What is a reverse osmosis water filter
What Is Reverse Osmosis and How Does It Work? (Simple Explanation)
Reverse osmosis pushes water through a membrane so fine that only water molecules pass through. Here’s how our 7-stage under-sink RO system works, explained simply, stage by stage.
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Reverse Osmosis Water Filtration: How It Works
Reverse osmosis (RO) is the most effective water filtration technology available for home use. It removes up to 99% of dissolved contaminants, including fluoride, heavy metals, dissolved salts, and chemicals that standard carbon filters can’t touch.
If you’ve ever wondered why under-sink reverse osmosis systems produce noticeably cleaner, better-tasting water than a standard filter, the answer is in the membrane. An RO membrane has pores so small, 0.0001 microns, that only water molecules can pass through. To put that in perspective: a human hair is about 75 microns wide. An RO membrane filters at a scale 750,000 times finer than that.
This guide explains how reverse osmosis works in plain language, walks through the 7 stages of our under-sink RO system, and covers what it removes, what it doesn’t, and who actually needs one.
Reverse Osmosis: The Simple Explanation
Start with osmosis, a natural process you might remember from school. When you have water on both sides of a thin membrane, the water naturally flows from the clean side to the contaminated side, trying to equalise the concentration. It’s why plant roots draw water up from the soil.
Reverse osmosis does exactly what the name suggests: it reverses that natural flow. Instead of clean water moving toward the contaminated side, mains water pressure pushes contaminated water through the membrane from the dirty side to the clean side. The membrane’s pores are so microscopically small that dissolved salts, heavy metals, fluoride, chemicals, and other contaminants cannot physically fit through. Only water molecules pass.
The contaminants that can’t pass through the membrane are flushed to drain as wastewater. The purified water goes to a small pressurised storage tank under your sink, ready to flow from a dedicated drinking water tap whenever you turn it on. A licensed plumber can install this system neatly under your sink and ensure the connections to your existing plumbing and drain line are done correctly.
That’s the core concept. But a modern RO system doesn’t rely on the membrane alone. It uses multiple filtration stages before and after the membrane to protect it, extend its life, and ensure the water that reaches your glass is as clean and balanced as possible. Many homeowners choose to have a professional plumber handle installation and maintenance to keep the system performing efficiently for years.
Interested in an Under-Sink RO System for Your Home? Our 7-stage reverse osmosis system installs under your kitchen sink and delivers purified drinking water on demand. Fully installed from $1,100 inc. GST by a licensed plumber. 0% finance available. ☎ 0420 646 641 | Get a Free Quote → |
The 7 Stages: How Our Under-Sink RO System Works
Each stage in a 7-stage reverse osmosis system has a specific job. Here’s what happens to your Melbourne tap water as it passes through each one:
Stage 1: Sediment Pre-Filter (5 Microns)
Job: Catch the big stuff before it reaches the delicate components downstream.
The first stage is a polypropylene sediment filter that traps particles like dirt, sand, silt, rust flakes, and pipe scale. If you live in an older Melbourne suburb with ageing pipes, this stage works especially hard. It protects the carbon filters and the RO membrane from physical damage and premature clogging.
Stage 2: Granular Activated Carbon (GAC) Filter
Job: Remove chlorine, which destroys RO membranes.
This is a critical protection stage. Melbourne’s water is treated with chlorine (and in western suburbs, chloramine ) to keep it safe during distribution. While effective as a disinfectant, chlorine degrades RO membranes over time, dramatically shortening their lifespan. The GAC filter adsorbs chlorine along with volatile organic compounds (VOCs), herbicides, pesticides, and chemicals that affect taste and odour.
Stage 3: Carbon Block Filter (CTO)
Job: A second line of carbon defence, targeting chloramine and finer chemical contaminants.
Where the GAC filter handles the bulk of chlorine removal, the carbon block provides additional contact time for chloramine reduction important because chloramine is harder to remove and requires denser media. This stage also catches any residual organic chemicals that passed through Stage 2. By the time water leaves Stage 3, virtually all chlorine and chloramine have been removed, and the RO membrane is fully protected.
Stage 4: Reverse Osmosis Membrane (0.0001 Microns)
Job: The core of the system. Removes dissolved contaminants at a molecular level.
This is where the real work happens. The semi-permeable RO membrane filters at 0.0001 microns, fine enough to remove dissolved salts, fluoride, lead, arsenic, mercury, nitrates, PFAS, pharmaceuticals, and virtually all dissolved inorganic contaminants. Standard carbon filtration cannot touch these because they’re dissolved at a molecular level. The RO membrane can.
Water that passes through the membrane is collected as purified permeate. Contaminants that can’t pass through are flushed to the drain. Modern systems are more efficient than older designs, typically producing around 1 litre of purified water for every 2–3 litres of feed water, rather than the 1:5 ratios of older systems.
Stage 5: Post-Carbon Polishing Filter
Job: Final taste and odour refinement after the storage tank.
After the RO membrane, purified water is stored in a small pressurised tank under your sink. When you open the dedicated drinking tap, water flows from the tank through this post-carbon filter before reaching your glass. It removes any residual taste or odour that may have been picked up during storage, ensuring the water is as fresh and crisp as possible.
Stage 6: Alkaline Mineral Restoration Filter
Job: Restore healthy pH balance and natural mineral content.
This is the stage that separates a good RO system from a basic one. The RO membrane is so effective that it strips virtually everything from the water, including beneficial minerals like calcium and magnesium. Post-membrane, the water’s pH drops to around 6.0, which is mildly acidic.
The alkaline restoration filter passes the purified water through mineral media that adds back calcium, magnesium, and potassium in controlled amounts. This lifts the pH to approximately 8.5–9.0, a slightly alkaline level that produces a naturally clean, smooth taste. You get the purity of reverse osmosis without the flat, empty taste that basic RO systems sometimes produce.
Stage 7: Far-Infrared and Hydrogen Enrichment Filter
Job: Final treatment for enhanced hydration properties.
The seventh stage uses far-infrared emitting ceramic media combined with hydrogen-generating media. Far-infrared energy helps inhibit bacterial growth in the stored water, extending its freshness, while hydrogen enrichment produces water with antioxidant properties. This final stage ensures the water you drink is not just purified and mineralised, but optimised for hydration.
All 7 Stages at a Glance
Stage | Filter Type | What It Does | Why It Matters |
1 | Sediment Pre-Filter (5µm) | Removes dirt, sand, rust, and pipe scale | Protects downstream filters and membranes from physical damage |
2 | Granular Activated Carbon | Removes chlorine, VOCs, herbicides, taste, and odour | Protects the RO membrane from chlorine degradation |
3 | Carbon Block (CTO) | Removes chloramine, residual chemicals | Essential for Melbourne’s western suburbs, where chloramine is used |
4 | RO Membrane (0.0001µm) | Removes fluoride, lead, arsenic, PFAS, dissolved salts, nitrates, and pharmaceuticals | The core stage removes dissolved contaminants that no carbon filter can |
5 | Post-Carbon Polishing | Final taste and odour polishing after storage | Ensures a fresh, crisp taste every time you use the tap |
6 | Alkaline Mineral Restoration | Restores calcium, magnesium, potassium; raises pH to 8.5–9.0 | Corrects the acidic pH of pure RO water and adds a natural mineral taste |
7 | Far-Infrared + Hydrogen Enrichment | Inhibits bacteria in stored water; adds antioxidant hydrogen | Extends water freshness and enhances hydration properties |
What Does Reverse Osmosis Remove?
A 7-stage RO system removes a wider range of contaminants than any other residential filtration technology:
Contaminant | Removed by RO? | Removed by Carbon-Only Filter? |
Chlorine | ✓ Yes (Stages 2–3) | ✓ Yes |
Chloramine | ✓ Yes (Stage 3) | Δ Partial (needs composite block) |
Fluoride | ✓ Yes (Stage 4 RO membrane) | ✗ No |
Lead, arsenic, mercury, copper | ✓ Yes (Stage 4) | Δ Partial (composite block only, limited) |
PFAS / PFOA / PFOS | ✓ Yes (Stages 2–4) | Δ Partial (carbon adsorbs some) |
Dissolved salts (TDS) | ✓ Yes (Stage 4) | ✗ No |
Nitrates/nitrites | ✓ Yes (Stage 4) | ✗ No |
Pharmaceuticals/hormones | ✓ Yes (Stages 2–4) | Δ Partial |
Sediment, rust, pipe scale | ✓ Yes (Stage 1) | ✓ Yes |
Herbicides/pesticides / VOCs | ✓ Yes (Stages 2–3) | ✓ Yes |
Bacteria/viruses | Δ Membrane blocks most; Stage 7 provides additional protection | ✗ No |
The key difference is the RO membrane (Stage 4). Carbon filters are excellent at removing chlorine, taste, odour, and organic chemicals, and that’s exactly what they do in our whole-house water filter system. But dissolved contaminants like fluoride, lead, salts, nitrates, and PFAS pass straight through carbon. Only the RO membrane’s 0.0001 micron pore size can physically block them.
For a detailed comparison of what whole-house carbon filtration removes vs what RO removes, see our whole-house vs under sink comparison
Reverse Osmosis vs Whole House Filter: Different Jobs
One of the most common questions we get is whether you need a reverse osmosis system, a whole-house filter, or both. The short answer: they do different things.
Feature | Under Sink Reverse Osmosis | Whole House Carbon Filter |
Where it installs | Under your kitchen sink dedicated drinking tap | On the main water line, every tap in the home |
What it filters | Everything, including fluoride, dissolved salts, heavy metals, and PFAS | Chlorine, chloramine, sediment, VOCs, and some heavy metals |
Removes fluoride? | ✓ Yes | ✗ No |
Flow rate | Slower is designed for drinking and cooking water only | Full house flow showers, laundry, appliances, all taps |
Best for | Purest possible drinking water at the kitchen tap | Clean water for the entire home, bathing, appliances, and all outlets |
Installed price | From $1,100 inc. GST | From $1,100 inc. GST |
Many of our Melbourne customers install both systems: the whole house filter handles chlorine, chloramine, and sediment for every tap, shower, and appliance, while the RO system delivers the purest possible drinking water at the kitchen sink. The combined installed price starts from $2,200 with 0% finance available. More details on when each option makes sense in our whole house vs under sink comparison.
Who Actually Needs a Reverse Osmosis System?
RO isn’t for everyone, and we’ll tell you that honestly. If your main concern is chlorine taste and you’re happy with Melbourne’s water quality otherwise, a whole-house carbon filter may be all you need, but reverse osmosis is the right choice if:
- You want fluoride removed — carbon filters don’t remove fluoride. Only RO or distillation can. If fluoride is a concern, RO is the practical residential solution.
- You have specific health concerns about dissolved contaminants — including lead, arsenic, nitrates, PFAS, pharmaceuticals, or dissolved salts. RO addresses all of these at once
- You want the purest possible drinking water — whether for general health, for infants and young children, or for specific dietary needs.
- You’re tired of buying bottled water — a household of four spends approximately $1,500–$2,500 per year on bottled water. An RO system eliminates this cost and produces water that’s cleaner than most bottled brands.
- You already have a whole-house filter and want drinking water purity taken to the next level — the dual-system setup gives you complete home coverage plus laboratory-grade drinking water at the kitchen tap.
Frequently Asked Questions
Is reverse osmosis water safe to drink?
Yes. RO water is among the safest drinking waters available. Our 7-stage system goes further than basic RO by restoring beneficial minerals and balanced pH (Stage 6), so you get the purity of reverse osmosis with a natural, healthy mineral content.
Does reverse osmosis remove fluoride?
Yes. This is one of the primary reasons people choose RO. The membrane (Stage 4) removes fluoride along with other dissolved contaminants that carbon filters cannot remove. It’s the most practical residential method for fluoride removal.
Does RO waste a lot of water?
Modern RO systems are significantly more efficient than older designs. Typical waste ratios are now around 2–3 litres of wastewater per litre of purified water (older systems could be 5:1 or worse). Since the system only produces water when you use the drinking tap, total water usage is modest for most households.
Does RO remove minerals from water?
The RO membrane removes virtually everything, including minerals. That’s why our system includes Stage 6 (alkaline mineral restoration), which adds back calcium, magnesium, and potassium and raises the pH to a healthy alkaline level of approximately 8.5–9.0. You get the purity without the flat taste.
How much does an under-sink RO system cost in Melbourne?
Our 7-stage reverse osmosis system is fully installed for $1,100 including GST. That includes the system, all cartridges, dedicated drinking tap, storage tank, and licensed plumber installation. If combined with a whole-house filter, the total is $2,200 with 0% finance available.
Do I need a whole-house filter AND an RO system?
Not necessarily it depends on your priorities. A whole-house filter handles chlorine, chloramine, sediment, and chemical contaminants at every tap. An RO system provides the purest possible drinking water at the kitchen sink, including fluoride and dissolved contaminant removal. Many customers choose both for complete coverage. We cover when each option makes sense in our whole house vs under sink comparison
Understanding Reverse Osmosis How Your: Water Gets Purified
