- What it removes: Up to 99% of contaminants — fluoride, heavy metals, PFAS, chlorine, nitrates, bacteria, microplastics
- Installed cost: $800–$2,000 for a quality 5–7 stage under-sink system in Australia
- Annual maintenance: $150–$300 for pre-filters, post-filter, and membrane replacement (averaged)
- Payback vs bottled water: Typically 6–12 months for a family of 4
- Key consideration: Requires minimum 40 PSI water pressure. Produces 3–4L of waste water per 1L filtered.
How reverse osmosis works
Reverse osmosis works by pushing water under pressure through a semi-permeable membrane — a thin film with pores 0.0001 microns in diameter. To put this in perspective: a water molecule is approximately 0.0003 microns; a chlorine molecule is 0.0012 microns; a fluoride ion is 0.0013 microns; a PFAS molecule is 0.0006–0.0040 microns. The RO membrane is small enough to allow water through while blocking virtually everything else.
The water that passes through the membrane — the "permeate" — collects in a pressurised storage tank under the sink. Water that doesn't pass through — the "reject" or "concentrate" — carries the removed contaminants down the drain. This is why RO systems produce waste water: every litre of filtered water typically requires 3–4 litres of input water.
What RO removes — and what it doesn't
| Contaminant | RO Removal Rate | Notes |
|---|---|---|
| Chlorine and chloramines | ~99% | Also removed by pre-carbon filters in the system |
| Fluoride | 90–96% | Standard carbon filters do NOT remove fluoride |
| Lead | 95–99% | Critical for older homes with lead pipe fittings |
| Arsenic | 92–99% | Relevant for bore and some regional water supplies |
| PFAS / PFOA / PFOS | 90–98% | Activated carbon also helps; combined system is most effective |
| Nitrates | 70–90% | Important for rural areas and bore water |
| Bacteria | 99%+ | Membrane physically blocks bacteria |
| Viruses | 99%+ | Combined with UV for absolute certainty on tank water |
| Microplastics | ~100% | Emerging concern; RO is highly effective |
| Beneficial minerals (Ca, Mg) | 95–99% | Remineralisation stage adds these back in premium systems |
Understanding RO stages
RO systems are described by the number of filtration stages they include. More stages generally means broader contaminant removal and better-tasting water — but higher cost and more cartridges to replace.
Standard 5-stage RO
Sediment pre-filter → carbon block pre-filter → carbon pre-filter → RO membrane → carbon post-filter. Handles chlorine, sediment, fluoride, heavy metals, and most contaminants. The baseline for most residential Australian installations. Installed cost: $800–$1,200.
Premium 7-stage RO with remineralisation
Adds an alkaline remineralisation cartridge (adds calcium and magnesium back to the water) and a final polishing stage. Produces better-tasting water and avoids the slightly flat taste of pure RO water. Preferred for those using the system as a complete bottled water replacement. Installed cost: $1,100–$1,600.
RO + UV system
For tank water, bore water, or any situation where biological contamination is a concern. The UV stage (after the RO membrane) kills any bacteria or viruses that might pass through the membrane in the event of membrane degradation. Not necessary for town mains water. Installed cost: $1,200–$2,000.
What it actually costs in Australia
| Component | Cost | Frequency |
|---|---|---|
| System + installation (7-stage) | $1,100–$1,600 | Once |
| Pre-filter cartridges (x2) | $40–$80 | Every 6–12 months |
| Post-filter / polishing | $25–$50 | Annually |
| Remineralisation cartridge | $30–$60 | Annually |
| RO membrane | $80–$180 | Every 2–3 years |
| Total annual cost (averaged) | $150–$300/yr | — |
A Melbourne family of 4 spending $40/week on bottled water spends $2,080/year. A 7-stage RO system installed at $1,100 costs $200/year to maintain. Year 1 total: $1,300. Year 2 onwards: $200. Over 10 years: RO costs $2,900 vs bottled water $20,800. Every litre of RO water costs approximately 1–3 cents. Every litre of bottled water costs $0.40–$2.00.
Frequently asked questions: reverse osmosis
Yes — RO water is safe to drink long-term. The concern sometimes raised is that RO removes beneficial minerals like calcium and magnesium. However, Australians get the vast majority of their mineral intake through food, not water. A well-balanced diet more than compensates for any minerals removed by RO filtration. For those who prefer mineralised water, a remineralisation stage can be added to the system — this also improves taste.
Standard tank-based RO systems fill a 250ml glass in approximately 30–60 seconds — noticeably slower than mains pressure. Modern tankless ("booster pump") RO systems significantly improve flow rate and are now available in Australia at a higher cost. For most users, the fill speed is adequate for drinking and cooking — the tank refills continuously when not in use, so there is always filtered water available.
RO membranes typically last 2–3 years under normal household use. Signs it needs replacing: water tastes different or starts to taste like unfiltered tap water again, water pressure from the RO tap drops significantly, or you're overdue based on the manufacturer's schedule. The simplest approach: use an inexpensive TDS (total dissolved solids) meter to test your RO water. If TDS rises significantly above baseline, the membrane is degrading.