Reverse Osmosis vs Deionized Water – Differences and More
Written by: Alexandra Uta // Last Updated: Sep 15, 2023
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Some people get confused about the difference between deionization and reverse osmosis when it comes to water treatment.
While there are some similarities between the two processes in terms of what they accomplish, the underlying mechanics and the intended applications of the two are quite different.
Here’s what you should keep in mind on that topic.
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Key Takeaways
- Water deionization is based on ion exchange resins which work similar to magnets that bind salts, minerals, dissolved metals, and other ions contained in water. As such, one type of water contamination not targeted by deionization are microorganisms – and there are several others.
- Reverse osmosis is much more broad. An RO membrane rejects almost all types of impurities (also but not exclusively based on size exclusion), achieving a higher level of water purity.
- Still, both processes have their right of existence and which to choose should mainly be based on your water quality and what you want to achieve.
- Other differences of RO vs deionization is that RO wastes water and is a much slower process.
Reverse Osmosis vs Deionized Water – Summary
Water deionization focuses specifically on removing ions from water. This mostly includes various minerals like calcium and magnesium but also heavy metals and salts. The process is highly effective at extracting those from water and replacing them with H+ and OH- ions, which eventually turn into pure water. It also works quite fast and is a good way to always have purified water available on demand.
Reverse osmosis is much more thorough. It can effectively remove almost all kinds of water contaminants not just ions, with some very minor exceptions like dissolved gases. And even then, those can be dealt with through the use of specialized pre and post-filters that come with most home RO systems. The final result is water with almost no contaminants/impurities left – as close to perfect purity as you can get. This makes reverse osmosis an attractive choice for people looking for extensive water purification, and it’s commonly used in places with heavily contaminated water supplies.
| Reverse Osmosis | Deionized Water | |
|---|---|---|
| Water Purity (Contaminant Removal) | Removes close to all contaminants, leaving only pure water | Doesn’t remove some contaminants (especially biological ones) |
| Wastewater | Wastes a lot of water | No water is wasted |
| Health Benefits | Pure water – no special health benefits other than lack of contaminants compared to regular water | Better than regular water due to reduced contaminants, but still not as good as RO water |
| Cost | Varies, both expensive and inexpensive models are available | Similar to reverse osmosis |
| Maintenance | Requires membrane and pre-/post-filter replacements | Ion exchange resins must be regenerated or replaced |
How Does Reverse Osmosis Water Purification Work?
Reverse osmosis is one of the most popular methods for purifying water, both on an industrial scale as well as in domestic setups. It’s a mechanically simple process, but at the same time it manages to achieve a very high level of purification, placing it far above most other water treatment approaches.
It all comes down to a special membrane lined with very tiny pores. The system forces water against this membrane at high pressure, and due to the size of the pores, only water molecules are able to pass through. All contaminants that were originally present in the water stream get sent away.
Pros and Cons
Pro: Never Worry About Fresh Water Again
With a reverse osmosis system at home, you’ll never have to worry about having access to fresh drinking water again. You will always have it on demand, and you’ll know that it’s as pure as it gets. You can even take this one step further with the help of a whole house reverse osmosis system, and switch to using purified water for everything in your household – washing your clothes and dishes, showering, etc.
Pro: Removes All Contaminants
It doesn’t really get any better than reverse osmosis if you’re looking for something as thorough and effective as possible. This is one of the very few processes that can remove practically all contaminants from your water supply. If you’re dealing with heavy contamination, this is one of the best options available for remedying the situation.
Pro: Stop Buying Bottled Water
You can also stop buying so much bottled water when you switch to reverse osmosis water. You’ll always have fresh water at home, making those annoying trips to the store unnecessary. As an added bonus, you will be able to stop contributing to global pollution so much.
Pro: Doesn’t Use Chemicals
No chemicals are involved in the reverse osmosis process. Some other methods for purifying water use chemicals to achieve their effect, and this tends to make some people wary. You can put your mind at ease in this regard when you’re using reverse osmosis.
Pro: Doesn’t Require Power
You don’t need to supply a reverse osmosis system with power, unless you need to use pressure pumps – but even that’s rare. By default, reverse osmosis runs without any external power, which is one of its most attractive features for some users.
Con: Wastes Some Water
Due to the way reverse osmosis works, some water gets wasted in the process. This is unavoidable, although it can also be remedied to some extent if you plan around it. Still, given the average price of water across the country, the final impact on your bill will be negligible even if you don’t take any special measures.
Con: Doesn’t Work Immediately
You need to wait some time for reverse osmosis to work its magic, and this can be annoying when you need to quench your thirst with a glass of water right now. Thankfully, most household reverse osmosis systems come with an integrated storage tank by default, which allows you to negate this downside.
Con: Healthy Minerals Get Removed Too
Reverse osmosis might be a little too thorough in some regards – particularly when it comes to the various minerals that are naturally present in water. All of those get stripped away in the process – so no magnesium, calcium, potassium, or any other minerals in your water. If you’re concerned about your nutritional balance, we recommend looking into options for reintroducing minerals to your diet, such as mineral drops or a remineralization filter.
Is Reverse Osmosis Water Deionized?
Technically, yes, reverse osmosis water is deionized – because all ions get removed during the purification process. The two terms are not interchangeable though. Deionized water will still contain various dissolved solids that were present in the input water, unlike reverse osmosis water. Some people categorize processed water in different grades, and in this context deionized water is type 2, whereas reverse osmosis water is type 3.
How Does Water Deionization Work?
Water deionization is a very different process compared to reverse osmosis, even though there is some overlap in the final outcome of both processes. During deionization, water is passed through a special ion-exchange resin, or multiple. This can either be a cation resin (containing positive ions) or an anion resin (containing negative ions). Most systems use a combination of both.
In any case, as water passes through the resin, positively charged ions in it are exchanged for H+ ions, while negatively charged ones are replaced with OH- ions. The original ions that get removed from the water include things like calcium, magnesium, iron, sodium, arsenic, chromium 6, and more. Meanwhile, the H+ and OH- ions eventually combine to form H2O, or pure water.
Pros and Cons
Pro: High Level of Purification
Water deionization may not be as effective as reverse osmosis when it comes to purifying water, but it still gets the job done to a good extent. Many contaminants do get effectively removed, and you can deal with the rest by using specialized pre and post-filters. For example, you can use a UV light filter to eliminate biological contaminants.
Pro: Fast Processing
Water deionization works relatively fast. While it’s still not perfect in this regard, it’s noticeably faster than reverse osmosis, making it a better option for getting a glass of water in a pinch.
Pro: Very Low Conductivity
This might not matter much to the average user, but deionized water is commonly chosen for its low conductivity in various industrial processes. What many people don’t realize is that water is not conductive by itself – it’s the various impurities present in it that give it its conductivity.
Con: Unusual Taste
Since deionization removes some contaminants but not all, it can lead to a strange taste in your drinking water. Whether that’s a positive or negative is up to you to decide – some people claim that they actually prefer the taste of deionized water compared to unfiltered water, or even compared to reverse osmosis water. Reverse osmosis water has no taste because it’s almost completely pure – but some people actually dislike that as they claim it tastes “flat”.
Con: Can Leach Materials from Storage Containers and Pipes
You should be careful with how you’re storing deionized water, especially over longer periods of time, because it’s prone to leaching materials from its storage containers and even the pipes it flows through. Normally this shouldn’t be a problem if you have a point-of-use system and avoid storing your filtered water for too long, but it’s something worth keeping in mind.
By the way, this is also a con of RO water.
What’s the Difference Between Reverse Osmosis and Deionized Water?
With all that said, what are the exact differences between reverse osmosis water and deionized water? It mostly comes down to the way the processes work, what contaminants are removed, and the intended use of both processes.
Treatment Process
Reverse osmosis and deionization work very differently. Reverse osmosis relies on pushing water through a thin, semipermeable membrane at a high level of pressure, while deionization uses a special ion-exchange resin to exchange specific ions with H+ and OH- ones.
Contaminant Removal
Deionization specifically focuses on ions present in the water, and it doesn’t remove anything else. This means that various categories of contaminants will still be present in the filtered water. Most notably, this includes biological contaminants like viruses and bacteria. Reverse osmosis, on the other hand, is very thorough and leaves almost nothing in the water other than pure H2O molecules.
Maintenance
The main area of maintenance for a deionizer is the resin. It needs to be occasionally regenerated – typically using hydrochloric acid and sodium hydroxide for the cation resin and anion resin respectively. Another option is resin replacement. That’s because the ions in the resin get depleted over time and need to be replenished.
With a reverse osmosis system, your attention should fall mainly on the membrane. It needs to be replaced on a regular basis – how often exactly depends on your usage habits and water contamination levels, but you should generally expect to do this once every 1-5 years.
In both cases, there will likely be additional pre and post-filters involved that will have to be maintained and replaced as well. The replacement frequency for those varies quite a lot across the board, but is usually between 6 and 24 months.
Cost
Deionizers and reverse osmosis systems both fall into a similar price range, although RO can seem like a more expensive option due to the sheer number of options available on the market and the differences in their individual prices. Generally, reverse osmosis is more flexible and customizable, which also allows you to more easily adapt it to your budget.
Applications
Deionized water has various applications. It’s frequently used in medical environments, for example, due to its high level of sterility. It’s also often found in various industrial machines and systems, where its low mineral content makes it valuable due to the lack of residue. You can also find deionized water used in many places where the conductivity of water is important.
Reverse osmosis water, on the other hand, is used for a wider range of applications. It’s the standard choice in environments suffering from heavy pollution where water must be treated thoroughly, and it’s also frequently used in industrial settings. If you’re concerned with contamination in your drinking water, reverse osmosis will likely be one of your top choices. Some people also prefer to use reverse osmosis water for cleaning, especially for washing windows, as its low TDS (Total Dissolved Solids) levels make it even better than soapy water in this regard.
Reverse Osmosis vs Deionized Water – Which Is Better for Your Use Case?
So, reverse osmosis vs water deionization – as you can see, which one is better for you is entirely subjective. If you’re only concerned with removing ions/minerals from your water and don’t care about other contaminants, then you will probably want to look into a deionizer.
If, on the other hand, you want to make sure that your water is as pure as possible, then reverse osmosis is the answer. You might also find yourself involved with various industrial and other processes that call for either of the two types.
If you have any questions about the difference between reverse osmosis and deionized water please don’t hesitate to leave a comment below!
About the Author Alexandra Uta
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