What is Reverse Osmosis and How Does it Work?

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By Eric Phillips


If you’re interested in finding the best possible way to filter your water at home, you’ve probably come across references to reverse osmosis or RO filters. These are seen by many as the creme de la creme of the water filter world, and with good reasons!

Reverse osmosis systems are some of the most efficient at-home water filters on the market. They get rid of absolutely everything in your water that isn’t water – that’s all chemicals, contaminants, sediment, VOCS. When I say everything I mean everything! 

In fact, some people would argue that RO filters are a little too efficient – they even strip your water of potentially healthy minerals. 

Before you make a judgement on these fantastically hard working filters yourself, let’s have a look at how they work, what they’re regularly used for, and how a reverse osmosis filter system might be able to enhance your at-home H2O experience.

What is Reverse Osmosis? Let's Start With The Process

Well, that’s fun to say, “The reverse osmosis process”! 

But water filtration is a serious matter. Especially if we’re talking something as potentially complex as reverse osmosis. 

Most RO filter systems will do more to filter your water than just employ reverse osmosis, but the technique itself, as explained in plans for a potentially life saving portable filtration unit, works by forcing water through a semipermeable membrane using hydrostatic pressure.

What is a semipermeable membrane, I hear you ask!

Well, according to a report from the Negev Academic College of Engineering, a semipermeable membrane is a membrane so fine that water molecules can get through it while almost nothing else can. In reverse osmosis systems, membranes are usually made of specially devised polymers. 

Once the water has been forced through the special membrane it is almost completely pure. This is what will eventually come out of your faucet if you have a reverse osmosis system installed under your sink or plumbed in where the mains supply enters your home, though most RO systems include one more step – they add the minerals lost through RO back into your water so that you don’t end up short on things like calcium and magnesium! 

Is It Like the Osmosis You Learned About in Biology Class?

Yes and no. As the name suggests, reverse osmosis is almost the opposite of osmosis.

In osmosis, two solutions either side of a membrane are compelled by osmotic pressure to equalise. 

So, if you have water in a vessel divided by a membrane (like gore-tex, for example) and pour a bunch of sugar into one half so it’s a water and sugar solution, water will begin to move through the membrane into the sugar-solution attempting to equalize the sugar to water ratio. The same thing would happen if other substances were dissolved in the water, too. 

That osmotic pressure happens naturally, and reverse osmosis filter systems have to counteract it by applying pressure of their own. This is why RO systems usually include a pump of some kind. 

What is Reverse Osmosis Used for? 

Worldwide, one of the most common (and innovative) uses for reverse osmosis is desalination. As noted in the Economist, there is plenty of water on Earth but a shocking 97% of it is seawater too salty for human consumption. Reverse osmosis is a relatively efficient and cheap way to turn seawater into drinking water, and according to an article from Fairleigh Dickinson University there are more RO desalination plants currently active than plants that use any other method to remove the salt from water. 

Reverse osmosis can also be used to treat water that is being reused, and groundwater that would otherwise not be suitable for human consumption. It has been suggested as a solution to current or imminent water shortages in Spain, Iran, Egypt and many other parts of the world, with studies published by universities in each of those countries outlining potential uses of RO nationally. (1, 2, 3)

Finally, reverse osmosis is also used for at-home water filtration systems. I will outline more about that below, but because of the amount of pressure needed for reverse osmosis you won’t find RO pitchers or bottles – they tend to be an undersink or whole house options.

One Stage vs Two Stage Reverse Osmosis Systems

Okay, you’re going to assume you know what this means, but I think I’ll surprise you with the actual explanation. You’re probably thinking of single or double pass systems, which I cover below.

A single stage reverse osmosis system is just your standard system. Water is forced through a membrane under pressure. The membrane only lets water molecules pass through, leaving wastewater (concentrate water) to be drained, and potable water (permeate water) to be sent to a tank and eventually a faucet.

In a double stage reverse osmosis system, on the other hand, begins with two standard RO systems. The potable water from these is sent straight to the tank, while the wastewater is sent to a third RO membrane. This membrane works in the same way as any other RO system, thus extracting more potable water from the wastewater of the first two membranes. Get it? Here’s a diagram to help:

In addition to this, some systems include ‘concentrate recycle’. This means some of the wastewater from the third RO system is sent back to meet the feed water, and cleaned by the first RO systems. Once again, this helps to reduce waste.

Single Pass RO systems vs Double Pass RO Systems

Okay, so this is where things double up in the way you might have imagined. Except that it’s usually a two stage system that is double passed. One two stage system feeds into a second, with all of the waste water from the final RO membrane sent back to join the feed water for the first membrane. Again, it might hurt your head a little bit to think about it… it looks like this:

Problems With Reverse Osmosis

There are two major problems with reverse osmosis. One applicable to the process on all scales (in fact, more so on a large scale) and the other specifically an issue when you’re using a home RO filter system.

Stripping Minerals

Firstly, reverse osmosis strips everything from the water it treats. That means it takes the good as well as the bad. According to a report from the Czech Institute of Public health, made public by the WHO, there are three reasons that the ultra-purified water produced by reverse osmosis might cause problems when introduced to the wider water supply or into your home water supply:

  • Demineralized water erodes pipes, storage tanks and all internal workings of a water system. It also leaches metals from pipes, which means if your pipes (or on a larger scale, a municipality’s pipes) contain any heavy metals they will make their way into the water supply.
  • Totally purified water doesn’t taste very good.
  • Minerals in naturally occurring water and tap water are good for us. Removing all the minerals from water means removing a major source of calcium, magnesium and more from our diets.

This issue is dealt with in at-home RO systems by the addition of a remineralization process after reverse osmosis has occurred. This involves small amounts of calcium and magnesium are added back into your water before it hits your pipes or comes out of your faucet. 

The Wastewater Problem

The second issue that hounds reverse osmosis is that it produces a whole bunch of wastewater. According to The Economist, this wastewater is returned to the ocean from large seawater desalination plants, and appears to return to normal levels of salinity around 1600 feet from its entrypoint into the sea. However, there haven’t been many long term studies on this wastewater, which is also high in contaminants like man made chemicals and heavy metals, and what it might do to the ocean environment.

At home all this wastewater is an issue because it has to be sluiced and flushed, but you’re still paying for it. That means higher water bills and also a bigger environmental impact – the wastewater from your RO system is part of your personal water footprint!

Not much can be done about this issue at the moment, but you should keep an eye on the water to waste ratio of the reverse osmosis system you purchase for your home. They generally range between 1:1 and 1:3.

How to Reduce Reverse Osmosis Wastewater

Obviously, wastewater is a big issue if you’re remotely environmentally conscious or worried about your bills. And who isn’t at least one of those two? No one I know, anyway! 

If you want the amazing filtration power of a reverse osmosis system, but you’re worried about wastewater, then you have a few options. None of them completely solve the problem, but they do make a difference. 

  • Add a permeate pump to your system. By making your RO system more efficient and faster, a permeate pump reduces wastewater by 70-80%. If you’re buying an RO system, make sure it either has a permeate pump (which is unusual) or will work with an additional pump.
  • When buying a new RO system, look for one with an automatic shut off valve. An ASO valve will automatically (as the name suggests!) shut off flow once the tank is full, saving waste water. 
  • Use the wastewater! Obviously, you don’t want to drink it as it has a very high level of total dissolved solids, potentially including heavy metals if they are present in your supply. But you can use it for gardening or similar if you need water for your plants or lawn.

Reverse Osmosis Filters For The Home

Whether you have an under-sink or a whole home reverse osmosis water filtration system, your at home RO filter almost certainly works something like this:


In most water filters, prefiltration consists of a simple sediment filter designed to capture larger particles like dust, rust and sand. In a reverse osmosis system, though, the pre-filtration stage might be equal to another system’s entire filtration process! Prefilters in an RO system usually consist of sediment and active carbon cartridges, and might even include a KDF media filter too.

Reverse Osmosis Filter

This is where the magic happens! If you consider water being forced through a super fine semipermeable membrane at pressure, leaving behind everything that was dissolved in it, to be magic. Personally, I think it’s more… well… science.

Remineralization and Alkalisation

Once it has been incredibly well filtered by the pre-filter system and the RO membrane, your water is a bit too pure. At this stage, most at-home RO systems will add some minerals, like calcium, magnesium and sometimes sodium back into your water. Some systems also raise the PH of your water slightly too, so that it’s easier on both your plumbing and you!

Tank Storage

Most reverse osmosis systems include a storage tank, where filtered water is kept until needed. These tend to have an antibacterial and antimicrobial coating, and occasionally they’re fitted with an additional UV filter to kill biological contaminants. 

Reverse Osmosis Upkeep

A reverse osmosis system shouldn’t require much upkeep apart from regularly changing pre and post filter cartridges, and, less often, changing the RO membrane. 

If your RO system doesn’t have a prefilter, you should install one. Without them, an RO system can become fouled (dirt, dust and rust or bacteria building up), scaled (calcium carbonate building up), or damaged by the chlorine in water. 

There are several ways to pretreat water for your reverse osmosis system if this feature isn’t inbuilt. You could use an ion exchange water softener, or add more basic filters like an active carbon filter, a fine sediment filter, or a microfiltration system, to your water supply before your RO filter. 

Final Thoughts…

Because of its efficiency, reverse osmosis filter systems are a very tempting prospect for anyone who wants constant access to perfectly pure water. But you do have to weigh up the pros and cons – RO systems are often quite expensive, and upkeep can be costly too with so many filter and post-filter cartridges to replace regularly. Add to that the potential environmental issues with all that waste water and many people choose a traditional high efficiency filter system over a reverse osmosis one.

That being said, no other at-home filter systems (in fact, no other industrial filter systems either) filter water as completely as an RO system. If you are dealing with very contaminated water – particularly water that is unsafe to drink straight from the municipal system, or water from a well, then I would advise looking into RO systems.

Beyond that, reverse osmosis looks like it is going to play an increasingly large role in the worldwide supply of water as increasing pressure is put on existing water systems and many water tables begin to drop. For that reason, a working knowledge of RO systems is definitely a good thing to have in your back pocket!

Eric Phillips

Meet Eric, the Water Treatment Specialist and founder of Dripfina, where he shares his wealth of expertise. With notable features in Realtor, ApartmentTherapy, FamilyHandyMan, and more, Eric is a renowned expert in water treatment industry. Join Eric on Dripfina and benefit from #AskDripfina community to make informed decisions for clean, refreshing water.

1 thought on “What is Reverse Osmosis and How Does it Work?”

  1. Again, chemicals and sediment will be targeted in this filtration stage. Reverse osmosis removes dissolved minerals and salts such as calcium, magnesium and potassium. Reintroducing these minerals can improve the alkalinity and taste of water, and make it less susceptible to future contamination.


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