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Home » Reverse Osmosis Systems » How Do Reverse Osmosis Water Filters Work?

How Do Reverse Osmosis Water Filters Work?

By: David Trinh
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how do reverse osmosis filters work

For almost 50 years, the process of Reverse Osmosis (RO) has been used to purify water in domestic, industrial and medical applications, and has even been used in desalination processes (removing the Salt from sea-water).

If you’re going to invest in a top-of-the-line RO system it’s important to know how they work and crucial features you need when selecting a system.

But what is RO, and how do domestic reverse osmosis water filters work?

What is Reverse Osmosis?

Reverse Osmosis is a water purification process, which removes contaminants from the water supply by applying pressure to force it through a partially permeable membrane.

Water on the supply-side of the membrane (known as the feed water) is pushed through, leaving the suspended contaminants on the concentrated side.  The small holes within the membrane (known as pores) will let water molecules through but not the molecules which make up the impurities within the water.  These impurities might include salt in the desalination process, or pollutants and sediments in a domestic reverse osmosis filter

RO results in two fluids being produced by the process – purified water and a waste water solution known as the brine, which contains all of the impurities from the supply water which weren’t able to pass through the membrane.

How does a Reverse Osmosis system work to filter water?

In the usual process of Osmosis, water molecules freely move across a membrane from areas of high concentration to areas of low concentration, in order to balance the distribution of molecules within the fluid. 

To make this process happen in the “reverse” direction, pressure is applied to the feed water resulting in a highly concentrated brine holding the filtered contaminants on one side of the membrane and fresh water on the other.

Stages of Reverse Osmosis Water Filtration

ro diagram

1. Sediment Pre-Filter

Dirt, rust, and sediment suspended within the feed water from the supply are filtered.

These pre-filters usually remove particles as small as 5 microns across (a human hair is roughly 50 microns across!) and a variety of different types of filter are available but they all work in the same way – when the feed water passes through the pores in the filter, the larger contaminant particles are caught up while water and smaller molecules can pass through.

2. Carbon Pre-Filter

In Stage 2, the water passes through an activated carbon filter, similar in to those used in under-sink or faucet water filters.  

The carbon filter absorbs pollutants, soaking them up within the pore structure.

This stage removes tastes, odours and chemicals such as chlorine. 

3. Semi-Permeable Membrane

The partially permeable RO membrane is made of a thin-film composite and can be expected to remove up to 98% of total dissolved solids (TDS) down to 0.001 microns.

This is the key part of the reverse osmosis filtration process and the element which sets it apart from other filtration systems. 

4. Storage

Once the fresh water has passed through the membrane it flows to a storage tank where it is held until needed.  

The RO process continues until this tank is full, at which point an automatic shut off valve closes, preventing any more water flowing across the semipermeable membrane and shutting off the flow of waste water to the drain.

5. Post-Filter

The filtered water passes through a second carbon filter between the storage tank and the outlet (also known as a polishing filter) removing any contaminants which may have made it through the other filters or the partially permeable membrane.

Why Do You Need a Storage Tank?

The RO process is very slow – you would not want to wait for a glass to fill!

This means a buffer is required to bridge the gap between the rate of at which the water will be drawn at the faucet and the speed which the same amount of water can be filtered through the semipermeable membrane during the RO process.

With a storage tank installed, water is pulled from the tank as it is required at the taps and the tank then continues to refill after the faucet has been shut off.  This gives the RO filtration system time to replenish the water used to ensure that filtered water is available whenever it is needed.

What does a Reverse Osmosis system remove?

The RO process removes sediment, sand and rust at the first prefilter stage.

At the second prefilter stage, chlorine, VOCs, sediment and odours are removed from the water.

Salt, minerals, fluoride, calcium, arsenic and herbicides and pesticides are all removed at the semipermeable filter, and directed to the drain suspended within the wastewater,

The significant elements which standard reverse osmosis water filtration systems do not remove from the water supply are bacteria and parasites.  It is possible to buy RO systems which incorporate ultraviolet filtration to destroy bacteria and parasites, or these UV filters can be added to the incoming water supply.

Reverse Osmosis Benefits


Reverse Osmosis filtration systems are some of the most thorough and comprehensive methods available for removing impurities and pollutants from the water supply.

RO systems can remove up to 98% of total dissolved solids (TDS) – comparable only with the distillation process in terms of the degree of drinking water purification which the systems offer.

Reverse Osmosis Systems can:

  • Improve taste, quality and purity of drinking water
  • Remove pesticides and pollutants from drinking water
  • Remove fluoride, chlorine and arsenic – unusual in other types of water filters 
  • Easy to install 
  • Cheap to maintain 
  • Fits discreetly into your home – usually under the kitchen sink 
  • Results in less plastic waste than bottled water

Why is Reverse Osmosis a good filtration option compared to others?

RO vs Water Softener vs Other Filters

Reverse Osmosis filtration is unique in the number and percentage of contaminants it can remove  from the water supply.

Water softeners do not remove contaminants from the water supply in the same way that water filtration systems do.

Water softeners neutralize the minerals in the water supply which cause scale in domestic and industrial plumbing systems.  They do not remove pollutants, contaminants, sediments, bacteria or parasites.

Water softeners are often fitted in areas where the water is very hard in addition to filtration systems however but they should not be installed as an alternative to water filters.

In terms of their filtration capability, the main downside to RO systems is their inability to remove bacteria and parasites from the water supply.  Where water is known to be at higher risk of contamination with these such as wells and boreholes, it is strongly recommended that UV filtration systems are installed as these are the most effective at dealing with organic contaminants.  Antimicrobial in-line filters can also be effective at removing bacteria and parasites. 

In areas with significant levels of industrial, mining, or agricultural activity, RO systems are the most effective at removing impurities and chemical pollutants from the water supply.

Under-sink and faucet water filters can be effective at providing clean, quality drinking water and can be more affordable than RO systems however it is rare that these units can compete with RO systems in terms of their filtration performance. 

Is Reverse Osmosis environmentally friendly?

Do They Waste Water?


RO systems do generate a wastewater product which is sent directly to drain however this water is immediately returned to the water treatment plant via the sewers.

Due to the higher levels of contaminants within the wastewater it is not suitable for domestic uses such as consumption or bathing however the wastewater byproduct can be used around the home, and for cooling or flushing in some industrial processes. 

The rate of waste can be as high as 75% (for every 1 gallon purified, 3 gallons is pumped to drain) and although this is high, for situations such as private boreholes where the water supply is not being charged the only additional resources used are the small amount of energy to run the pumps and the RO process itself.

How to reduce wastewater

Wastewater from the Reverse Osmosis process cannot be reduced as such because the amount generated is a direct relation to the amount of filtered water required, but it can be used elsewhere around the home to replace water which would otherwise have been drawn form the supply.

Greywater Systems 

Although the brine ejected from the RO process cannot be used by people for drinking or washing, it can still be used around the home.

Greywater systems are typically a totally separate water service installed in the home for processes such as flushing toilets, supplying washing machines and serving outside taps for car washing and watering plants.

Waste water from the RO process can be directed to a greywater storage tank rather than the drain, and used for these processes instead of feed water.  If the tank becomes full it will simply redirect the wastewater to the drain, as it would have been otherwise.

Household Cleaning

Everything from cars to floors to decking and patios around the home need cleaning on occasion, and the waste water from RO can be used for these purposes – simply redirect the drain from the RO unit into storage instead of the drain and use it when it is needed.


Waste water can be used for pre-rinsing dirty clothes to reduce the water required on a normal laundry cycle.


Watering plants, feeding sprinklers and cleaning decking are all safe uses for wastewater once it has been collected from the RO process.

RO vs Bottled Water

In much of Europe it is illegal to produce bottled water using the Reverse Osmosis process, meaning only mineral, filtered and spring water can be sold in bottled form.

Mineral water has several benefits, as it still contains many of the beneficial minerals which are removed from feed water during the RO purification process.  Where RO is used to treat drinking water, it can be necessary to find another dietary source of these minerals as sodium and calcium in particular are vital for the body to carry out its normal functions.

In the US however most bottled water is produced with via RO but owing to the inefficiency of the process the degree of wastage is much greater than in domestic Reverse Osmosis systems.

The main benefit of RO vs bottled water is the amount of plastic waste saved by avoiding bottled water.  Given the huge problems with plastic waste polluting the oceans, harming sea birds and its survivability in the environment meaning that it does not break down naturally for hundreds of years, any solutions which reduce the amount of plastic waste being generated will help while a permanent solution to the plastic problem is found.

Is RO water good for you?

The Reverse Osmosis filtration process removes almost all contaminants within the feed water.  While we usually think of these contaminants as being the heavy metals, sediments, bacteria and pesticides that pollute our drinking water, some of them are beneficial for our health.

Salt, magnesium and calcium in particular are critical for the body’s processes however these are all minerals which can be primarily found in foods or supplements.  Given the minimal quantities which are found in a healthy water supply, the benefits of these compounds is minimal.

There are however potentially severe consequences from drinking polluted water.  Heavy metals have been linked to brain diseases and dementia in later life and there are many ongoing court cases linking pesticides with cancers and birth defects.  

Although it is not accurate to say that reverse osmosis filtered water is good for you as it is just water (the purest available by domestic filtration), our bodies are reliant on water to carry out essentials functions and when the potential for harm from contaminants within the water supply is considered, there is a far greater chance that you will be harmed by drinking unfiltered water than water which has been purified using reverse osmosis.

Where are Reverse Osmosis Systems used?

Reverse Osmosis systems can be used literally anywhere that water purification is required however when considering them for use in the home there are applications where they’ll be more effective than others.


Installing an RO system under the sink to supply drinking water is an excellent deployment of the technology.  By installing under sink the RO plant remains out of sight and makes the installation as simple as possible, whilst still keeping easy access for maintenance.

Ice Machines

Although it may seem different ice is still going to be drunk from the glass and so ought to be filtered in the same way as your drinking water.

A refrigerator with inbuilt ice machine is usually not too far from the kitchen sink and so connecting the RO unit to the back of the ice machine should be quite simple and unobtrusive.

Well Water

Drawing water from a well is one of the most natural ways to obtain water, and humans have been hydrating this way for thousands of years.

Unfortunately mining and pesticides have been in use for the past 200-300 years and so the groundwater from which your well draws from is not as pure as it was previously.

Although bacteria and parasites need to be treated separately from the RO filtration system, Reverse Osmosis is well suited to well water feeds.


Fish and other aquatic creatures can be extremely sensitive to the conditions of their environment, and some species are notoriously difficult to keep alive in tanks.

By using reverse osmosis filtration, almost all contaminants can be removed from the tank water, which can then be resalinated  (for salt water fish) using a remineralisation filter to create the perfect environment for your fish with no pollutants… you’ll just have to remember to feed them! 

Industrial Processes

RO filtration can be employed in any setting where water purity is important.  Many industrial processes rely on clean water to operate and RO can be invaluable in these situations. 

To ensure that a reverse osmosis filtration system is correctly specified for the processes being carried out a specialist contractor should be engaged to carry out the and guarantee the works.

How do you install an RO system?


Once you have bought your RO system, you should refer to the manufacturer’s guide.  This will show a schematic of the items to be fitted and include an overview of the tools required.

For this example we will assume that you will be installing below the kitchen sink and installing a new faucet for the RO supply.  We also assume that you are reasonably competent to undertake general maintenance around the home – if you do not feel confident please do not take the risk and contact a plumber who will be able to complete the installation in just a couple of hours.

  1. Firstly, make a hole in your sink or worktop for the new faucet.  The process for this will depend on the type of sink and materials involved – we recommend looking up  a guide for this task separately as there are many options.
  2. Next, cut a hole into the existing pipework to suit your drain saddle (which will take the waste water from your new RO unit to drain) and fix the saddle across the hole using the clamps provided.  This ought to be fitted as far away from any appliances as possible, and upstream from the sink trap.
  3. Switch off the water supply and drain down the domestic services.
  4. With the water services drained, cut in the RO feed valve and tighten the nuts using a wrench.
  5. Site the storage tank within the sink unit and connect to the new faucet.
  6. Position the RO unit, remembering that access will be required for maintenance, filter changes etc in the future 
  7. Using the hoses provided, match up the colour coded quickfit connections on the RO unit and complete the connections to the plant
  8. With all connections made, insert the prefilters, polishing filter and membrane into their respective positions as the manufacturer’s instructions direct.
  9. Slowly switch on the water supply again, checking all connections and joints for leaks.  Should you find any leaks, switch the water off again, tighten/check the connection and start again.
  10. You may hear gurgling and hissing as the system fills with water – this is completely normal and is just caused by air escaping. 
  11. Leave the system for 4-8 hours to fill.  When you hear water running into the drain, the system is full.  Open the drain valve on the storage tank and completely flush the system to empty.
  12. Leave the system overnight to fill again.
  13. In the morning with the system filled, fully drain the storage tank and flush out the system again.
  14. Once filled for a third time, the system is ready to use.

How do you maintain an RO system?


Filters will generally  need to be replaced every 6-12 months depending on the system you install.  Some filters can last up to two years however the filter life is usually prominently displayed on the product packaging. 

The semipermeable membrane will usually last up to 3 years but should be inspected annually.

Annual Maintenance

Once a year it is recommended that the RO system is fully serviced in order to inspect the condition of filters, seals, valves and other items which could cause damage to the property or the system in the event of a failure. The manufacturer’s instructions will usually advise this and many have Approved Installer schemes so you can be sure of a professional service from a knowledgeable tradesman.

How long do RO systems last?

WIth the correct maintenance, there is no reason that a good quality reverse osmosis filtration system cannot last for 20 years or more, however you should expect your RO unit to be operational for at least 10-15 years.

As with all filtration plant, careful, routine maintenance using approved components is key to the system’s longevity and if the manufacturer’s instructions are followed your reverse osmosis system should be something which you can fit and almost forget about!


Easy to fit, affordable to buy and the most effective means of domestic water filtration available, an RO system should be high in the list of considerations for anyone looking for a permanent, long term filtration solution that will remove almost every impurity from your drinking water.

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David Trinh
David is an expert in all things plumbing, heating, cooling, and water treatment. He got his start in the plumbing business working on fixing all types of home improvement issues including water leaks, broken toilets, appliance installation, and more. Over time, he learned a ton about installing and choosing the correct water treatment products for homeowners.

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