[Infographic] How Does Reverse Osmosis Work?

Interested in buying a reverse osmosis (RO) filter, or simply curious to learn how reverse osmosis works? I’ve covered everything you need to know in this handy guide.

What is Reverse Osmosis?

Reverse osmosis is the most effective form of water purification, and removes more than 99.9% of total dissolved solids from unfiltered water. If you’re looking for the cleanest, purest water, you’ll get it with reverse osmosis.

Most reverse osmosis filters are installed at your property’s main waterline, almost as soon as it enters your home, or near your kitchen sink. Your water supply will feed into the reverse osmosis system, where it is filtered before being sent back into your waterline. Many RO filters also come with their own faucet to ensure no nasties are added to your water after purification.

reverse osmosis process

How Does a Reverse Osmosis System Work?

A reverse osmosis (RO) system typically consists of a series of filters and a semi-permeable membrane, which has small pores made from a thin film composite that trap dissolved solids. When you turn on your tap, water passes through the filters before being forced at a high pressure against the semi-permeable membrane at a constant rate. Water molecules are small enough to pass through the pores, so only clean water makes it out of the other side.

The impurities left in the RO chamber, known collectively as the RO concentrate, are sent straight down the drain, leaving only clean, pure drinking water. As I mentioned, all of this happens at a constant rate – water is forced against the membrane while emptying the concentrate out of the drain. This means that there is some water loss associated with the reverse osmosis process.

The best RO filtering systems feed water through a number of varying filters and membranes for optimal results. Because not all contaminants are the same size, this helps to remove as many different types of impurities through a combination of filtration steps.

You may also find that a reverse osmosis water filtering system features a remineralization filter as its final filtration stage. That’s because reverse osmosis is so effective that it doesn’t only remove the harmful impurities from water – it also removes healthy minerals, like magnesium and calcium. A remineralization filtering cartridge can re-introduce these minerals into water after filtration, giving it a more pleasant alkaline taste and boosting its health properties.

What Are the Stages of an RO System?

There are four common stages of all reverse osmosis systems: a sediment pre-filter, an activated carbon filter, a reverse osmosis membrane, and a post or polishing filter.

1. Sediment Pre-filter

Though we can’t usually see them ourselves, water contains a number of large sediment particles such as dust, rust, salt or sand. These particles may enter a stream or reservoir naturally, and may not be fully filtered during municipal treatment. You’ll likely find that your water contains a particularly high sediment content if you source from a private well.

A reverse osmosis system will first feed water through a sediment filter to remove these large particles, which could damage the later-stage filters or quickly clog them up, resulting in the need for more frequent cartridge changes. Most sediment filters are 5 microns in size, meaning they can remove impurities as small as 5 microns. The aim of a sediment pre-filter is to target these larger particles, and having relatively large pores works well in this case.

2. Activated Carbon Filter

Because sediment pre-filters allow smaller impurities to pass through with water, the next-stage filtering cartridge in a reverse osmosis system needs to have smaller pores. Activated carbon filters, which are almost always a feature of a reverse osmosis system, have a pore size as small as 0.5 microns (one-tenth of the size of a sediment pre-filter).

This type of filtering cartridge uses a process known as adsorption to trap particles in its pores. An RO system will feed water through a carbon filter, and contaminants will stick to its surface and are unable to pass through. While carbon filters are best if you’re looking to remove chlorine and chloramines – thereby improving water’s taste – they also remove heavy metals.

3. Reverse Osmosis Membrane

After passing through two filtration stages already, you’d assume that water was, for the most part, now clean enough to taste the difference. This is certainly true, and the majority of filters stop there. But there are many contaminants that can’t be removed by a typical sediment or carbon filter – and that’s where the reverse osmosis membrane comes in.

There are hundreds of trace contaminants in both city and well water sources, and reverse osmosis membranes can remove near enough all of them. These contaminants include fluoride, arsenic, radium, calcium, sulfate, VOCs, magnesium, pharmaceuticals, nitrate, and potassium. The semi-permeable membrane’s pores are so tiny that only water molecules can pass through.

4. Post or Polishing Filter

The final stage of a reverse osmosis system is the post-filter, otherwise referred to as a polishing filter. Just in case any small contaminants had managed to pass through the reverse osmosis membrane, the post-filter should get rid of them.

If you have a reverse osmosis system with a water storage tank, the post-filter will be placed after this tank, and will feed water through before it reaches your tap, removing any bacteria that water may have accumulated while sitting in the tank before drinking. This filter is usually a form of carbon, such as coconut carbon.

how does reverse osmosis work

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What Are The Components Of A Reverse Osmosis System?


A reverse osmosis system couldn’t operate without its filters. While the RO membrane does a thorough job of removing contaminants, if there were no filtration stages before water reached the RO membrane, it would contain a much broader range of contaminants that would damage the membrane and wear it out at a faster rate.

RO membrane

The RO membrane gives reverse osmosis systems their name, so this is another important component that the unit couldn’t operate without.

Filter housing

To protect the filters and RO membrane from the world outside, filter housing is used to encase the entire system. Most filters can be quickly clicked or twisted into place in this housing.

Bypass valve

The majority of RO systems come with an included bypass valve, which is positioned at your waterline before the reverse osmosis water filter to feed water into, or away from, the system. You may not want to use your reverse osmosis system all the time, and having a bypass valve means you can divert water away when you’re performing system maintenance, such as changing the filters.

Backflow prevention valve

Water is sent through a reverse osmosis water filter at a high pressure, and if backflow occurred, it could damage the bypass valve. The backflow prevention valve lets out backwashed water to prevent damage.


You can’t install a reverse osmosis system without a drain line. When water is sent through the reverse osmosis membrane, contaminants begin to accumulate inside the RO chamber. That’s why water needs to be able to leave via a drain line to remove these contaminants during filtration.

Flow Restrictor

At a reverse osmosis system’s drain line is a flow restrictor, which prevents excess water from being wasted due to poor flow. The flow restrictor usually comes with a pressure pump that keeps water pumping through the system at the desired pressure. This prevents too much water from being in the RO chamber, thereby preventing how much water is wasted.

Water storage tank

Purified reverse osmosis water is usually sent into a holding tank, which keeps it pressurized until you turn on your faucet. When a faucet is opened, pressure forces water straight to the faucet, so there’s no waiting around to fill your glass.


Finally, it’s common for reverse osmosis filtration systems to come with an included faucet. It may be optional whether you choose to use this faucet or not, but the obvious bonus is that you can be certain it won’t leach heavy metals into your water thanks to its lead-free design.

RO system diagram

What Affects the Performance of an RO System?

Water Pressure

Water pressure might just be the most important factor of all when it comes to the operation of an RO system. If your water pressure is too low (below 40 PSI), it won’t pass through your RO system at a fast enough rate, which means it’ll be inefficient and waste a lot of water.

60 PSI is optimal for a reverse osmosis filtration system – you can use a pressure gauge to measure your water pressure if you’re unsure. If your water pressure is lower than this, just make sure you buy an RO system that comes with an included pressure pump, or buy a pressure pump separately, which will keep your water flowing through the system at a speedy rate.

TDS type & Quantity

All water sources are different, and there are multiple factors that affect the TDS level of your water – your region, whether you use city or well water, and so on. TDS, or total dissolved solids, is a measure of how much organic and inorganic matter your water contains. Common total dissolved solids are chloride, potassium, sulfate, magnesium, and calcium.

You can measure your water’s TDS using a TDS meter. The best TDS meters have a range of 0 to 5,000 PPM (parts per million). Drinking water usually has a TDS of between 200 and 600 PPM, and a reverse osmosis system can usually reduce PPM to about one-tenth of its original PPM (so drinking water with a TDS rating of 300 PPM would be reduced to around 30 PPM).

A TDS reader can tell you your TDS level, but it can’t tell you exactly the types of total dissolved solids you have in your water. The best way to learn this is to check out your local water quality report (if you use city water) or send off your water to a lab for thorough testing. You could also buy a test kit online, which can indicate whether your water contains contaminants such as copper, iron, chlorine, hardness minerals, sulfite and fluoride.

You may wish to use an RO system regardless of your TDS level or quantity, but it’s helpful to know which issues you need to tackle before making a purchase, so you can be 100% sure that your preferred system can handle it.

TDS in water

Temperature of Water

If your water temperatures drop in the winter and rise in the summer, they may be responsible for the changing efficiency of your RO system. With a higher water temperature, water will be thinner (a less concentrated solution), speeding up its flow rate through the semi-permeable membrane. Equally, with a lower water temperature, it’ll be thicker (a highly concentrated solution), which will slow down the rate at which water flows through the semi-permeable membrane. Even when pressure is applied, this may affect the performance of a system.

It’s not possible to do anything about your incoming water temperature – and by the way, it’s not recommended to use water from your heater for the sake of efficiency, as it’s too hot for an RO system to handle.

But if you do notice that your system appears to be less efficient in winter months, and nothing appears to have caused this, note that your water flow is most likely a result of your water temperature – so hold off before you assume your semi-permeable membrane has worn out and buy a replacement. When your water becomes less concentrated again, it’ll probably improve the efficiency of filtration once more.

System & Filter Quality

The quality of your RO system overall will affect its performance. A high-quality system with strong, durable components is more likely to not only remove a greater level of total dissolved solids from water, but also do it at a faster rate and waste less water.

Price and quality are usually linked, so think twice before you consider investing in a deal that seems too good to be true, as it probably is. Check out customer reviews if you want to see whether a manufacturer’s big claims live up to reality when customers use the product in day-to-day life.

How Much Water Does Reverse Osmosis Waste?

Traditionally, an RO system would have a 1:4 purified water to wastewater ratio, meaning that for every 1 gallon of filtered water produced, 4 gallons is wasted. This seems like quite a lot, but remember that this water is only going to be wasted when the machine is in action – so, for the ten or so seconds you spend filling up a pitcher or glass.

Most RO water systems are now much more efficient, and have a 1:3 or a 1:2 pure water to wastewater ratio. Some even have a 1:1 ratio, meaning that for every gallon of fresh water for drinking that’s produced, only one gallon is wasted, which is about as good as it’ll get. Reverse osmosis couldn’t work without some water waste, unfortunately, but some RO systems will cycle water around the system multiple times before disposing of it, which is more efficient.

What Does Reverse Osmosis Remove?

Thanks to the effective semipermeable membrane, reverse osmosis works to remove nearly 100% of total dissolved solids from drinking water. These contaminants include:

  • Protozoa (e.g Cryptosporidium, Giardia)
  • Bacteria (e.g Salmonella, Campylobacter, E. coli, Shigella)
  • Viruses (e.g Hepatitis A, Enteric, Norovirus, Rotavirus)
  • Chemicals and metals (e.g sodium, copper, chloride, chromium, arsenic, lead, fluoride, magnesium, potassium, sulfate, nitrate, radium, calcium, phosphorous)
  • Pharmaceuticals (e.g bisphenol-A, caffeine, antibiotics, opiates)

It’s easier to talk about what reverse osmosis doesn’t remove: some pesticides, volatile organic chemicals (VOCs) and solvents.

How to Add Minerals to Reverse Osmosis Water?

As I mentioned earlier in this guide, the reverse osmosis semipermeable membrane doesn’t just remove the bad stuff from water – it also removes healthy minerals like calcium and magnesium. Luckily, reintroducing these minerals into water, improving water quality overall, is usually straightforward.

Remineralizing Filters

If you buy an RO system that comes with a remineralization water filter already included, there’s no work needed on your part. You’ll just need to change the filtering cartridge as advised in the user manual (usually once every 6 to 12 months). These filters add a specific quantity of minerals to drinking water before it leaves your tap. Alternatively you can use a specific alkaline water filter to accomplish this.

Trace Mineral Drops

Trace mineral drops are the cheaper and more convenient option if you don’t have an RO system that comes with a remineralizing filter. These drops work very similarly to remineralization filters, improving tap water quality by introducing a host of healthy minerals. The only difference is that you have to add them to your water yourself once you’ve filled your glass.

Benefits of Reverse Osmosis Systems

Improved water taste

Water reverse osmosis systems can greatly improve the taste of tap water by removing the contaminants that are to blame for poor taste (namely chlorine, lead and sulfur). You won’t need to buy bottled water when you use an RO system – which brings me to my next point.

Long-term penny saving

While reverse osmosis water systems are expensive to buy upfront, they can help you to save money in the long run. Instead of having to buy bottled water for its improved taste and mineral content, you can get the same benefits from RO remineralized water per day. If you haven’t worked out how much you spend on bottled water every year, do it now. It’s easy to see how much money you can save simply by switching from bottled water to reverse osmosis drinking water from your own tap.

Healthier, cleaner tap water

The drinking water that comes out of our faucets contains a range of unhealthy contaminants, including chlorine, bacteria and viruses. When you purify your drinking water using RO systems, you have the peace of mind that it’s much healthier and cleaner for you and your family.

More effective than other water systems

RO systems provide a water filtration process that’s one of the best in the industry. If you’re looking for bottled water quality, you might not achieve it from a single water filter. But with water reverse osmosis, which removes a high concentration of contaminants, you can achieve the cleanest, purest drinking water possible per day.

Offer immediate access to water

A huge benefit of most RO systems is that they have a water storage tank, which means that can provide you with instant access to clean drinking water whenever you need it. Some water systems, such as pitcher filters, require that you wait up to 15 minutes for access to one batch of drinking water, but reverse osmosis is immediate.

Are All Reverse Osmosis Filtration Systems the Same?

The short answer is no. There are so many factors that can make a water reverse osmosis system unique from its competitors: quality of RO membranes, materials used in filters, wastewater ratio, storage tank capacity, and additional handy accessories, such as a pressure pump, to name a few.

For that reason, it’s important to do two things before looking at reverse osmosis water systems to buy. First, make a list of the features you’re looking for in a filtration system for your home. For instance, you may prioritize systems that are more efficient, or you may even prefer a tankless system that takes up less space.

Second, read reviews – both customer reviews and in-depth impartial reviews online. It’s important that you’re familiar with all the features of an RO water treatment system before you make a purchase. That way, you won’t be surprised – either positively or negatively – when you receive your RO filtration system in the post.

How Long Do Reverse Osmosis Systems Last?

A reverse osmosis water treatment system itself can last for an impressively long time; virtually forever if you keep up with maintenance and replace worn-out parts.

RO membranes typically last for 2 years, though some RO membranes can even last for 4 years or more nowadays. The sediment filter, carbon filter and post lasts for around 1 year, though this depends on your water source – a higher concentration of contaminants will clog membranes at a faster rate.

Frequently Asked Questions

Where do I buy replacement RO membranes?

You can usually buy them directly from the manufacturer’s website or Amazon shop – you can be assured that these membranes reverse osmosis will fit your specific RO water system, as they’ll most likely be the same membrane product as the one you currently use.

Can reverse osmosis be used to remove water hardness?

The membrane in an RO water system can effectively remove hardness ions as drinking water flows through it, but it can’t be considered a water softener. While a water softener is designed to specifically treat hard water issues, a high concentration of water hardness will most likely deteriorate the filtration capabilities of the RO membrane at a faster rate. In fact, if you have particularly hard water, many manufacturers recommend installing a water softener before your reverse osmosis system, which will protect every stage of the filtration process and ensure each filtering cartridge and membrane has the longest lifespan.

Is reverse osmosis the best solution for removing chlorine?

The reverse osmosis process is certainly an effective solution when it comes to chlorine, but this level of filtration will do much more than removing chlorine and one or two other common contaminants. Consider your needs carefully before making a purchase, as it may be that, if you’re only interested in chlorine removal, you can opt for a more affordable option that removes chlorine with a more basic filtration process.

What happens to the RO concentrate?

After failing to pass through the RO membrane, this concentrate gets sent down your drain along with the wastewater from the rest of your home.