content is free. We earn a commission at no extra cost when you purchase through referral links on our site.
Learn more

Does Reverse Osmosis Remove Bacteria From Water? Latest Guide 2023

Does Reverse Osmosis Remove Bacteria


Around 23 million people rely on private wells for drinking water in the United States, which are at higher risk of bacterial contamination. Bacteria can enter a well through surface runoff carrying contaminants, such as animal waste or fertilizers, and specifically poorly maintained plumbing systems. 

In water purification systems, reverse osmosis water filters work incredible for all hidden impuritites. But does it would remove tiny harmful villains, “bacteria” from drinking water? Yes, it can effectively remove 90% bacteria including total coliform, pathogenic, heterotropic, while iron and sulfur bacteria to some extent as they can clog the membrane.

In this article, we’ll explore how and what bacteria reverse osmosis can remove; also, we’ll describe the application of these systems for your home use. 

Can Reverse Osmosis Remove Bacteria?

Can Reverse Osmosis Remove Bacteria

Yes, reverse osmosis (RO) is an effective method for removing 99% bacteria from drinking water. Its semi-permeable membrane removes a wide range of contaminants, including bacteria such as coliform and e-coli, viruses such as hepatitis c, parasites including amoeba, minerals like calcium and magnesium, salts like sodium, and other impurities.

The semi-permeable membrane known as the heart of an RO system has tiny pores. When the water is pushed through this membrane under pressure, the filtered water is passed through it, and all contaminants, including bacteria, are left behind.

However, the effectiveness of the RO system may depend on factors such as the quality of the membrane, water pressure, and the initial bacterial load in the water.

How Does Reverse Osmosis Remove Bacteria?

How Does Reverse Osmosis Remove Bacteria

In a reverse osmosis water purification system, the contaminated water travels through multiple stages before reaching the purified state.

A basic reverse osmosis filtration system works by passing the water through a pre-filter (sediment filter), which removes sediments and dirt. This filtration protects the RO membrane from clogging and damage.

The RO membrane is the main part of the reverse osmosis filtration system, which can remove contaminants larger than the water molecules (0.0001 microns), including bacteria and pathogens. 

After trapping various impurities, including bacteria, the water is allowed to pass through post-filter media such as activated carbon or ion exchange. This media helps to enhance the water quality by removing hardness, chlorine, foul odor, and unpleasant taste. Lastly, The filtered water is stored in a reverse osmosis tank before the water goes through a faucet.

What Bacteria Does Reverse Osmosis Remove?

What Bacteria Does Reverse Osmosis Remove

Various types of bacteria can be present in the drinking water, which reverse osmosis can remove as follows: 

1. Coliform Bacteria or E.coli

The presence of coliform bacteria, particularly E. coli, in well water, is a strong indicator of fecal contamination. If your water contains E. coli, it can lead to gastrointestinal illnesses, including diarrhea, abdominal cramps, and, in severe cases, more serious illnesses.

The size of Escherichia coli (E. coli) is about 0.5 to 2 micrometers (μm) in diameter and 2 to 6 μm in length, which is large enough to trap by RO membrane. As a result, RO systems can achieve over 99% removal rates for bacteria, like E. coli.

2. Total Coliform Bacteria

They are a broader group of bacteria that includes coliform bacteria and fecal bacteria. If your well water is contaminated with total coliforms, it suggests your well water has surface water or fecal matter along with other various pathogens. 

The size of individual total coliform bacteria typically ranges from about 1 to 5 micrometers (µm) in length. So, reverse osmosis (RO) can effectively block and remove them from its extremely small pores. 

While not all total coliforms are harmful themselves, their presence indicates a potential risk of other harmful microorganisms that you may need to remove.

3. Fecal Streptococci

Fecal streptococci bacteria are commonly found in the intestines and wastes of warm-blooded animals, including humans. Its presence in well water indicates fecal contamination, whose consumption can lead to gastrointestinal illnesses and many more.

The U.S. Environmental Protection Agency (EPA) developed the MCL level to be zero—no detectable coliform bacteria, including fecal coliforms, should be present in drinking water.

They are relatively small in size, ranging from 0.5 to 1.5 micrometers (µm) in diameter. RO is highly efficient at removing them along with a wide range of other contaminants, making it an excellent choice for improving water quality. 

4. Iron Bacteria

When bacteria start living with bacteria, they form a colony of slimy deposits. It can be seen in the form of slime-sludge-like material on the pipes, tanks, and fixtures, leading to clogs, unpleasant odors, and yellowish water. They are not harmful to human health but are crucial to avoid rusty issues. 

The iron bacteria grow in various sizes, but they are relatively large compared to some other bacteria mentioned above. However, their average size is 1 to 10 micrometers (µm) in size. Generally, RO can remove them, but not in a very effective way. 

Iron bacteria often form biofilms that can adhere to surfaces of RO components such as tanks, tubings, and connectors, making it challenging for an RO membrane to eliminate. We suggest using an RO system without a tank to avoid high-maintenance issues and bacterial build-up.

We recommend you disinfect your well with a high concentration of chlorine or an oxidizing agent such as greensand media, and potassium permagnet. It will help oxidize the iron bacteria, reducing their composition and precipitating the iron, which a RO system can easily filter out.

5. Sulfur Bacteria

Sulfur bacteria are known for producing hydrogen sulfide gas, which can result in a rotten egg-like odor in well water. While they are not harmful in themselves, the presence of hydrogen sulfide gas can make the water unpleasant to drink.

Sulfur bacteria are typically quite small, approximately 0.5 to 2.0 micrometers in diameter.  You can use a reverse osmosis system to fix them but they are not specially designed to remove sulfur bacteria. Regardless, if sulfur compounds are your main concern, various iron filters whole-house water filters, or carbon filters can effectively remove them.

6. Heterotrophic Bacteria

Heterotrophic bacteria are mostly found in the environment, including well water. These bacteria can obtain energy and carbon by breaking down organic matter or consuming other microorganisms.

In well water,  the maximum allowable level of heterotrophic bacteria is 500 CFU/ml, which is typically harmless, but higher levels are associated with aesthetic and health issues. Pseudomonas, Aeromonas, Favobacterium, and Acinetobacter are the most common types.

Heterotrophic bacteria vary in size, typically ranging from 0.5 to 5 micrometers (µm) in diameter.  Reverse osmosis membrane can easily remove these bacteria by its small pores ranging from 0.0001 to 0.001 µm, which is significantly smaller than the size of most heterotrophic bacteria.

Sometimes, if the incoming water contains a high bacterial load, it starts growing in reverse osmosis (RO) tanks or systems. They adhere to surfaces and form biofilms where water remains stagnant or has low flow rates, such as in the RO tank, tubing, and connectors. 

You may need to maintain the system and clean the RO tank regularly to prevent the growth of heterotrophic bacteria. Clean and sanitize the components while ensuring flushing of the system to prevent bacterial buildup. 

7. Pathogenic Bacteria

These bacteria include Salmonella, Campylobacter, Shigella, Campylobacter, and Vibrio, which can cause foodborne illnesses.

When they are present in well water and ingested through drinking or consuming contaminated food prepared with contaminated water, they can cause symptoms such as diarrhea, fever, abdominal cramps, and vomiting. In severe cases, it can lead to dehydration and hospitalization.

Pathogenic bacteria vary in size, approximately 0.5 to 5 micrometers (µm) in diameter. If these bacteria are your main concern, then yes, reverse osmosis is the best choice to remove them up to 99%.

Recently, in 2022, a non-spore-forming, gram-negative bacterium that appears as a “safety pin,” Burkholderia pseudomallei, was found only in parts of Southern Asia and Australia and for the first time in soil of the U.S. water samples. Its exposure is associated with the illness called melioidosis. It measures 2–5 μm in length and 0.4–0.8 μm in diameter, which is large enough to trap by a reverse osmosis membrane. 

Applications of RO System— Which to Consider?

You can install reverse osmosis (RO) systems in various settings such as under the sink, over the counter, or even whole-house systems. 

Whole house RO filters are relatively larger and more costly than others but efficient in providing the quality water used for cooking, drinking, bathing, gardening, and others. 

Countertop RO systems are typically easy to install and do not require any plumbing modifications. They connect directly to the kitchen faucet and are portable to move easily, which is ideal for apartments and rental homes. Unlike under-sink systems, countertop RO systems do not require drilling holes and are more affordable than under-sink models. 

Similarly, Under-sink RO systems are known for their exceptional water purification capabilities. They remove a wide range of contaminants, including bacteria, viruses, minerals, and dissolved solids. They are installed under the sink, maximizing counter space in the kitchen. Once installed, it delivers the purified water directly to a faucet, providing bacteria-free water.

Besides this, you can also consider the reverse osmosis water filter with UV lights, which is highly effective at inactivating harmful microorganisms like bacteria, viruses, and protozoa. 


In the end, your reverse osmosis can significantly reduce the presence of bacteria in well water. Its semi-permeable membrane is highly efficient in removing impurities, including bacteria, viruses, and parasites. However, it may not eliminate 100% of all microbial organisms, as some smaller ones pass through the membrane. 

Therefore, we suggest you disinfect or boil your water at high temperatures for 15 minutes to make water safe for drinking. Overall, RO is a reliable and efficient means of enhancing the safety and quality of drinking water.

Scroll to Top