What Does an RO Membrane Remove from Water?
Written by: Alexandra Uta // Last Updated: Mar 24, 2023
This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no extra cost to you. Learn more.
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!
Contents
show
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.
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.
Further Reading:
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
Information provided on BOS is for educational purposes only. The products and services we review may not be right for your individual circumstances.
We adhere to strict editorial guidelines. Rest assured, the opinions expressed have not been provided, reviewed, or otherwise endorsed by our partners – they are unbiased, independent, and the author’s alone. Our licensed experts fact-check all content for accuracy. It is accurate as of the date posted and to the best of our knowledge.