What is reverse osmosis and how does it work?

Reverse osmosis technology uses a semi-permeable membrane to remove up to 99% of contaminants found in water. The membrane acts as a barrier and only pure water molecules can pass through, leaving contaminants behind.

Reverse osmosis is a tried and tested way to ensure you have access to clean and pure drinking water.

While reverse osmosis systems can be a powerful and beneficial water purification method to remove impurities, contaminants and potentially harmful bacteria it’s only suitable for very specific requirements such as:

• Managing health conditions where medical advice is to remove as much from the water as possible.
• Tropical fish tanks.
• Where there is no other way of accessing potable water such as private water supplies.
• Reducing elevated sodium levels in softened water.

This article explores what reverse osmosis is, how it works and why reverse osmosis may or may not be the right choice for your circumstances.

So, what is reverse osmosis and how does reverse osmosis work?

Understanding reverse osmosis: what it is and how it functions

At the heart of any reverse osmosis water filter is the semi-permeable membrane. (add image)

Before water reaches the all-important membrane, it flows through pre-filtration where spun polypropylene or pleated sediment cartridges capture larger particles that may be present in the water such as dirt and rust. This helps protect the membrane from clogging and damage and is a crucial step in any reverse osmosis system.

Water then passes through a 5 micron carbon filter cartridge which removes chlorine and other chemicals. Removing these further protects the reverse osmosis membrane. Chlorine can be highly damaging to the thin film composite (TFC) membranes commonly used in reverse osmosis systems. The taste and smell of the water is improved when carbon block filtration is used.  

And then it’s time for the semi-permeable reverse osmosis membrane to work it’s magic.

Water is pushed through the membrane, which acts as a barrier, allowing only water molecules to pass while rejecting contaminants. The membrane is designed to remove up to 99% of dissolved salts, heavy metals, and other impurities.

Water pressure plays a key role in the membrane, as it forces water molecules through the tiny pores of the membrane while leaving contaminants behind. The purified water is then stored in a holding tank, ready for use, while the rejected contaminants are flushed away as wastewater.

*TOP TIP* The general rule of thumb is that if your water pressure is below 3 bar, you’ll need a pumped reverse osmosis water filter. If your water pressure is 3 bar or above, you can choose an unpumped reverse osmosis system.

Finally, water passes through reverse osmosis post-filtration using an inline carbon filter. This is often described as a polishing stage to ensure that the water coming out of your tap is as crisp and as clean as possible.

Some reverse osmosis systems have a 6^th remineralisation stage which re-introduces important minerals back into the water such as calcium carbonate.

The key stages of the reverse osmosis filtration process

Reverse osmosis technology comprises of three main stages.

• Pre-filtration: the removal of particles, dirt, rust, sediment, chlorine and chemicals.
• Membrane: separates dissolved solids, salt and other impurities from water.
• Post-filtration: the polishing stage where the taste and odour of the water is further improved. Some reverse osmosis water filters also have a remineralisation cartridge which adds calcium carbonate back into the water.

 

It’s important to talk about the wastewater, highlighted on the above image as ‘impurities out to drain’. All the contaminants, which do not pass through the semi-permeable membrane go down the drain. Yes, you’re literally throwing water away! A reverse osmosis water filter wastes more water than the amount of clean water produced.

What impurities does reverse osmosis remove from water?

A reverse osmosis water filter is highly effective at removing many impurities that may be present in water such as those found in the table below.

 

Category	Examples of impurities removed by RO
Dissolved Solids (TDS)	Minerals: Calcium, Magnesium, Sodium, Potassium Salts: Chloride, Sulphates, Nitrates Organic matter: plant or animal matter
Heavy Metals	Lead, Arsenic, Copper, Manganese, Mercury, Cadmium, Chromium
Chlorine & Chloramines	Chlorine, Chloramines
Pesticides & Herbicides	Agricultural chemicals (weed killers, pest control agents)
Bacteria & Viruses	Bacteria: E. coli, Salmonella, Campylobacter, Shigella, Cholera, Legionella Viruses: Norovirus, Rotavirus, Hepatitis A, Adenovirus, Enteroviruses
Fluoride	Naturally occurring mineral or artificially added to water in areas of England that have a water fluoridation scheme.
Pharmaceutical residues	Hormones, Anti-biotics
Asbestos	Asbestos fibres from asbestos cement pipes, landfills containing asbestos waste, industrial waste, demolished buildings.
Forever Chemicals	Persistent “forever chemicals” used in manufacturing and consumer products.
Particulate Matter	Sediment, silt & other suspended solids
Radiological Elements	Radium, Uranium
Microplastics	Plastic particles from household and environmental sources.

 

 You can enjoy further reading below about some of the contaminants we’ve mentioned in the table above.

Water Fluoridation in the UK

How to remove fluoride in UK water

How to remove forever chemicals in UK tap water

How to filter microplastics from water

There is no 100% removal with any water filtration method. For example, reverse osmosis systems don’t remove the chemical element boron, so if boron is present in your water you may end up with an artificially increased boron level.

Benefits and drawbacks of reverse osmosis filtration

How does reverse osmosis work in terms of its benefits and drawbacks?

Because a reverse osmosis removes up to 99% of contaminants in water, let’s look at some of the benefits.

 

Pros	Cons
Removes up to 99% of contaminants.	Wastes more water than it produces. Not suitable for whole house.
Produces clean and safe water that tastes and smells good.	Removes healthy minerals resulting in bland tasting water.
Reduces the need for bottled water.	Higher upfront cost than other water filters.
Useful for certain health conditions and aquariums.	Needs regular cartridge/membrane changes.
Effective as a final purification stage after other filtration methods.	Takes up valuable under-sink space.

 

How reverse osmosis compares to other water filtration methods

Reverse osmosis is a bit of an ‘eliminate everything’ approach to water filtration and we can see why many people think it’s an effective way to filter tap water.

But if you’ve perused the drawbacks above, you’ll probably be considering if reverse osmosis is overkill and what other water filtration methods there are that you could consider.

It all boils down to what you want to change about your water. This is one of the first questions we ask people when they get in touch with us.

This table helps compare reverse osmosis to other water filtration methods.

 

Filtration Method	What it removes	Cost range	Maintenance	Best suited for
Reverse Osmosis (RO)	Dissolved salts, heavy metals, fluoride, PFAS, microplastics, pesticides, bacteria & viruses (with pre / post stage)	Upfront cost £300+   Replacement cartridges from £75 for a 5 stage system.	Cartridge change every 6-12 months.   Membrane change every 2-3 years.	Maximum purification, aquariums, medical/food uses.
Activated Carbon	Chlorine, Pesticides, herbicides, trace pharmaceuticals, PFAS, hormones, particulate lead, and more.	Basic system including tap from £100.   Replacement cartridges vary in price depending on system.	Cartridge changes every 6-12 months.	Suitable for multiple filtration problems.   Improves taste and smell of drinking water.
Ultraviolet (UV) Filtration	Kills bacteria, viruses, protozoa.	Systems from £400	UV lamp must be changed every 12 months.   Quartz sleeve change every 3 years (depending on condition)	Private water supplies (wells, boreholes, springs).
Ion Exchange (Water Softener)	Hard water minerals (calcium, magnesium), iron, manganese.	Systems from £700	Regular salt refills, resin top-ups.	Households in hard water areas wanting limescale reduction.   Private water supplies with high iron/manganese.
Ceramic Filters	Bacteria, cysts, moulds, sediment.	From £100.	Replace cartridges every 6-12 months.	Off-grid setups, emergency filtration.
Distillation	Heavy metals, salts, bacteria, parasites, viruses.	From £100 for countertop units.	Regular descaling.	Small scale laboratory, or emergency use.
Conditioners	Scale and chemical treatment.	Systems from £100.	Media cartridge renewals every 6-12 months.	Domestic water.
Surface / Depth	Particles, metals, sediment, colloids.	Systems from £80.	Cartridge renewals every 6-12 months.	All water applications.

 

There is no denying that reverse osmosis water has gone through a comprehensive purification process, but is it right for you? We’ll look at this in more detail later.

Practical uses of reverse osmosis in homes and businesses

Reverse osmosis systems can be installed in homes and across a wide range of industries and sectors in a commercial set up.

Reverse Osmosis – in homes?

A reverse osmosis system is normally installed under the kitchen sink as a point of use system. Other locations for a reverse osmosis water filter could be under a bathroom sink, to a refrigerator, in a garage, or basement.

Additionally, a reverse osmosis water filter is often installed on a fish tank as it helps control the balance of minerals present in the water. Some fish are very particular about the water they live in.

If you’re looking for a whole house water filter, then we wouldn’t normally recommend installing a reverse osmosis system as it wouldn’t provide a good enough flow rate. In this instance it would be better to treat your water using one of the alternative water filtration methods mentioned above and install a reverse osmosis water filter at point of use. Chatting to a water filtration specialist would ensure you get a system that’s fit for purpose.

Reverse Osmosis – in commercial settings?

Reverse osmosis water filtration is also used in commercial and industrial settings.

• Food and beverage – reverse osmosis systems help maintain consistent water quality. This ensures the quality and safety of products.
• Food processing – produce can be soaked and rinsed in reverse osmosis water to ensure it is free of contaminants.
• Medical & Pharmaceutical – purified water is used for sterilisation of equipment in hospitals and clinics, dialysis machines, and in scientific laboratories where testing equipment needs to be free on any impurities that may impact on results.
• Water treatment & purification – in large industrial settings such as power plants, metal finishing, and semi-conductor manufacturing; reverse osmosis is used to produce purified water for cooling and steam generation and reduce limescale build-up that will eventually corrode equipment. Reverse osmosis also helps recover and recycle water in water treatment plants.

Lifespan and replacement schedule for reverse osmosis filters

Water quality across the country varies, as does water usage and this will have an impact on the lifespan and replacement of reverse osmosis filter cartridges.

As a rule of thumb, the filters in a reverse osmosis water filter should be changed as follows:

• Pre-filtration cartridges: the spun polypropylene (melt blown) or pleated cartridge and the carbon block cartridge should be changed a minimum of every 12 months. These cartridges are responsible for the removal of particles, dirt, rust, sediment, chlorine and chemicals.
• Membrane: the semi-permeable membrane should be changed every 2-3 years. This cartridge separates dissolved solids, salt and other impurities from the water.
• Post-filtration: the inline carbon cartridge and remineralisation cartridge (if there is one) should be changed a minimum of every 12 months. These cartridges further improve the taste and odour of the water and if remineralising, calcium carbonate is added back into the water.

Replacement cartridges for reverse osmosis systems are normally sold in packs which means you get everything you need in one go.

Maintenance tips for efficient reverse osmosis performance

Here are our top two maintenance tips for your reverse osmosis system.

Tip One – Change the cartridges

It goes without saying but the first maintenance tip for ensuring your reverse osmosis system is running efficiently and effectively is to replace the cartridges when you need to.

If you don’t do this the performance of your system over time will diminish. For example, you may notice a drop in water flow, or a change in the taste and smell of you water. We encourage you to make a note in your diary or set a reminder on your phone, so you don’t forget. Our customers love that we send them a cartridge reminder email every six months.

Tip Two – Sanitise your system

You should also sanitise your system by cleaning out the filter housings every now and again to prevent mould and bacterial growth. We recommend you use something like Milton Sterilising Fluid to do this. If it’s good enough for babies bottles, it’s good enough for your reverse osmosis water filter.

This process involves disassembling the system, cleaning each component, and rinsing thoroughly before reassembly. Be sure to read the detailed instructions in the systems instruction manual!

Is reverse osmosis the right choice for your water needs?

There are several factors to consider when determining if purchasing a reverse osmosis water filter is going to be right for you. Or, whether another type of water filtration system is going to work just as well.

Reverse osmosis may be right for you if:

• You need to remove a broad range of contaminants in one go. For example, heavy metals, fluoride, PFAS, bacteria, microplastics.
• You have specific health requirements or a weakened immune system and need the purest drinking water possible.
• You keep tropical fish or have an aquarium that needs controlled water quality.
• You’re on a private water supply (well, spring, or borehole) where other filtration methods alone aren’t enough.
• You already use a water softener and want to reduce sodium levels in your drinking water.
• You want an alternative to buying bottled water for taste, safety, and environmental reasons.

Reverse osmosis may not be the best option if:

• You only want to improve taste and remove chlorine. A carbon filter would be sufficient for this.
• Water scarcity is a concern, as RO systems waste more water than they produce.
• You prefer mineral-rich, naturally balanced drinking water.
• You want a whole house solution. RO is usually recommended for point-of-use, not entire households.
• You’re looking for the cheapest or simplest water filtration option.

How Fountain Filters can help you with Reverse Osmosis systems

At Fountain Filters we are passionate about equipping you with useful information when it comes to water filtration methods.

We hope that this article has helped you understand reverse osmosis; what it is and how it works.

If you believe that installing a reverse osmosis system is going to be the best system for you, but you have questions; please chat your project through with us as we are expert problem solvers when it comes to water filtration.

Contacting us is easy, either by calling us on 01352 838 281 or sending us a message via our contact page. 

FAQs

What is the point of reverse osmosis water?

The point of reverse osmosis water is the ability to remove a high percentage of contaminants from drinking water.

Is it healthy to drink reverse osmosis water?

It’s safe to drink reverse osmosis water but as to it being healthy or natural; that’s open to debate. Removing good minerals such as calcium and magnesium may not make the water as healthy as unfiltered tap water. Reverse osmosis water is often not the only option when it comes to water filtration.

What is the disadvantage of reverse osmosis water?

Some disadvantages of reverse osmosis water include the bland taste of the water and that the process wastes more water than is produced. It’s important to learn about the reverse osmosis process before determining if installing a reverse osmosis system is going to be right for you.

Is reverse osmosis water the same as distilled water?

No, reverse osmosis water is not the same as distilled water. The two methods are entirely different. Both methods remove a high percentage of contaminants, but distillation involves boiling the water to create steam and then turning it back into liquid. Reverse osmosis does not.

Can I use reverse osmosis water for coffee/tea?

No, reverse osmosis water is not recommended for brewing coffee or tea. The lack of minerals in reverse osmosis water can give hot drinks a bland taste with no depth of flavour.

Is reverse osmosis water good for plants?

Yes, reverse osmosis water can be good for plants, but depending on the type of plant you may need to ensure you are feeding your plants at regular intervals with nutrients to ensure healthy growth.

Reverse osmosis vs bottled water: which is better?

From an environmental, cost and health perspective reverse osmosis water is better than bottled water. Bottled water should only be bought and consumed if you have no other options available.