Reverse osmosis: what it actually removes, and whether you need it
It is the most thorough domestic filtration there is, and for a lot of Australian households on mains water it is solving a problem they do not have. The useful question is never "is RO good" but "what is in my water".
Reverse osmosis pushes water under pressure through a membrane with openings so fine that most dissolved substances cannot follow it through. That is what separates it from every other filter under a kitchen sink: a carbon filter adsorbs things onto a surface and a sediment filter strains particles out. But neither can remove something genuinely dissolved in the water. Reverse osmosis can, and that is both the whole argument for it and the source of its drawbacks.
Whether you need one is not a question about reverse osmosis. It is a question about your water, and it cannot be answered by a website, a salesperson or a taste test.
How does reverse osmosis actually work?
Osmosis is water moving naturally across a membrane towards the saltier side. Reverse osmosis applies pressure to drive water the other way, from the side with more dissolved material to the side with less, leaving the dissolved material behind. Mains pressure does that work in a domestic system. That is why RO units are sensitive to supply pressure. And why a household on a tank and pump can run into trouble that a household on mains never notices.
A domestic unit is a train of stages, not a single device, and this matters because people blame "the RO" for problems belonging to one stage:
- Sediment pre-filter. Strains out particles that would otherwise clog the membrane. It is protecting an expensive component with a cheap one.
- Carbon pre-filter. Removes chlorine and chloramine, which is not optional housekeeping: the membrane material in many units is degraded by chlorine, so the carbon stage is what keeps the membrane alive.
- The membrane itself. Where dissolved solids are rejected.
- Storage tank. RO produces water slowly, so it accumulates it. Without the tank the tap would trickle.
- Post-filter, and often remineralisation. A final polish for taste, and on many units a stage that deliberately puts some minerals back.
What does reverse osmosis remove that a normal filter does not?
The short version: a carbon filter is very good at the things you can taste and smell, and largely powerless against the things you cannot. Reverse osmosis is the reverse of that emphasis.
| Filter type | What it is actually for | What it will not do |
|---|---|---|
| Sediment | Particles, rust, silt, grit. Protects everything downstream | Nothing dissolved; no effect on taste chemistry or microbiology |
| Activated carbon | Chlorine, taste, odour, many organic compounds | Dissolved salts and most dissolved metals; does not disinfect |
| Reverse osmosis | Dissolved solids broadly: salts, nitrates, fluoride, many dissolved metals | Runs slowly, sends some water to drain, strips beneficial minerals too |
| UV | Disinfection. Inactivates micro-organisms | Removes nothing physical or chemical; water must already be clear |
| Water softener | Hardness, i.e. scale in kettles, pipes and hot water systems | Not a purifier at all. Adds sodium, and is aggressive towards hot water tank anodes |
The practical consequence is that these are not ranked from worst to best. They address different problems, and buying the most thorough one is only sensible if the problem you have is the one it solves.
What are the downsides of reverse osmosis?
There are three real ones and they are inherent to the method rather than faults of any particular brand.
It sends water to drain. Rejecting dissolved solids means flushing them away. So a domestic RO unit produces a quantity of drain water alongside the filtered water it delivers.
The ratio varies considerably between units and with supply pressure and temperature, and it is a fair question to ask about a specific model before buying. In much of Australia, where water restrictions are a normal part of life rather than an emergency, that is a genuine consideration and not a technicality.
It removes minerals you were not trying to remove. The membrane does not distinguish between a contaminant and calcium or magnesium. That is why remineralisation stages exist, and why RO water tastes flat to people used to mains water: the taste they are missing is dissolved mineral content.
It is slow, and it has ongoing maintenance. Production is measured over hours, which is why the storage tank exists. Pre-filters need changing on schedule or the membrane pays for it, and the membrane itself is a periodic replacement. An RO unit that has been ignored for years is not a neutral object, because a neglected filter housing is a place where water sits.
Do I need reverse osmosis on Australian mains water?
For most households on a reticulated mains supply, honestly, probably not. It is worth saying that plainly on a page about reverse osmosis. Mains water in Australia is supplied against the Australian Drinking Water Guidelines, and your water authority publishes water quality data for your area. If your complaint is that the water tastes or smells of chlorine, that is exactly what activated carbon is for, at a fraction of the complexity and with nothing going to drain.
Reverse osmosis earns its place when there is something dissolved that carbon cannot address and you have evidence of it. The common real cases are water that is genuinely saline or brackish, bore water with dissolved metals or elevated nitrates. Or a specific measured result you are trying to bring down. In every one of those, the distinguishing feature is the same: somebody measured the water first.
If you are on tank, bore or spring water, start at what a laboratory test actually looks for rather than at a product. Choosing filtration before testing is how people end up with an expensive system aimed at the wrong problem. And, more seriously, how people end up believing a microbiological issue has been dealt with by a device that never claimed to disinfect anything.
Under-sink or whole-house?
Reverse osmosis is almost always installed at a single point of use (typically a dedicated tap at the kitchen sink) and that is a design decision rather than a cost-saving compromise.
Producing every litre a household uses through an RO membrane would mean filtering the shower, the laundry and the garden to drinking standard, at the drain-water and throughput penalties described above, for water nobody drinks. Whole-house treatment is usually sediment and carbon, sometimes softening, sometimes UV. Drinking-water polishing is done at the kitchen.
The other half of the decision is physical: an under-sink RO system needs cupboard space for the stages and the storage tank, a drain connection. And a hole in the benchtop or sink for its own tap. Those constraints decide more installations than the specification does.
What is genuinely yours to do
- Find your water quality report if you are on mains. Your water authority publishes it, and it tells you what is actually in the supply rather than what a product page implies.
- Get a laboratory test if you are on tank, bore or spring water, from a NATA-accredited lab. This is the step that decides everything after it.
- Write down what you are actually trying to fix (taste, smell, staining, scale, a measured result) because each points at a different technology.
- Ask about drain ratio, production rate and filter change intervals for the specific model, not the category.
- Check the cupboard, the drain and the benchtop before choosing a unit.
- Diarise the pre-filter changes. The cheap stages are what keep the expensive one alive.
Frequently asked questions
What does reverse osmosis remove from water?
It removes dissolved solids broadly (salts, nitrates, fluoride and many dissolved metals) which is the category that sediment and carbon filters cannot touch, because those work by straining particles or adsorbing substances onto a surface. The trade-off is that the membrane does not distinguish between contaminants and beneficial minerals, so calcium and magnesium go as well.
Is reverse osmosis better than a carbon filter?
They are not ranked, they do different jobs. Activated carbon handles chlorine, taste and odour very effectively and simply. Reverse osmosis handles dissolved material that carbon cannot, but it runs slowly, sends water to drain and strips minerals. If your complaint is that the water tastes of chlorine, carbon is the right tool and RO is solving a problem you do not have.
Do I need reverse osmosis if I am on mains water in Australia?
Usually not. Mains supply is provided against the Australian Drinking Water Guidelines and your water authority publishes water quality data for your area, which is the honest place to start. Reverse osmosis earns its place when something dissolved has been measured that carbon cannot address. Saline or brackish water, or bore water with dissolved metals or elevated nitrates.
How much water does a reverse osmosis system waste?
Rejecting dissolved solids means flushing them to drain, so every RO unit produces drain water alongside the filtered water it delivers. The ratio varies considerably between models and with supply pressure and temperature, so it is a question to ask about the specific unit rather than the category. In a country with routine water restrictions it is a genuine consideration.
Does reverse osmosis make water safe to drink?
No filter makes an untested supply into a tested one, and this site does not tell anyone their water is safe to drink. Reverse osmosis is a filtration process, not a disinfection process: UV is what inactivates micro-organisms. Whether a supply is fit to drink is a laboratory result from a NATA-accredited laboratory read against the Australian Drinking Water Guidelines, specific to your property and season.
Why does reverse osmosis water taste flat?
Because the taste people associate with water is largely dissolved mineral content, and the membrane removes that along with everything else dissolved. This is why many domestic units include a remineralisation stage after the membrane, which deliberately puts some mineral content back for taste.
Can I install a reverse osmosis system myself?
Connecting a filtration system to the drinking water supply is licensed plumbing work in every Australian state and territory, and that includes the drain connection and the backflow protection that keeps waste water from reaching the drinking supply. The regulator for your state or territory licenses and inspects that work.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Sydney Water. Water supply, wastewater and stormwater services