Does Reverse Osmosis Remove Bacteria from Drinking Water?

Does Reverse Osmosis Remove Bacteria from Drinking Water?

Do you want to make sure your drinking water is free of bacteria and viruses?

Reverse osmosis removes bacteria and viruses by filtering them out via the RO membrane. The pore size of the membrane is too small for these pathogens to pass through. This makes reverse osmosis an effective way to improve the quality of your drinking water.

You can feel confident that you’re giving your family safe, healthy water with an RO filtration system.

How Reverse Osmosis Works to Remove Bacteria and Viruses

Reverse osmosis is a filtration process that uses a special membrane perforated with thousands of tiny pores. These pores are typically 0.0001 microns. A micron (µ) is one-millionth of a meter.

RO filters work by pushing water molecules through the pores of the membrane. The smaller, more harmful particles—including bacteria and viruses—are left behind, while the larger water molecules pass through.

RO membranes can be used to filter out a variety of contaminants, including arsenic, fluoride, lead and mercury. They also remove bacteria and viruses from drinking water. This makes reverse osmosis an ideal choice for households that want to ensure their water is free of harmful contaminants.

Pathogens that can be filtered out by reverse osmosis include:

  • bacteria
  • viruses
  • protozoan cysts

Membrane pore size is the key

The membrane is the heart of the reverse osmosis filter. The thousands of tiny pores are designed to filter out contaminants while allowing water molecules to pass through. Design engineers specify the pore size to be sure they are small enough to remove the specific contaminants that are being targeted.

A typical residential RO membrane has a pore size of 0.0001 microns. This opening is smaller than most of the contaminants, including bacteria, viruses, and protozoan.

RO Filter size chart
Comparison of RO membrane pore size and common contaminants.

Why not just make the pore size super small? That way you can remove everything.

The problem with having too small of a pore size is it creates a significant pressure loss. Since most RO filters use the water line pressure to drive the filtration, this situation would result in very low water pressure for the treated water or no flow through the filter.

Another problem with small pore sizes is they tend to foul very quickly. This would result in having to change the membrane frequently, which would be very expensive.

What happens to bacteria in reverse osmosis filter?

When water containing bacteria contacts the membrane, they are unable to pass through because of their relatively large size.

The typical bacteria have a length of about 1 to 10 microns and a width of 0.2 to 1 micron. Compared to the pore size of 0.0001 microns, they are too big to pass through. As a result, they are removed by the RO filter.

Reverse osmosis removes bacteria from drinking water very effectively.

What happens to viruses in reverse osmosis filter?

Viruses that come in contact with a reverse osmosis membrane are filtered out because they are larger than the membrane pore size.

A typical virus ranges in size from 0.004 to 0.1 microns. This is very small – too small to see with the naked eye – but still significantly larger than the membrane pore size of 0.0001. Viruses are 40 to 1,000 times bigger than the pore size of the RO membrane.

As a result, viruses are filtered out by the RO membrane.

Reverse osmosis removes viruses from drinking water very effectively.

What happens to protozoan cysts in reverse osmosis filter?

If water containing protozoan cysts comes into the RO filter, the membrane filters them out because the cysts are too large to pass through its pores.

Protozoan cysts range in size from 2 to 50 microns in diameter. Compared to the pore size of 0.0001 microns, they are much too large to pass through and are filtered out.

Reverse osmosis removes protozoan cysts from drinking water very effectively.

Reverse osmosis filter system

I have a great reverse osmosis filter system under my kitchen sink. It was easy to install and very easy to maintain. It removes all of the contaminants I’m worried about, so my family and I can enjoy the clean water it produces for us.

You may want an RO system for your home.

Check Pricing on Amazon

Comparison of Pathogen Size to RO Membrane Pore Size

The typical RO membrane has a pore size of 0.0001 microns. This is much smaller than the size of pathogens that cause illness and disease. The following table presents the typical size of harmful pathogens and how much larger they are than the membrane pore size.


Ratio (Amount larger than Pore size)
Bacteria0.2 – 1 micron wide2,000 – 10,000
Virus0.004 – 0.1 microns40 – 1,000
Protozoan cyst2 – 50 microns20,000 – 100,000


Features to Include with Your Reverse Osmosis Filter

Reverse osmosis filtration systems come with various components and options. You should become aware of these features to select the system that works best for your needs.

Important components for pathogen removal

There are several components your RO system should have to ensure it removes all pathogens from your drinking water.

  1. Particle filter. A well-made RO filtration system should include pre-filtration to protect the membrane from fouling. This first stage filter is designed to remove suspended solids that might clog the pores of the RO filter. A high-quality filter should have a rating of 0.1 microns to remove turbidity, color, and particles that are in your water.
  2. Activated carbon filter. The RO membrane is made of a specialty plastic material that can be damaged by chlorine and organic compounds in the water. A carbon filter can remove both of these and protect the membrane from oxidation and degradation.

Ultraviolet light option for bacteria and virus disinfection

UV Light Disinfection module
UV light modules like this can effectively disinfect drinking water contaminated with bacteria or viruses.

A reverse osmosis filter can remove all bacteria, viruses, and pathogen cysts from your drinking water. It is recommended that you include an ultraviolet light (UV) disinfection module on your RO system as “insurance” that all pathogens have been removed.

Under unusually high pressure, the RO membrane can stretch. This can cause the pores to become larger which might allow bacteria or viruses to pass through.

If the membrane is damaged or degraded in some way, untreated water can bypass the membrane, allowing contaminated water to pass through. Either of these scenarios would result in pathogens in the treated water.

Since bacteria are living organisms, they can reproduce in downstream plumbing and tanks, causing illness and other problems. It is critical to keep up with the maintenance on your RO system so that issues like these can be addressed before your water is contaminated.

To protect against this situation, it is recommended that you include a UV disinfection module on your RO system. This will kill any bacteria or viruses that might have passed through the membrane.

RO systems with UV disinfection are available from many manufacturers. If your RO system does not have a UV disinfection option, you can simply purchase a standalone UV unit and install it downstream of your RO unit. Either configuration will provide the required disinfection.

Risks Associated with Bacteria and Viruses in Drinking Water

If your drinking water contains bacteria, viruses, or other pathogens, there are several risks you might face.

  • Gastrointestinal illness. Drinking water that contains bacteria can cause gastrointestinal illnesses such as diarrhea, vomiting, and cramps. These illnesses are often caused by consuming food or drink that has been contaminated with fecal matter.
  • Hepatitis A. This is a serious liver infection that can be caused by drinking water that contains the hepatitis A virus. The virus is usually spread when an infected person does not wash their hands properly and then contaminates food or drink.
  • Cryptosporidium. This is a protozoan parasite that can cause severe diarrhea, vomiting, and fever. It can be contracted from drinking water that contains cryptosporidum.
  • Other parasites and protozoa. There are several other types of parasites and bacteria that can be found in drinking water such as giardia, amoeba, or shigella. If your drinking water contains any of these pathogens it could result in severe illness if you drink the contaminated water.

Each of these pathogens can cause serious illness and should be taken seriously. It is important to have a good quality RO filtration system in your home to remove these contaminants from your drinking water.

How do bacteria and viruses get into my drinking water

Bacteria and viruses can enter your drinking water in a number of ways.

They can be present in the water source. Surface water that is used for drinking, such as rivers and lakes, can be contaminated by agricultural runoff, sewage discharges, and wildlife.

If you live in a rural area, bacteria and viruses can enter your well water from wild animals that have contaminated the groundwater supply. These might include rodents, birds, deer, etc. If your septic system fails or is too close to your well, bacteria and other pathogens can enter your water system.

Bacteria and viruses can also be introduced into the water system during treatment. Pathogens can be released into the water when it is treated at a municipal plant or if it is disinfected with chlorine.

And finally, they can come from the plumbing in your home. If you have a leaky faucet or pipe, bacteria and viruses can be released into the drinking water.

Pathogens to worry about

Some of the more dangerous microbial contaminants that might be present in your water include:

  • E. coli
  • Giardia
  • Cryptosporidium
  • Hepatitis A

These pathogens can cause gastrointestinal problems, jaundice, nausea, and flu-like symptoms. In some cases, they can lead to death if the person is not treated.

Final Take on Reverse Osmosis to Remove Bacteria and Viruses

Reverse osmosis is a water purification process that can remove bacteria, viruses, protozoan, and other pathogens. The special RO membrane pores are too small for these microorganisms to pass through, so the only thing that passes through the filter is clean drinking water.

Even if you don’t live in an area with contaminated groundwater or improper disinfection practices, it never hurts to be safe about what you drink. If your home has city-provided tap water, but you want extra protection against bacteria like E coli or rotavirus, consider installing an RO system. Key features to include with your reverse osmosis system include sediment filters, activated carbon, and a UV light module.

Boch Richard

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

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