Reverse Osmosis vs Desalination – Differences and Similarities
Written by: Gene Fitzgerald // Last Updated: Aug 25, 2023
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Reverse osmosis and desalination are two methods of water treatment.
Reverse osmosis is found in both commercial and home environments. Desalination is more of an industrial process.
Let’s take a look at what the two are about, their similarities, and their differences.
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Key Takeaways
- Reverse osmosis is one way to desalinate water.
- Other desalination methods include forward osmosis, distillation, microbial desalination, electrodialysis, and freeze-thaw.
Reverse Osmosis vs Desalination – Differences and Similarities
First of all, what are the differences and similarities between reverse osmosis and desalination?
Well, reverse osmosis is one method of water desalination (there are several others as well). So both reverse osmosis and desalination can be used to remove salts from water. Other forms of desalination include forward osmosis, distillation, microbial desalination, electrodialysis, and freeze-thaw (more on these later).
What Is Reverse Osmosis and How Does It Work?
The main purpose of reverse osmosis is thorough water purification. This is accomplished by forcing water against a thin, semipermeable membrane at high pressure. Water molecules pass through the tiny pores of the membrane, while pretty much all impurities get left behind.
The purification thoroughness of reverse osmosis is almost unmatched – and this makes it one of the most popular methods for water filtration, both on the consumer market as well as in commercial environments.
The only major downside to reverse osmosis is that it wastes a lot of water in the process, but this can be mitigated in various ways. For example, wastewater can be reused for different purposes.
What Is (Seawater) Desalination and How Does It Work?
Desalination refers to removing salt from water – typically sea salt, as this is part of the process of converting seawater into drinkable water.
Water desalination can be accomplished with reverse osmosis, and in fact, this is one of the most commonly used approaches in industrial desalination plants around the world. Other than that, most of the alternative approaches to desalination rely on evaporation or thermal treatment of the water.
Distillation is perhaps the simplest example. Water is simply allowed to evaporate, and the vapors are condensed back into liquid form. Since salt doesn’t evaporate (most contaminants don’t, in fact), it gets left behind in the original container. This idea can be expanded to involve multiple stages, or to rely on artificially introduced heat to accelerate the process. More advanced forms of desalination might involve compressing the vapors or even using specific microbes with electrochemical properties suitable for desalination.
Reverse Osmosis Desalination
Reverse osmosis, as described above, is very effective at removing salt from water. Since salt crystals are too large to pass through the pores of a reverse osmosis membrane, they get left behind and sent away with the reject water stream. As an added benefit, various other contaminants are removed as well.
Forward Osmosis
Forward osmosis, like reverse osmosis, relies on a semipermeable membrane with fine pores to accomplish its effect. However, the process is used in reverse here. Instead of forcing water against the membrane at high pressure, the other end of the membrane is filled with a heavily concentrated solution. The important point is to ensure that the concentration on that side is significantly higher than that on the feed side. This creates a water flow which separates the feed water from its contaminants, including salt.
Distillation
Distillation is one of the oldest techniques for purifying water, and it’s still very relevant today. The process is simple – water is allowed (or forced) to evaporate, leading it to a separate container where it condenses back into liquid. All contaminants – including salt – get left behind in the original container.
Microbial Desalination
Microbial desalination relies on the specific electrochemical properties of certain microbes. Through careful design of the system, water can be purified by having bacteria act as cathodes and anodes, not too different from what you’d find in a regular ion exchange water filter.
Electrodialysis
Electrodialysis uses pairs of membranes which charge passing through them. By forcing salt to bounce between alternating membranes, large amounts of it become progressively trapped.
Freeze–Thaw
A very slow method which relies on natural freeze/thaw cycles, freeze-thaw desalination involves spraying salt water into a container and letting it freeze, then drawing desalinated water once the ice starts to melt. The process can be improved by utilizing vacuum, although that also drives up the cost significantly.
Advantages, Disadvantages, and Effectiveness of Reverse Osmosis Desalination
Pro: Very Effective Desalination
Reverse osmosis is very thorough when it comes to removing impurities from water – and that includes salt. It’s one of the best water purification methods on the market, and this also makes it very effective as a desalination tool. If performance is one of your key concerns, you should definitely consider reverse osmosis.
Pro: Gets Rid of Contaminants
Reverse osmosis doesn’t just remove salt, it takes away all the contaminants normally present in water. This can include a wide range of elements, some of which can be quite harmful in larger concentrations. With reverse osmosis, you can handle the complete purification of your home’s water in one centralized place.
Pro: Low Maintenance Requirements
Once you’ve installed the membrane and everything is set up, you don’t have to put a lot of effort into maintaining your reverse osmosis system in the long run. You just have to remember to swap out the filter elements on a regular basis.
Con: Wastes Water
The main disadvantage of reverse osmosis is that it wastes water – and quite a lot of it. Be prepared to lose 3-4 gallons of water for each gallon you’ve desalinated. On the bright side, there are options to improve the efficiency of a reverse osmosis system in this regard, including by reusing wastewater for other purposes.
Con: Slow
There is no way around it: Reverse osmosis is a slow process when used at home. That’s because you can only increase feed water pressure to a certain level. Slow filtration speed is also why RO systems often come with a storage tank that will be filled for later use.
Further Reading:
The Pros and Cons of Other Desalination Methods
Pro: Easy to Deploy
Many alternative desalination methods are quite easy to deploy and get them running, especially when it comes to something like distillation. True, some approaches do require a bit more work, especially more mechanically/chemically advanced ones, but there are also many simple solutions available.
Pro: Low Maintenance Requirements for Some Approaches
Some approaches to water desalination also require less maintenance than reverse osmosis – but once again, that’s not universal. In some cases, the opposite is true. But when you take something like distillation, for example, there isn’t much maintenance needed in the long run to keep the system running optimally.
Con: Difficult to Scale in Some Cases
Many of the alternative approaches to desalination are difficult to scale up. Distillation is especially notorious in this regard. It works well on a small scale, but it’s very challenging to turn it into an actual production chain.
Con: May Require Additional Filtration
Unlike reverse osmosis, other approaches to desalination might not remove additional contaminants. Distillation is one of the exceptions here, but in general, you should keep in mind that you are likely going to need additional filters if you want to thoroughly purify the water you’re dealing with.
If you have any questions about desalination vs reverse osmosis please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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