Watertechadvice.com is supported by readers. If you purchase through referral links on our site, we make a commission at no extra cost to you. Learn more.

Home » Reverse Osmosis Systems » Does Reverse Osmosis Remove Bacteria?

Does Reverse Osmosis Remove Bacteria?

By: David Trinh
Last Updated:

Most individuals tout reverse osmosis (RO) as the silver bullet for water purification.

RO is considered an effective method for removing impurities and contaminants from drinking water.

But does reverse osmosis remove bacteria?

Yes, reverse osmosis removes bacteria from water.

But before installing an RO unit, you need a water treatment specialist educating you on how reverse osmosis works.

Below is vital information on reverse osmosis and its efficacy in water filtration.

Other Helpful Resources:

How Does Reverse Osmosis Work?

RO’s basic filtration process consists of 5 main stages:

  1. Sediment and Carbon Pre-Filtration
  2. Semi-permeable .0001 micron RO membrane
  3. Storage tank
  4. Post-Filtration and Remineralization

Basically, a reverse osmosis filter moves water through these filtration stages to separate the impurities and particles in water that are larger than h20 molecules from the pure h20.

For a more in-depth guide review our page here.

How Does Reverse Osmosis Remove Bacteria?


The average diameter of common bacteria like Escherichia Coli ranges from 0.2 to 4 microns.

This size is too large to pass through the RO membranes which have pores of 0.0001 microns.

When contaminated water passes through the semipermeable membrane, pure water emerges as the filtrate, while bacteria are held up as the residue because of the .0001 micron filter.

Reverse osmosis doesn’t kill or inactivate bacteria. It only sieves bacteria off the contaminated water.

You get pure water while the filter membrane retains the biological contaminants.

Which Microorganisms Does Reverse Osmosis Remove?

According to the Center for Disease Control and Prevention (CDC), reverse osmosis systems can remove protozoa, bacteria, viruses from water.

RO can effectively filter out microbes like:

  • Salmonella
  • E. coli
  • Campylobacter
  • Shigella
  • Giardia
  • Cryptosporidium
  • Enteric
  • Rotavirus
  • Hepatitis A
  • Norovirus

How Effective Is RO Against Bacteria?

removes over 99

Once you properly install your reverse osmosis system, it should be capable of removing 99.9% of organic contaminants.

Organic material in water comprises pathogenic microbes like bacteria, viruses, fungi, and protozoa.

Keep in mind that membrane fouling and degradation are some of the aspects that might lower the effectiveness of RO against bacteria.

It’s a good practice to replace RO membranes every 2-3 years. A faulty membrane is unlikely to operate effectively.

Other Contaminants RO Systems Remove

RO systems are also effective in removing solid particles and chemical contaminants from water. The efficacy against individual pollutants varies, but the average is between 95-99% of total dissolved solids (TDS).

RO membranes can filter out ions and metals like Aluminum, Barium, Iron, Chromium, Chloride, Arsenic, Fluoride, Manganese, Copper, Magnesium, Calcium, Lead, Cadmium, Zinc, Magnesium, Mercury, Nitrate, Sulfate, Sodium, Potassium, Silver, Radium, Chromium.

The membranes can also remove organic compounds like benzene, toluene, trihalomethanes, carbon tetrachloride, trichloroethylene, and dichlorobenzene.

Pesticides like Pentachlorophenol, Endrin, Trichlorobenzene, Lindane, and Atrazine can also not penetrate these membranes.

What Is Not Removed?

Reverse osmosis isn’t a fool-proof water treatment solution—there are minute contaminants that bypass filter membranes.

Dissolved Gases

Most dissolved gases are small enough to pass through RO membranes. Hydrogen sulfide is one of the gases that easily escapes through reverse osmosis systems. It’s responsible for the rotten egg odor present in water. But, hydrogen sulfide isn’t a health risk in small concentrations.


RO membranes might not effectively remove chlorine. Chlorine and chlorides are detrimental to these membranes. Chlorine quickly oxidizes polyamide membranes making them lose their ability to reject salts.

Modern reverse osmosis systems come with Granular Activated Carbon (pre-filter) to capture chlorine. These filter traps absorb chlorine and other impurities like fluorides before they reach the semi-permeable membrane.

Disadvantages of Reverse Osmosis Systems

Despite being effective against bacteria and other contaminants, RO has some limitations.

Removes Useful Minerals


Water contains both healthy and harmful minerals.

Although humans get most of their minerals from food, they can find extra minerals in drinking water. RO removes all minerals from water, including healthy ones.

If you use reverse osmosis for purification, you won’t get helpful minerals like zinc, potassium, phosphorus, calcium, magnesium, fluoride, copper, and sodium.

Purified water also lacks natural taste. You’ll notice water from your RO system tastes “flat.”

Wastewater Generation


Reverse osmosis isn’t an overly efficient water treatment method. For every 4 gallons of wastewater, the system generates a gallon of pure water (4:1).

The wastewater contains contaminants, including bacteria that are unable to flow through the membrane.

Stagnant wastewater is the perfect breeding ground for microorganisms. The buildup of microbes and impurities will eventually damage the membrane.

Wastewater carrying these contaminants needs to be flushed away from the system.

Some systems re-circulate wastewater back into the system as feedwater. However, this process overworks filters and might hasten their degradation.

Choosing an RO System

Before buying and installing an RO system, there are vital things you should consider.

Type of RO System

ro systems

Reverse osmosis systems come in many forms. The final choice often narrows down to either under sink RO units or whole house RO units.

Under-sink tap water units are common in many homes since they purify water at the point of use. These units only serve a limited section of your home. They come in handy when disinfecting water from municipal sources.

If you intend to purify all the water in your home, install the point of entry system. This system is necessary if you depend on groundwater or live close to compromised water sources.

If you rent or move a lot, the portable countertop RO unit is what you need. These units receive water from the faucet then disinfect the water. Some come with tanks where you can fill contaminated water and later purify.

Efficiency Ratio

The efficiency of traditional reverse osmosis systems isn’t great. Manufacturers are turning to innovation to increase the efficiencies of these systems. You will find units with 3:1, 2:1, and 1:1 efficiency ratios.

These units don’t eliminate wastewater production. Instead, they increase the amount of pure water they generate. They cost more compared to standard units.

Water Quality

Water quality is perhaps the most important thing you should consider before purchasing an RO unit.

If you receive water pre-treated with chlorine, you should opt for a system with carbon filters to absorb chlorine. As earlier mentioned, chlorine damages reverse osmosis membranes.

Highly contaminated water is likely to overwork your RO system. This will, in turn, lower the lifespans of the filters, membranes, and the whole system. Consider installing some pre-treatment system before allowing overly contaminated water into a reverse osmosis unit.


RO units are ideal for filtering out suspended particles in water. Yet, they do not kill microorganisms. There is always the risk of bacteria buildup in your plumbing system and the unit’s membrane. Your whole water filtration system could turn out to be an incubator for bacteria biofilm.

To avoid such problems, consider getting a UV water purifier. UV light will kill any bacteria the reverse osmosis system filters out.

How Do You Maintain a Reverse Osmosis System?


RO units require regular maintenance to function seamlessly. You need to inspect each component frequently and replace worn-out parts. A well-maintained unit can serve up to 15 years.

Each unit often comes with a manual that indicates how to remove and replace parts. It’s advisable to read your manual before embarking on any maintenance project.

Overall, reverse osmosis systems require servicing at least once a year.

Ensure you wear sanitary gloves before undertaking this process.

Start by shutting off the water supply. Afterward, drain, clean, and sanitize the water tanks. You’ll then need to depressurize the system before dismounting filters and membranes from their housing. Inspect whether the mentioned components are in top shape.

The three filters present in most RO units include:

  • Sediment filter (pre-filter)
  • Carbon filter (pre-filter)
  • Polishing filter (post-filter)

Sediment Filters

Always replace your sediment filters annually. This period might be shorter depending on the amount of silt, sediment, and dirt the filter handles. Failure to change this filter will allow dirt to clog your system’s membrane.

Carbon Filter

As a water treatment specialist educating individuals, I recommend replacing carbon filters every 6-12 months. These filters come as carbon block filters or Activated Granular Carbon (AGC). Their longevity depends on water quality, usage, carbon quality, and humidity.


The semi-permeable membrane is responsible for filtering out most contaminants, including bacteria. Although it can last up to five years, it’s advisable to replace this membrane every 1-2 years. A compromised membrane is likely to leak contaminants such as bacteria to your drinking water.

The best indicator your system requires a membrane change is when the water output levels drop below 80%.

Polishing Post-Filter

AGC mainly acts as a polish post-filter. It removes any taste and odor present in the water. A polishing filter can last up to 2 years, but to be safe, replace it every 12 months.

Photo of author
David Trinh
David is an expert in all things plumbing, heating, cooling, and water treatment. He got his start in the plumbing business working on fixing all types of home improvement issues including water leaks, broken toilets, appliance installation, and more. Over time, he learned a ton about installing and choosing the correct water treatment products for homeowners.

Learn More About The Water Tech Editorial Team