What Does an RO Membrane Remove from Water?

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Reverse osmosis systems have taken at-home water filtration to the next level.

Touted as one of the very best methods available, you may be asking, is it really that good? What does an RO membrane actually remove? And what doesn’t it remove?

Let’s find out!

Key Takeaways

A reverse osmosis membrane can remove pesticides, dirt, asbestos, sediment, bacteria, cysts, and viruses from your water supply; plus:

  • Aluminum
  • Ammonium
  • Arsenic
  • Barium
  • Cadmium
  • Calcium
  • Chloride
  • Chromium 6
  • Copper
  • Cyanide
  • Fluoride
  • Iron
  • Lead
  • Manganese
  • Magnesium
  • Mercury
  • Nickel
  • Nitrates
  • Phosphate
  • Potassium
  • Radium
  • Selenium
  • Sodium
  • Sulfate
  • Sulfite
  • Zinc

What Does an RO Membrane Remove from Water?

So, what does an RO membrane remove from water?

A reverse osmosis membrane removes harmful pesticides, dirt, sediment, asbestos, volatile organic compounds, microorganisms such as bacteria and viruses, proteins, and more. It can also remove up to about:

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

And that is only the reverse osmosis membrane itself! Meaning, RO systems that come with additional pre and post-filtration stages, as they usually do for home use, will remove even more contaminants.

Does a Reverse Osmosis Membrane Remove Bacteria?

Reverse osmosis membrane pores are so tiny, that they will inadvertently remove most viruses and bacteria from water. However, this is not the purpose they were designed for, and the bacteria may proliferate on the RO membrane and in the pre-filters. Most systems will be labeled to be used with “biologically-safe” water.

If you have water that is highly likely to be biologically contaminated, then you may need to look at additional filtration steps such as UV purification or chlorination.

blue reverse osmosis membrane

What Does an RO Membrane Not Remove?

An RO membrane removes contaminates from the water that are too large to fit through its pores. The pores on the membrane are so small that this covers a wide range of things, but not everything.

  • Some pesticides, such as atrazine, may get through.
  • As will some organic compounds, such as benzene.
  • Dissolved gasses such as chlorine, radon, and carbon dioxide also won’t be effectively filtered by reverse osmosis alone. That’s because these gases are neither ionized nor very heavy.

This is why the pre-filtration in an RO system is essential.

How Does Reverse Osmosis Work?

Reverse osmosis is primarily a mechanical filtration method, which means that impurities are rejected based on size exclusion. Here is how it works:

Pressure is utilized to push contaminated water through a semipermeable membrane.

This membrane has tiny pores that are big enough to allow the very small water molecules to diffuse through, but not most contaminants with larger structures, and those are blocked from following the H2O past the membrane.

Instead, that wastewater is sent through a separate tube/line and down a drain.

Pros and Cons of RO Water Purification

With any water filtration setup, there are pros and cons to consider before deciding if it is right for you. In the case of reverse osmosis, you can expect the following:

  • High-level filtration – Reverse osmosis is one of the most thorough contaminant removal systems out there, providing almost pure H2O drinking water.
  • Improved drinking water – Your RO water will have little to no taste and smell, and most people report a considerable improvement in their water quality, even if they thought there was nothing wrong with it, to begin with.
  • Easy to install and maintain – Particularly with countertop units and even with under-the-sink RO systems, all you really need is a few tools and a bit of patience to get your system set up and ready to go. Maintenance can also be performed easily by yourself at home.
  • Customization – Under sink and whole house RO systems can have extra filtration steps or lines added to them to suit your specific needs. Want a remineralization filter? Sure! Want to run a line to the ice maker on your refrigerator? No problem.
  • Loss of minerals in the water – Reverse osmosis systems eliminate most things, and as you can see from the list above, some of those minerals it filters are good ones! If this is a concern for you, then a remineralization filtration step may be something you want to consider.
  • Slow speed – Unfortunately, RO can be a slow process. If you are using a lot of filtered water and you end up emptying the storage tank, it can be hours before it sufficiently fills again.
  • Creates wastewater – The most significant downside for a lot of people is the wastewater the RO system creates. It can be really high in some cases; think up to 5 gallons of waste for every 1 gallon purified. Still, you can make sure to invest in a more water-efficient system (do check before you purchase), or you can add pumps to increase the pressure in the system and reduce the wastewater.

If you have any questions about what an RO membrane removes from water 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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