliferate on the membrane and deteriorate it, sooner or later finding their way into your glass.

It is important to know that most RO membranes are designed to be used with water that is biologically safe to drink. If you think it’s likely for your water to be contaminated with microorganisms, UV filtration may help.

Under Sink Reverse Osmosis System

Other Limitations of Reverse Osmosis

Reverse osmosis does come with some more limitations:

Loss of Minerals

The reverse osmosis membrane rejects anything larger than water molecules. And while this is great for unwanted compounds, it also gets rid of the good stuff. Any minerals, including calcium, magnesium, or iron, found in your feed water will not get through the membrane.


Remember the two streams of water that get separated by the membrane during the filtration process? Well, the one loaded with the contaminants, unfortunately, ends up down the drain.

Wastewater is a part of RO treatment, but know that a decent and up-to-date system can significantly reduce this.

RO Benefits

When it comes to water filtration, reverse osmosis allows two huge advantages:

Great Filtration

One of the best methods available, you can rest assured that your feed water will be thoroughly filtered using a reverse osmosis system. Furthermore, if you wish to take things to an even higher level, you can install additional pre and post-filters for an even more detailed purification.

Great Water Quality

Clean and pure water with great taste with just a semi-permeable membrane and water pressure. It’s really as simple as that.

If you have any thoughts about the question, what is not removed by reverse osmosis, 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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