What Does Reverse Osmosis Not Remove from Water?

What Does Reverse Osmosis Not Remove from Water?

Reverse Osmosis systems are popular across the globe for filtering out most contaminants from your water source and delivering safe-to-drink water to your tap. However, if you are wondering whether there are some contaminants that an RO system cannot remove – you are not alone. Let’s explore the answer to this question in this article.

RO systems cannot remove pesticides, herbicides, dissolved gasses such as hydrogen sulfide, fungicides, and certain Volatile Organic Compounds (VOCs) from water. Regular RO systems can also not remove higher concentrations of chlorine molecules from water.

You may have recently installed an RO system or are planning to do so. If this is the case, then wondering whether it can eliminate all kinds of contaminants is a good thought. RO systems are highly effective against most contaminants, but some can still slip through their membranes. We’re going to discuss these contaminants in detail in this article.

Read my comprehensive article about reverse osmosis.

4 Contaminants That an RO System Cannot Remove

RO systems utilize semipermeable membranes with tiny pores to filter most contaminants, like lead, iron, sulfur, chlorine, etc., from water sources. However, there are some contaminants whose molecules are smaller than that of water and can slip through the membrane.

It is also important to remember that not all water treatment options can remove contaminants completely. Let’s look at the contaminants that can pass through an RO system:

Dissolved gasses

Gases such as Carbon Dioxide and Hydrogen Sulfide have smaller molecules than water molecules. These gases are not removed effectively from RO systems and can still be present in the water you drink.

Hydrogen Sulfide, in particular, has a rotten egg smell and can impact the taste of your water if not removed effectively.

Using an oxidizing filter, carbon filter, and proper chlorination of water in addition to the RO system can remove or reduce the quantities of Hydrogen Sulfide gas in your water.

Volatile Organic Compounds

Volatile Organic Compounds are commonly found in the atmosphere. They have extremely low boiling points and can dissolve in water after converting into a gaseous state.

VOCs have a carbon foundation, so the most effective way to remove them is through carbon filters that can easily attract and absorb these substances.

Agricultural treatment chemicals

Agricultural Chemical products such as pesticides, insecticides, herbicides, and fungicides keep crops safe from diseases. Unfortunately, these chemicals can seep into ground and surface water and dissolve in drinking water sources.

These chemicals are hazardous to human health and can cause acute symptoms such as dizziness, diarrhea, and headaches to chronic illnesses such as cancer.

Regular RO systems cannot completely remove these products, and some of them can still become a part of the water you drink. However, using an active carbon filter with your RO system can effectively remove these contaminants.

Read my article Does Reverse Osmosis Remove Glyphosate from Drinking Water?

Resistant microorganisms

RO systems can easily remove bacteria that are larger in size than water molecules. However, there are certain resistant strains of microorganisms like bacteria and viruses that are smaller than water molecules that can enter your drinking water. Pairing an RO system with a UV filter can ensure all kinds of these microorganisms are filtered out.

Below is a table that summarizes the contaminants a regular RO system can and cannot remove:


Contaminant Removed by RO System
Sulfate Yes
Sodium Yes
Calcium Yes
Potassium Yes
Nitrate Yes
Iron Yes
Zinc Yes
Mercury Yes
Selenium Yes
Phosphate Yes
Lead Yes
Arsenic Yes
Magnesium Yes
Nickel Yes
Fluoride Yes
Manganese Yes
Cadmium Yes
Barium Yes
Cyanide Yes
Chloride Yes
Agricultural Treatment Products No
Resistant Bacteria Strain No
Volatile Organic Compounds No
Dissolved Gasses No


Limitations of an RO System

RO systems are promoted as a universal solution to all your water purification problems. However, just like any water purification system, you should be aware that it has its limitations. If you are considering buying one, you should have a complete picture of all the contaminants an RO system can and cannot remove before making that purchase:

1.   Removes all minerals from water

Water has certain trace minerals such as calcium, sodium, magnesium, and potassium that are important for blood circulation. RO systems are designed to remove minerals when water passes through the semipermeable membrane.

These healthy minerals are also removed from the water, and you are left with drinking water that is neither harmful nor very beneficial to your health in terms of mineral intake.

Read my article about minerals that are removed by reverse osmosis.

2.   Wastes a lot of water

As a rule of thumb, three liters of water are wasted to get one liter of purified water. This is a considerable loss, and an expensive one at that!

Read my article about sustainability of reverse osmosis.

3.   Too many options can be overwhelming

All RO systems are not that similar and don’t remove the same contaminants either. For example, VOCs are only removed by RO systems with an active carbon filter, and others remove bacteria with a UV filter. The homeowner must do extensive research to see the kind of filtration offered by an RO system before making a purchase decision.

4.   Dependency for maintenance and installation

RO systems are sensitive and require expert assistance when relocating or servicing them. You may find yourself depending on a service technician as well as an expert to help you with any changes to your RO system, including cleaning out the system.

5.   RO filtration process is slow

It takes time to filter water through the RO, and if you need a lot of water at the same time (and you are also emptying the holding tank to fulfill that need), you will need to wait a while before the tank fills up again.

Frequently Asked Questions

What are the side effects of regularly drinking RO water?

Drinking RO water can lead to low bone mineral density, weakness, muscular cramps and headaches. RO water is void of all the essential minerals usually present in water which are beneficial for the human body.

Read my article about whether reverse osmosis water is good for you.

Can you drink RO wastewater?

RO wastewater has a very high concentration of contaminants and should not be used for drinking purposes as it can make you very ill.

Is it better to drink RO water or tap water?

Tap water that comes directly from the source often has contaminants that are harmful to your health. RO removes the majority of these contaminants and ensures you have clean and safe drinking water at your disposal.

Final Thoughts

RO systems are effective in removing contaminants. However, they need help from other filtration systems like carbon and UV filters to remove all impurities from water. RO filters work best with different water treatment options to produce water that is truly free from all contaminants.

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.

Recent Posts