What Does Reverse Osmosis Remove?

Written by: James Smith

Updated: April 10, 2023

Are you thinking of investing in a reverse osmosis water filtration system? Before you do, consider what they can and can’t remove, as outlined in this guide.

man placing RO system under sink

You have probably already heard of, and may even have used, a reverse osmosis water filtration system. Aside from realizing that it involves the reduction and removal of contaminants present in water through pressurized filtration, you may not be aware of much beyond that. In this guide, we want to give you a detailed overview of reverse osmosis filtration.

As well as briefly discussing what reverse osmosis is and how it works, we will also look closely at the contaminants it can remove from your drinking water.

As there is no filtration system that will completely clean water of all contaminants, reverse osmosis does have its limits. With that in mind, we will also discuss the contaminants that can’t be removed using this system.

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What is Reverse Osmosis?

As touched upon above, reverse osmosis filtration systems involve the process of removing or reducing the volume of contaminants present in water. It does this by forcing water to move in the opposite direction to its natural flow. Although it may feel like a relatively new technology to some, it’s been around for over 45 years.

The concept was first used as a method of removing salt from seawater. It began being used commercial once it was realized it could remove other contaminants too and not just salt. And as far back as the 70s, these kinds of systems were installed in people’s homes to help clean and sanitize the water consumed and used for various applications.

ro water filtration device

How Does it Work?

Using a semi-permeable filter membrane, reverse osmosis filtration systems allow water to flow through them, while many contaminants are prevented from going any further. The water molecules are pushed through the filter’s membrane by way of reverse osmosis, which separates the water from the contaminants.

Now that you understand better how the process of reverse osmosis water filtration works, it is worth discussing what it can and what it can’t do. As it is by no means a perfect filtration system, there are various contaminants it can remove with relative ease, but others that it can’t. In which case, you’d need to utilize another form of filtration.

Ro system

What Does Reverse Osmosis Remove?

Let’s start with the positives first. Some of the most common contaminants that reverse osmosis systems can remove include:

  • Protozoa, like Giardia and Cryptosporidium
  • Bacteria, such as E-coli, Shigella, Salmonella, and Campylobacter to name a few
  • Viruses, like Rotavirus, Norovirus and Hepatitis B

Chemical contaminants including:

  • Lead
  • Chromium
  • Copper
  • Chloride
  • Sodium

Although it may not be as effective, it could still remove the following:

  • Phosphorous
  • Nitrate
  • Potassium
  • Magnesium
  • Calcium
  • Sulfate
  • Radium
  • Fluoride
  • Arsenic

reverse osmosis tank

What is Not Removed by Reverse Osmosis?

Now for the downsides. There are always exceptions with water filtration systems, and reverse osmosis systems are no different. Therefore, some of the contaminants you cannot remove using reverse osmosis includes:

Chlorine – Okay we mentioned that it can remove some chlorine. That, however, does leave some chlorine still being allowed to be passed through to your water. It’s important to note this is usually a sign that the membranes are either damaged or just old. However, the percentage of chlorine that gets passed, even in situations like this, is too small to be considered dangerous.

Carbon-based filtration is the most effective method to absorb (and adsorb) chlorine from water. It just so happens that home reverse osmosis water filtration systems will always have a carbon stage incorporated for this reason.

Viruses & Bacteria – Reverse osmosis removes these two types of contaminants to a high degree but even just a small amount passing through can pose problems to health. Where with contaminants like chlorine, a fractional amount doesn’t pose much of a health risk, with bacteria and viruses, this just isn’t true.

Thankfully, there’s a solution to this problem too and it’s incorporated into some reverse osmosis system packages. The answer is ultraviolet light filtration. This is the most effective method of filtration against harmful bacteria and viruses and works as a disinfectant treatment, much like how water chlorination was designed. The big difference is that there are no harmful by-products added to the water like with chlorination.

Some reverse osmosis set-ups come equipped with UV lights built-in. This is an example of one.

The truth is RO filtration is very effective. In fact, it’s probably the most effective form of water filtration available in the home. When combined with other filtration methods like carbon media and UV lights you have a pretty formidable weapon against water contamination.


Should You Choose a Reverse Osmosis Water Filtration System?

As with any major purchase, this will really depend on various factors such as your own preference, your needs, the application it will be put to and the quality of the water source it will be filtering.

If you are wondering if a reverse osmosis system is ideal based on its abilities, then we would highly recommend it. It can be something of a hassle because you need to invest in additional filters for things that reverse osmosis doesn’t stop. As you can see, though, there are not many of those, making it the ideal choice to have cleaner water in your home or office.

Just remember to get your water tested before making a decision on a water filter. It’s essential that you know which contaminants need to be targetted before thinking about things like installation.

Also, look out for certifications to prove a water filter’s effectiveness in removing certain contaminants. Find certified models here.

Systems are available for the whole home or just to be fitted under the kitchen sink. You can check out our guide to installation here.

We’d be happy to answer any questions that you might have. Just leave us a comment down below and we’ll get back to you promptly!

About the Author James Smith

James is the chief water geek here at His mission is to empower the consumer and allow people to take control of their health. His passion for water health is contagious, hopefully unlike your tap water!

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  1. I am using a well for water, unfortunately if has a moderate level of sodium. Which unit should I be looking at for a whole house usage?

  2. my towns water supply company is overpriced and they are ripping me off and i cant afford it. i have a creek in my backyard, what do i need to filter it for safe drinking and home use? and will it be a cheap to do in the long run?

    1. I wouldn’t recommend taking water from the creek even if it gets filtered. The water would be vulnerable to whatever gets dumped upstream. Things like fertilizer run-off from a field or even dead animals could be dangerous. If you really need to go this route then the first thing you would want to do is to get a thorough lab analysis of the water. This would tell you the quality of the water and what contaminants might be present (at the time of testing). You can find certified labs here: Only with this information would you be able to think about types of water filter, installation, price etc.

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