Reverse Osmosis (RO) Membrane Pore Size – What You Need to Know

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The main idea behind a reverse osmosis membrane is that its pores are small enough to only allow water molecules through and reject almost everything else.

However, some people don’t seem to realize just how fine RO membrane pores are.

Let’s discuss RO membrane pore size!

Key Takeaways

  • The pores on a reverse osmosis membrane are in the range of 0.001-0.0001 microns.
  • This is more than enough for rejecting most types of contaminants found in household water supplies.

What Is the Pore Size of RO Membranes in Micron?

The pore size of a reverse osmosis membrane usually falls in the range of 0.001-0.0001 microns. This size allows RO filters to reject most contaminants that you would find in the typical household water supply.

For reference, pesticides are usually around 0.001 microns in diameter, most bacteria are between 0.2 and 2.0 microns, and viruses are typically between 0.2-0.4 microns. As you can see, the pore size of an RO membrane is small enough to easily reject those contaminants + many more.

How Does a Reverse Osmosis Membrane Work?

Reverse osmosis works by forcing water through a semipermeable membrane at a high pressure. Thanks to the small size of the membrane’s pores, almost only water molecules can make it through the membrane. Other molecules and contaminants get “rejected” and end up in the concentrate line.

Before water is fed through an RO filter, it’s usually treated by several pre-filters to ensure that the RO membrane can work as efficiently as possible. Post-filtration may be applied as well, especially if a storage tank is used.

blue reverse osmosis membrane

What Water Contaminants Does an RO Membrane Remove?

Reverse osmosis is very effective at removing certain types of contaminants. It can remove the following and some others:

  • Arsenic
  • Barium
  • Cadmium
  • Chloride
  • Colloids
  • Cyanide
  • Fluoride
  • Iron
  • Lead
  • Manganese
  • Mercury
  • Microorganisms
  • Nickel
  • Nitrate
  • Pesticides/herbicides
  • Phosphate
  • Proteins
  • Sediments
  • Sulfate
  • VOCs

The filtration effectiveness for most of these is above 90%, with some exceptions like nitrate which is removed at around 70%.

Pros & Cons of Reverse Osmosis Systems

Pro: Clean, Fresh Water

You will always have access to clean, fresh water with close to no contaminants. Using a storage tank means that you won’t have to wait.

Pro: Effective at Removing Many Contaminants

Reverse osmosis systems, especially when used with a good set of pre and post-filters, are excellent at removing a wide range of water contaminants.

Pro: Low Cost and Maintenance Requirements

It doesn’t cost much to purchase and install a reverse osmosis system, and they require relatively little maintenance in the long term too.

Con: Water Gets Wasted

Some water will get wasted as part of the process. The amounts can be quite significant – in some cases, you may waste up to 75% of your water in the purification process.

Con: Useful Minerals Get Removed Too

Reverse osmosis gets rid of almost all contaminants in your water, which unfortunately also includes some useful elements like minerals. You can add a remineralization stage to your system to avoid this.

Con: Slow Speed

Reverse osmosis works slowly by default, which is why you’ll commonly see systems using a storage tank as a buffer. However, this may require additional post-filtration.

If you have any questions about RO membrane pore size 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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