What Is Reverse Osmosis? How It Works and What It Removes

Reverse osmosis explained: what it is and how it works

Reverse osmosis provides the highest level of home water purification, removing up to 99% of dissolved contaminants, including PFAS, fluoride, heavy metals and mineral salts. It also removes calcium and magnesium, making it the only under-sink system that provides deep filtration and soft drinking water at the tap.

For most Perth homes on mains water, standard sediment and carbon filtration is enough. Reverse osmosis becomes worthwhile when your water needs a deeper level of filtration and contaminant removal. This guide explains how reverse osmosis works, what it removes and when it may be the right choice for a home in Western Australia.

Key facts:

• Reverse osmosis reduces up to 99% of dissolved contaminants, including PFAS, E. Coli, fluoride, heavy metals, along with naturally occurring minerals such as calcium and magnesium
• Most reverse osmosis systems use multiple stages of filtration including sediment and carbon filters, the RO membrane and a final carbon polishing filter
• RO is the only under-sink system that both purifies and softens drinking water at the tap
• Systems come in a range of capacities such as 50, 100 and 200 gallons per day (GPD), to suit different household sizes and water use
• Sediment and carbon pre-filters and the taste and odour active granulated post filters need replacing every 6 to 12 months while the membrane typically lasts 2 to 4 years
• Reverse osmosis systems should be professionally installed to ensure correct operation and compliance

What is reverse osmosis?

Standard twin under-sink filter systems use sediment and carbon filters to reduce chlorine, organic compounds and suspended particles. They improve the taste and smell of tap water, but they are not designed to remove dissolved minerals, fluoride, PFAS or many heavy metals. 

Reverse osmosis works differently. Rather than passing water through filter media, it pushes water under pressure through a semi-permeable membrane that allows water molecules to pass through while reducing a wide range of dissolved contaminants, minerals and salts. The process reduces naturally occurring minerals, including calcium and magnesium, resulting in soft drinking water at the kitchen tap.

Reverse osmosis is intended to treat drinking water at the point of use, not across the whole house. The wastewater it produces and the reduction in flow rate make it unsuitable for treating the full home supply. It belongs under the sink, not at the point of entry.

How does reverse osmosis work?

Most reverse osmosis systems use multiple stages of filtration before water reaches the tap.

• Sediment pre-filter. Removes sand, silt, rust and suspended particles that could damage the membrane downstream.
• Carbon filter. Reduces chlorine, chlorine by-products and other contaminants that affect taste and odour. Removing chlorine is important because it can shorten the life of the RO membrane.
• RO Membrane. Water is forced through the semi-permeable membrane under pressure. Dissolved solids, minerals, fluoride, PFAS, E. Coli and other contaminants are removed before treated water is fed to the pressurised storage tank. 
• Post-RO carbon “polishing” filter. Removes any residual taste or odour before the water reaches the tap.

The membrane is the heart of an RO system, and regular replacement of the pre-filters helps maximise its lifespan and maintain performance. 

What does a reverse osmosis system remove?

A properly maintained reverse osmosis filtration system removes up to 99% of dissolved contaminants, including PFAS, heavy metals such as lead and arsenic, nitrates, dissolved mineral salts including calcium and magnesium, chlorine and chlorine by-products and fluoride.

Because it reduces calcium and magnesium, reverse osmosis produces soft drinking water at the point of use. That makes it the only under-sink filter system that purifies and softens drinking water at the point of use. 

Reverse osmosis also provides significant reduction of many micro-organisms, bacteria and waterborne viruses. However, bore water with microbial contamination may require additional treatment such as UV disinfection alongside RO.

Fluoride is a dissolved mineral, removed by the RO membrane. If you want to keep fluoride in your drinking water, a standard twin filter system may be the better choice. 

Every household and water source is different, so the right filtration system depends on the quality of your incoming water and what you want to improve. The reverse osmosis systems we install across WA range from standard household RO units to high-capacity commercial RO systems for regional businesses and large households with challenging source water.

Reverse osmosis vs standard twin filter system

A standard sediment and carbon twin under sink filter system removes chlorine, chlorine by-products, organic chemicals and sediment. For Perth homes on mains water where the main concern is taste, smell and sediment, it does the job well and is a cost-effective option. Reverse osmosis goes further by removing dissolved contaminants that standard twin filter systems cannot remove.

The honest verdict: for most Perth metro homes on mains water, a standard sediment and carbon filter system is usually the right and most cost-effective way to improve drinking water quality. Reverse osmosis is more valuable where there are specific concerns about your water quality, like relying on bore water, a high calcium content, the presence of PFAS or heavy-metals or high levels of dissolved solids. 

Reverse osmosis produces purified water more slowly than a twin under sink system and discards up to 3 litres of water for every litre it produces. You can collect the wastewater and use it for the garden or laundry rather than send it down the drain. Our under-sink vs reverse osmosis guide takes a closer look at how the two systems compare in everyday use, filtration performance and maintenance.

When is reverse osmosis the right choice?

Reverse osmosis is particularly useful in four situations.

1. Bore and collected rainwater in the Pilbara and regional WA. Bore and mains water in Karratha, Tom Price, Paraburdoo and other parts of the Pilbara can contain elevated mineral levels, iron, manganese, bacteria and dissolved solids. Water quality varies significantly by location and source, and standard sediment and carbon filtration may not be adequate. Reverse osmosis provides a deeper level of filtration and may be combined with UV disinfection in areas with poor, unreliable water quality. 
2. PFAS concerns. A 2019 DWER investigation of ambient PFAS across the Perth metropolitan area detected PFAS at all surface water sampling locations and 91% of groundwater sampling locations. Perth municipal mains water remains compliant with the Australian Drinking Water Guidelines. Households using bore water near Jandakot Airport, Bullsbrook or former defence sites may face higher exposure. Some under-sink systems provide partial PFAS reduction, while reverse osmosis offers a deeper level of filtration to remove PFAS. 
3. Older homes with copper plumbing. The World Health Organization confirms lead can contaminate drinking water through plumbing systems containing lead or copper pipes, joins and fittings. A reverse osmosis system treats water before it reaches the kitchen tap, helping reduce lead, copper and other dissolved metals that may be present in the old plumbing system.
4. High dissolved solids. Some parts of WA, including areas of the Wheatbelt and the Pilbara have source water with high levels of total dissolved solids that affect drinking water taste and quality. Reverse osmosis is particularly effective at removing these contaminants drinking and cooking water.

For filtered water across the home, reverse osmosis is not the solution. A whole-home water filtration system treats water before it enters your property so you can enjoy filtered water from every tap, shower and appliance such as a coffee machine. Pairing a whole house filter system with a water softener removes contaminants, sediment and calcium and magnesium to treat water hardness and provide genuine complete home filtration. Reverse osmosis treats drinking water at a single tap, which makes it excellent for what comes out of that tap, but it does not extend to the rest of the home.

Before you choose a reverse osmosis system

Reverse osmosis systems are available in a range of capacities to suit household size and source water quality.

RO system capacity varies from 50, 100 or 200 gallons per day (GPD), with larger capacities designed to serve commercial and industrial RO systems. Water2water’s 4-Stage RO systems typically produce 50 GPD which is around 189 litres and is adequate to meet most household needs.

Before installing an RO system, it is important to consider:

• The quality of your source water
• How much water your household uses each day
• Available space under the sink to install the filter system and water storage tank
• Benchtop material if you want an additional sink top tap 

Stone, marble and granite benchtops require specialist drilling, while some older properties may need additional plumbing work, which must be carried out by a licensed plumber.

How often should you service a reverse osmosis system?

Sediment and carbon pre-filters need replacing every 6 to 12 months depending on usage and source water quality. In Karratha, Tom Price, Paraburdoo and across the Pilbara, homes that use bore water or have higher sediment and mineral levels may need to replace the pre-filters more frequently.

The RO membrane typically lasts 2 to 4 years under normal conditions. Regular pre- and post-RO filter replacement and system servicing helps protect the delicate membrane and maximise its life expectancy. 

We take care of ongoing reverse osmosis servicing and maintenance, including:

• Regular sediment and carbon pre-filter replacement
• Inspection of the RO membrane, filter housing and connections 
• System sanitisation and flushing
• Updating the service log history. 

Some households add a remineralisation filter after the membrane stage to reintroduce healthy minerals such as magnesium and calcium that the membrane has removed. Mineralising cartridges don’t improve the level of filtration and are an option based on your personal preference.

Water2water installs and services filtration systems across Perth and regional Western Australia. See our home filtration options for systems suited to different water sources and treatment goals.