What Does Reverse Osmosis Not Remove From Water

What Does Reverse Osmosis Not Remove From Water?

Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove toxins, bacteria, ions, chemicals, and larger particles like sediment from drinking water. The end result of the process is supposed to be pure water because smaller particles like hydrogen and oxygen (H20) pass through.

Despite that claim, many people wonder what does reverse osmosis not remove from water?

While RO is a very efficient process for removing common chemical contaminants, some bacteria, viruses, and pharmaceuticals still pass through the filters into the water. In combination with other stages, the percentage of contaminants that pass the filtration is so small that they could barely cause any harm.

Fortunately, the ultimate protection comes with other filtering stages and methods that can produce cleaner water. I summarize the research and clarify what remains in water after RO.

The RO Membrane Explained

Reverse osmosis can remove many types of chemical compounds as well as biological species (like bacteria) through the use of a selective membrane with pores of 0.001 micrometers in size.

The membrane works to filter water by blocking large molecules/ions and allowing smaller components like water (0.00028 micrometers) to pass through freely.

The “osmosis” part of the RO process occurs when water migrates between the weaker of two salt solutions of different concentrations via the semi-permeable membrane. The “reverse” part means that pressure is applied to reverse the natural flow of the water.

This results in the contaminants ending up on one side of the semi-permeable membrane. At the same time, the pure water is on the other side.

Standard RO systems are effective at removing most common contaminants like metal ions, aqueous salts, sodium, chloride, copper, chromium, lead, arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorous.

Despite that, many people wonder about other contaminants like viruses, bacteria, and pharmaceuticals. Does their molecular structure allow them to pass through the filters? The following section will answer that question.

What Contaminants Can Pass RO?

RO-processed water is considered safe to drink, however some systems do not remove impurities because many contaminants are smaller than the filter in molecular size. Without adequate additional processing, these contaminants may be allowed to pass through. They include:


Chlorine is used to sterilize drinking water and to disinfect swimming pools. Unfortunately, it’s also dangerous. In its gas form, it is a respiratory irritant that can lead to breathing difficulties. As a liquid, it has been shown to negatively affect intestinal bacteria that cause gas, bloating, and digestive difficulties.

While RO removes a lot of chlorine, some inevitably pass through, most particularly if the system is old and not adequately maintained.

To combat this issue, system maintenance is necessary, along with the use of a pre-filtration carbon-based system.


Pesticides are some of the most dangerous chemicals known to humans, and should be removed from the water whenever possible.

Since they are smaller than water in terms of molecular size, they can pass freely through the pores of the RO filters.

Fortunately, many RO systems have a carbon-based filtration stage (or equivalent) that is effective in removing pesticides. This ensures that the water is much safer to drink than standard tap water.


Herbicides are chemicals that destroy weeds and are commonly used in agriculture to make the process of picking and sorting the plants faster and more efficient. Unfortunately, they are smaller in molecular size (like pesticides) and dangerous to humans.

UV light pre-treatment (prior to RO), along with carbon filtration is an effective way to remove these chemicals from the water.

Can Reverse Osmosis Remove Bacteria From Water?

All water contains some microbes and other organic elements that cannot be detected through our senses. Not all of them cause illness, and many live naturally in our bodies. If you consider that humans once drank from rivers, then it is easy to see that we are adapted to consume many natural organic microbes.

The ones to be concerned about include E. coli, Giardia, and Cryptosporidium. When people vacation in foreign countries, they often encounter these new bacteria in the food and water that can cause gastrointestinal problems and flu-like symptoms.

Here is a quick summary of what can be found in our water, and how to deal with it:


Humans are full of bacteria. Some live happily in our bodies and help us digest food, while others are parasitic and cause infections. They range in size from about 1 to 10 microns in length and from 0.2 to 1 micron in width, which means many get eliminated in the RO process.

Smaller bacteria can easily be dealt with using ultraviolet (UV) light treatment that exposes water to light at a specific wavelength that kills bacteria, viruses, fungi, protozoans, and cysts that may be present in the water.

The effectiveness of UV treatment is dependent on the level of water contamination, strength, and intensity of the light, and time the water has been exposed.


Viruses are microscopic parasites that can only live inside a host body. At a size about 100 times smaller than bacteria (0.004 to 0.1 microns), they cause diseases in humans by replicating in the cells.

This is why fevers kick in when we are sick, as the high temperatures destroy the viruses as part of the healing and strengthening process.

While viruses are found in water, UV, RO & chlorination kills them off, rendering the water safe to drink.


Some protozoans and bacteria produce special sacs similar to cocoons that they create before reproducing or resting. While some people are concerned about their presence in the water, they shouldn’t be. At 2 to 50 microns in size, they are easily filtered out using RO, and can even be removed using standard fine filtration.

Ultraviolet (UV) Light Treatment

The UV disinfection method, which does not involve chemicals, has long been popular for commercial use and is becoming more common in homes.

UV systems expose water to light at just the right wavelength for killing microbes. It’s a way to kill bacteria, viruses, fungi, protozoans, and cysts that may be present in the water.

Does Reverse Osmosis Remove Pharmaceuticals?


An Associated Press investigation reported findings by scientists of dozens of pharmaceuticals in tap water, ranging from tranquilizers to painkillers to antibiotics.

This has sparked huge concerns among people about the possibility of being “medicated” beyond their control.

Concerning pharmaceutical removal using RO, they respond much like pesticides and herbicides and are mostly removed. In combination with ozonation and activated carbon filters, the removal rate is even higher.

Ozonation is considered to be an effective way to remove pharmaceuticals without the use of harsh chemicals like chlorine.

The process involves the use of electrolytic and chemical reactions to create energy referred to as ozone generators. An ozone concentration is then created using clean, dry air, and then water is passed through a tube.

At the same time, the ozone gas is pulled through the water using a vacuum. This process treats the water, pulling out many contaminants.

At-home ozonation systems are available and are a complement to the RO process.

Final Words

RO is a very efficient process to remove common chemical contaminants and most minerals. Despite its effectiveness, some bacteria, viruses, and pharmaceuticals can also pass through.

Anyone wanting extra protection from contaminants can use UV filters and ozonation. In combination, this trio can produce super-clean water that is ultra-safe to drink.

Many people have tried this protocol with great results, so now I turn it over to you:

  • What is your experience with bacteria or viruses in the water?
  • Have you tried ozonated water?
  • What carbon filters do you use?
  • How do you find the taste? Is it “flat” as some people have described?

I love hearing your input, so leave a comment below!

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