Ion Exchange vs Reverse Osmosis – Similarities and Differences

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Ion exchange and reverse osmosis are two approaches to water treatment that are both very common in household environments.

Both are designed to address different issues, and while reverse osmosis is considered to be one of the most thorough methods for water filtration in general, ion exchange does have its advantages which are important to consider as well.

So, ion exchange versus reverse osmosis – let’s discuss how they work, the similarities, and the differences.

Key Takeaways

  • Reverse osmosis uses a fine semipermeable membrane to physically remove water impurities.
  • Ion exchange is based on ionically-charged resin beds. Cationic resins bind positively-charged ions present in water, anionic resins bind negatively-charged ions.
  • Both methods are commonly used in home environments – cation exchange for water softening and reverse osmosis for drinking water filtration.
  • Other differences between reverse osmosis and ion exchange is that only RO can be used for removing microorganisms and sediments from water. However, RO produces significantly more wastewater.

Comparison: Ion Exchange vs Reverse Osmosis

Reverse osmosis works by forcing water through a membrane lined with very fine pores. The pores are so small that pretty much only water molecules can effectively pass through, and everything else gets left behind – which means all contaminants including ions.

Ion exchange filters, on the other hand, work by percolating water through a tank filled with small resin beads. The existing ions on these beads force an exchange with the ions in the water being filtered, swapping them out for alternatives.

One prominent example of ion exchange is traditional water softening. Here, hard water minerals, mostly calcium and magnesium, are removed in a cation exchange process. The hardness minerals stick to the resin beads, while sodium or potassium ions go into solution.

Aside from softening, ion exchange can be used for removing water contamination. There are two main types of ion exchange methods in this regard: cationic and anionic. Using anionic resin allows you to focus on contaminants like chlorides, nitrates, and fluoride, while with a cation ion exchange system, you can filter out chromium and iron among other stuff.

The two processes, reverse osmosis and ion exchange, are quite different in their core operation, and so they are commonly used in combination for maximum effectiveness. It’s not rare to see household water filtration setups running water through a reverse osmosis stage as well as an ion exchange system.

Under Sink Reverse Osmosis System

What Is Reverse Osmosis? How Does It Work?

Reverse osmosis is a mechanical process that relies on a thin, semipermeable membrane lined with very fine pores. This filtration method relies on high pressure, which is necessary to push water up against the membrane with a lot of force. As a result, water molecules pass through the membrane’s pores, while all contaminants get ejected with the wastewater stream.

This is a highly effective approach to purifying water, and it’s used both in domestic as well as commercial environments. It does have some minor downsides however, mainly the fact that it tends to waste a lot of water.

What Is Ion Exchange? How Does It Work?

Ion exchange filters can be split into two general categories – ones that utilize anionic resins, and ones that use cationic resins – but the underlying idea is essentially the same: Water passes through filtration media, typically based on resin. The important point is that strongly binding ions can force an exchange with weakly bound ones as water passes through the resin beads, taking undesirable materials out of the water and replacing them with better alternatives.

The main difference between anionic and cationic resin systems is the range of contaminants they remove. With an anionic resin, you can remove contaminants that include:

  • Arsenic
  • Cyanide
  • Chlorides
  • Nitrates
  • Fluoride
  • Sulfates

It’s important to note that the type of resin used in an anionic ion exchange system can further affect the type of purification that’s accomplished. With strong-base anions, you can remove various kinds of minerals from water and increase its acidity. On the other hand, with weak base anions, you can do the opposite – reduce the acidity of your water.

Cation ion exchange systems are more about softening water, as they primarily deal with the following:

  • Calcium
  • Chromium
  • Iron
  • Magnesium
  • Manganese
  • Sodium

Similarities Between Reverse Osmosis and Ion Exchange

What are the similarities between reverse osmosis and ion exchange?

Both are very effective at dealing with their respective contaminants, but there are various differences between the two that are important to understand. You can use a reverse osmosis system in combination with an ion exchange water filter, as long as you know what types of contaminants they filter out and you’re familiar with the quality of your own water.

Removing Hard Water Minerals

Both approaches can be very effective at removing minerals that contribute to water hardness. In the case of cation ion exchange systems, that’s actually one of the main purposes of the setup. Ideally, reverse osmosis isn’t used for water softening because it will clog the RO membrane in no time.

limescale on heating element of washing machine

Suitable for Domestic Filtration

While both ion exchange and reverse osmosis systems are used industrially, they are also quite popular among domestic users. In fact, these two are among the leading water treatment methods on the consumer market, partly due to their thoroughness and in part due to the relatively low maintenance requirements.

pH Level Decrease

This depends on what type of ion exchange system you’re using, but if you’re looking to decrease the pH level of your drinking water, both systems are viable options. Just make sure to run some tests first to find out what the current state of your water supply is – otherwise you run the risk of lowering the pH level too much, especially when using both systems in combination.

Differences Between Reverse Osmosis and Ion Exchange

There are also various differences between reverse osmosis and ion exchange:


One of the main downsides to reverse osmosis is that it produces a lot of wastewater – the ratio can sometimes be 4 gallons of wastewater for each purified gallon. And while ion exchange filters also suffer from this problem to some extent – think regeneration – they are much more efficient and waste significantly less water in the process.

Bacteria Removal

Reverse osmosis is great at removing many kinds of bacteria. While it’s not 100% thorough in this regard, most of the shortcomings of the process can be compensated for by using a UV filter as an optional post-filtration step.

Ion exchange systems, on the other hand, do nothing at all against bacteria. If you have biological contaminants in your water, you’re going to need to invest in specific filters that can target them.


Ion exchange filtration also isn’t very good at removing sediments, while reverse osmosis does a great job in this regard.

If your water is heavily contaminated with suspended solids, however, you’re going to want to invest in sediment pre-filtration. Otherwise, your RO membrane will clog immediately.

Filtration Media

Reverse osmosis uses a thin, semipermeable membrane to accomplish its goal. On the other hand, ion exchange filters rely on resin made from specific types of tiny beads with suitable ion properties.

Physical vs Physio-Chemical Method

Reverse osmosis is a completely physical mode of filtration. It doesn’t rely on any chemicals to accomplish its task. Comparatively, there is some chemistry involved in ion exchange. While it’s not technically a 100% chemical process, it’s also not entirely physical, unlike reverse osmosis.

If you have any questions about the difference between reverse osmosis and ion exchange please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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