What Contaminants Do Reverse Osmosis Systems Remove from Drinking Water?

Reverse osmosis is one of the most powerful types of water filtration available to homeowners.

By pressurizing water and pushing it through membranes with microscopic pores, reverse osmosis systems can remove a wide variety of commonly occurring metals, minerals, and chemicals from the water supply.

Most reverse osmosis systems also contain additional filtration stages – carbon blocks and sediments screens – to catch any remaining contamination that’s resistant to reverse osmosis treatment.

So what exactly do reverse osmosis systems remove from water, and are any contaminants that can’t be filtered using reverse osmosis?

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Health risks

Does reverse osmosis remove microorganisms?


Bacteria, viruses, and protozoa can exist in water that hasn’t been disinfected – or water that’s been reinfected after disinfection. This can occur due to flooding, cracked pipes, contaminated water heaters and other fixtures, or when water is sourced from untreated lakes and wells.

Most reverse osmosis systems have membranes with almost unimaginably small pore sizes: between 0.001 and 0.0001 microns. This is so small that it approaches the size of water molecules themselves.

The smallest bacteria measures around 0.4 microns, while viruses can measure as small as 0.02 microns. As a result, reverse osmosis systems effectively retain all types of microorganisms.

This means you can technically use a reverse osmosis system to protect drinking water from bacteria like Salmonella and E. coli, viruses like Norovirus, and protozoa like Giardia.

We say “technically” because, all reverse osmosis systems are able to remove these contaminants, they are not disinfection devices.

Reverse osmosis systems don’t destroy microorganisms, only filter them from the water supply. Using a reverse osmosis filter on heavily contaminated water can therefore result in a build-up of bacteria inside the filter housing and on the membrane itself. That’s why standard RO systems are listed for use on “biologically safe water” only.

Does reverse osmosis remove heavy metals? (Lead, arsenic, mercury)


Again, the pore size of all reverse osmosis systems is far smaller than the size of metal molecules, meaning that reverse osmosis systems retain all heavy metals.

Check the specifications of individual systems for the lead removal rating. Most products are rated to remove 95 – 99% of lead and other metals.

Protection against dangerous metals like lead and radon is one of the more common use cases for reverse osmosis filters. It’s relatively easy to install a reverse osmosis system under any faucet used for drinking, to protect the water supply without the need for complex plumbing alterations.

Remember that lead and many other metals have no safe consumption level, meaning that water at risk of heavy metal contamination should always be treated before consuming.

Aesthetic contaminants (smells, tastes, clouding)

Does reverse osmosis remove minerals?


Reverse osmosis systems can remove dissolved mineral compounds from tap water, such as sodium, potassium, calcium, and magnesium. Removing minerals is also known as water softening.

Water with a high mineral content can appear cloudy or milky, have a stronger taste, and may cause scale to accumulate inside appliances. Many people also find that showering with hard water has a drying affecting on skin and hair.

As is the case with microorganism removal, however, using a reverse osmosis filter to retain minerals may not be the most effective approach. As minerals are caught by the filter, they can quickly clog the membrane and reduce the lifespan of the system. This is more likely to happen if your water is hard.

That’s why most filtration experts recommend using water softeners to deal with hard water – as well as adding a water softener to your setup if you’re planning to use a reverse osmosis system with very hard water. Water softeners use a treatment process called ion exchange, where pure salt attracts and exchanges places with minerals dissolved into the water.

Does reverse osmosis remove sediment?


Sediment such as sand, silt, rust, brick, dirt, or other visible particulates can be easily removed by a reverse osmosis system.

While reverse osmosis membranes can block sediment, most systems contain a dedicated sediment stage, containing a replaceable material screen. This helps to protect the delicate membrane screens that are prone to damage and clogging by larger contaminants.

Does reverse osmosis remove chlorine?


Reverse osmosis systems can remove large amounts of chlorine from water – mostly through the carbon cartridges that are included alongside the osmosis membrane.

Activated carbon is a simple and effective chlorine removal filtration method. Chlorine molecules are attracted and adsorbed onto the surface of the filter media as they pass through the cartridge.

As is the case with other contaminants above, reverse osmosis membranes can remove chlorine, but allowing the chemical to build on the membrane is likely to degrade the sensitive material.

Other contaminants

Does reverse osmosis remove fluoride?


Fluoride is a naturally occurring mineral that can be found in water. Most municipalities also add extra fluoride to water in an effort to improve the population’s dental health.

Because fluoride molecules are larger than the pore size of osmosis membranes, fluoride can be removed from water using a reverse osmosis system.

As fluoride has become a more common ingredient in toothpaste and other dental hygiene products, many people believe that drinking fluoridated water is no longer necessary and wish to filter it.

What doesn’t reverse osmosis remove?

With reverse osmosis being such a powerful water treatment technique, it’s easy to miss the fact that some contaminants are resistant to membrane filtration, and will pass through a reverse osmosis system.

These resistant contaminants are all organic gases and chemicals and can slip through membranes thanks to their size, or the way they bind with water molecules.

Here are the most common drinking water contaminants that can resist reverse osmosis filtration:

  • Pesticides and herbicides. These chemicals often enter the water supply from runoff after rainfall in agricultural areas.
  • Hydrogen sulfide. This natural gas has a distinctive odor and is common in well water.
  • Methane. Like sulfur gas, dissolved methane is unlikely to be filtered by reverse osmosis.
  • Other organic solvents.