Does Reverse Osmosis Remove Bacteria from Water? Find Out Here!

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Reverse osmosis is an effective water purification method trusted by millions of people worldwide.

It eliminates floating and dissolved contaminants alike and is the source of clean drinking water for many homeowners.

But how does reverse osmosis fare against biological contaminants like bacteria? Does reverse osmosis remove bacteria from water? Let’s find out!

Key Takeaways

  • Reverse osmosis removes bacteria from water (examples of such bacteria are Campylobacter, Salmonella, Shigella, and E. Coli).
  • Reverse osmosis does not kill bacteria in water however.
  • To destroy bacteria in water, you must add a UV light to your RO system.

Does Reverse Osmosis Remove Bacteria from Water?

So, does reverse osmosis remove bacteria from water?

Yes, reverse osmosis does remove bacteria from water!

Reverse osmosis systems filter water using a semipermeable membrane with a pore size of around 0.0001 microns. However, the average bacteria is about 1 to 2 microns wide and 5 to 10 microns long, which is a lot bigger than the membrane’s tiny pores.

Because bacteria cells are larger than the RO membrane’s pores, they cannot diffuse through the membrane during filtration and are rejected.

Common bacteria removed by reverse osmosis include Campylobacter, Salmonella, Shigella, and E. Coli.

bacteria in water

To What Percentage Does Reverse Osmosis Remove Bacteria?

Reverse osmosis removes 99.9% of bacteria from water. This is a stellar level of effectiveness, but don’t get carried away. While reverse osmosis can remove bacteria, it wasn’t designed for that purpose. In fact, most RO systems are to be used with biologically-safe water (it says so on the label). Why?

Bacteria are not like inorganic contaminants; they grow and multiply constantly. After some time, the bacteria will accumulate on the RO membrane surface. With an increased bacterial load, they begin obstructing and damaging said membrane, and slip through the pores and into your water eventually.

The only way to get rid of bacteria with a reverse osmosis system is to combine it with a UV water purifier. Unlike RO systems, UV purifiers are actually designed to kill bacteria in water.

What Else Does Reverse Osmosis Remove from Water, and What Not?

Besides bacteria, reverse osmosis also removes other biological contaminants like viruses, protozoa, and even cysts. Reverse osmosis also shines when it comes to inorganic contaminants. It removes sediment, dirt, harmful pesticides, volatile organic compounds (VOCs), and asbestos. Plus:

  • Aluminum (98%)
  • Ammonium (98%)
  • Arsenic (96%)
  • Barium (96%)
  • Cadmium (96%)
  • Calcium (96%)
  • Chloride (88%)
  • Chromium 6 (94%)
  • Copper (97%)
  • Cyanide (88%)
  • Fluoride (88%)
  • Iron (96%)
  • Lead (96%)
  • Manganese (96%)
  • Magnesium (96%)
  • Mercury (97%)
  • Nickel (97%)
  • Nitrates (68%)
  • Phosphate (97%)
  • Potassium (90%)
  • Radium (80%)
  • Selenium (95%)
  • Sodium (90%)
  • Sulfate (97%)
  • Sulfite (97%)
  • Zinc (97%)
  • And more.

A reverse osmosis membrane removes all the impurities listed above. A complete reverse osmosis system, however, with its pre-filters and post-filters can remove even more contaminants, like chlorine!

Bottom line: RO membranes filter out a long list of contaminants, but on their own, don’t get everything. Here’s a list of things that may slip through:

  • Pesticides like atrazine
  • Organic compounds like benzene
  • Dissolved gasses like radon, carbon dioxide, and chlorine (this is because they’re not very heavy and are also not ionized).

How Does Reverse Osmosis Work?

Reverse osmosis works by pushing water under high pressure through a semipermeable membrane with very tiny pores.

Water molecules are small enough to pass through the pores, but contaminants (biological, physical, and chemical) are too large and remain stuck on the membrane surface. From here, they are washed away in a wastewater stream through a separate tube and down the drain.

Pros and Cons of RO Water Purification

Let’s see the pros and cons of RO water purification:

Pro: Advanced Filtration

Reverse osmosis is up there when it comes to filtration results. RO is thorough, and a standard reverse osmosis system provides almost 100% pure water.

Pro: Quality Drinking Water

Besides making your water safe to drink, reverse osmosis also improves your experience with drinking water. RO water has almost no taste and odor, and it’s a massive improvement on awfully tasting or smelling water.

Pro: Easy Installation and Maintenance

For all its thoroughness, reverse osmosis systems can be pretty simple to install and maintain. This particularly goes for countertop and under sink RO systems. You can install them at home with a few tools and a manual by your side. You don’t need to factor in installation costs for these reverse osmosis systems because you can DIY.

Like installation, maintenance is also straightforward, and you can carry it out without help. All you need to do is replace filters and other elements when due.

plumber installing reverse osmosis system under sink

Pro: Customizable

You can customize your RO system, adding extra filtration steps to cater to your water’s needs. You can also add a remineralization stage or run a line to the icemaker in your fridge. It’s all up to you to decide what extras you want!

Cons: Mineral Loss

Reverse osmosis removes impurities from water. Unfortunately, this also means healthy minerals. If this bothers you, don’t worry. You can include a remineralization step to add these minerals back in.

Cons: Slow Filtration

Reverse osmosis is a slow process and this can be a challenge to some people. If you have a large household and consume a lot of water, you may empty the storage tank frequently, and you’ll have to wait hours for it to fill again. Not ideal.

Cons: Wastewater

This is the biggest downside of reverse osmosis. A lot of wastewater is created in the process of filtering water (up to 5 gallons of wastewater for every gallon of filtered water). Fortunately, there are ways around this. For example, you could invest in a pump to increase the pressure in the RO system, thereby cutting down wastewater by 90%.

Other Ways to Remove Bacteria from Water

Reverse osmosis removes bacteria from water, but there are other efficient ways to do this as well. Let’s see some of them!


Unlike RO systems, UV purification is specifically designed to kill microorganisms in water. These microorganisms include bacteria, viruses, protozoans, algae, and fungi.

UV light works by disrupting the bacteria’s DNA, making it impossible for it to grow and multiply. However, for the UV water purifier to work smoothly, it has to be used on already filtered or clear water. This is because bacteria can hide behind sediments and other impurities in water and escape the UV rays.

As long as you don’t use UV on murky water, you’ll get some really good results.

UV purifiers are inexpensive, easy to use, quick, and chemical-free. There are also no risks associated with using them.

Tip: Periodically test your water after disinfection to see if the UV water purifier is working as intended.

Carbon and Ceramic Water Filters

Ceramic water filters have very tiny pores, too, that can filter out bacteria and other contaminants. But they only do this if the micron rating is low enough (meaning if the pores are small enough).

There are different kinds of ceramic water filters, and you can choose one you find most convenient. Ceramic filtration can be found in under sink systems, countertop/gravity-fed units.


Shock chlorination is another way to remove bacteria from water. It kills them by disrupting their cell membrane. After the membrane is weakened, chlorine enters the cell and disrupts cellular respiration and DNA activity.

You can chlorinate your water using household chlorine bleach or chlorine dioxide tablets.

Chlorine dioxide tablets can help kill tough microbes like cryptosporidium if you carefully follow the manufacturer’s instructions.

Health Effects of Bacteria in Drinking Water

Depending on type, bacteria in drinking water can cause the following health effects:

  • Diarrhea
  • Cramps
  • Vomiting
  • Nausea
  • Fever
  • Headache
  • Fatigue

In severe cases, death may occur. Children, older adults, and the immunocompromised are more likely to get sick after drinking contaminated water.

If you have any thoughts about the question, will reverse osmosis remove bacteria, 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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