- ✓RO is the only residential technology that reliably removes fluoride, PFAS, nitrates, dissolved heavy metals and dissolved salts from drinking water. It does this by filtering at the molecular level through a semi-permeable membrane.
- !RO strips all minerals including beneficial ones — calcium, magnesium, potassium. A remineralisation post-filter is strongly recommended. Without it, RO water tastes flat and is slightly acidic.
- →Modern RO systems waste 1–2 litres per litre produced — roughly a toilet flush per day for a typical household. Much less than older 1:4 systems. Waste water is not contaminated.
- ✓NSF 58 certification is the standard to verify. Check the exact product model at nsf.org — not just the brand. Certified membranes in uncertified systems are not the same as NSF 58 certified complete systems.
- ✗RO is not necessary for every household. If your main concern is chlorine or chloramine taste, a carbon block filter does this at a fraction of the cost. RO is most justified for fluoride, PFAS, or high-mineral water.
How reverse osmosis actually works
Reverse osmosis uses pressure to force water through a semi-permeable membrane with pores at the 0.0001-micron scale. To put that in context: a human hair is approximately 70 microns wide. A standard sediment filter removes particles down to 1–5 microns. An RO membrane removes dissolved ions and molecules — things that have already dissolved into the water and become part of it, not just particles suspended in it.
When pressurised water contacts the RO membrane, water molecules pass through but larger dissolved molecules cannot. The contaminants collect in a concentrate stream that is flushed to drain. The purified water (permeate) collects in a pressure tank or, in tankless systems, flows on demand from the post-filter stages.
A complete residential RO system typically has 3–5 stages in sequence: sediment pre-filter → carbon pre-filter → RO membrane → carbon post-filter (polishing) → optional remineralisation. Each stage protects the next. The carbon pre-filter is critical — chlorine and chloramine degrade RO membranes rapidly if not removed first.
What RO removes — and what it doesn’t
| Contaminant | RO removal rate | Standard | Notes |
|---|---|---|---|
| PFAS (PFOA, PFOS) | 90–96% | NSF 58 / 53 | Most effective residential technology for PFAS. Membrane quality matters significantly. |
| Fluoride | 90–96% | NSF 58 | Only reliable residential method. See our fluoride guide. |
| Lead | 95–98% | NSF 58 / 53 | Highly effective. Important for homes with old copper plumbing. |
| Arsenic (As V) | 85–95% | NSF 58 / 53 | pH affects removal. Effective at typical residential pH levels. |
| Nitrates | 85–95% | NSF 58 | Important for rural areas with agricultural runoff. Carbon alone does not remove nitrates. |
| Chlorine / chloramine | 95%+ (with carbon pre-filter) | NSF 42 | The carbon pre-filter does this work, not the membrane itself. |
| Dissolved salts / TDS | 90–97% | NSF 58 | Produces very low TDS water. Remineralisation is recommended for taste. |
| Bacteria / viruses | 99%+ | NSF 58 | Membranes physically exclude microorganisms. UV adds further assurance. |
| Microplastics | 99%+ | NSF 58 | RO membranes block particles at sub-micron scale. |
| Calcium / magnesium (hardness) | 90–97% | NSF 58 | Completely removes hardness from drinking water. Remineralisation stage adds back beneficial amounts. |
| Hydrogen sulphide (rotten egg) | Partial | N/A | Carbon pre-filter does most of this work. Aeration more effective for high H₂S. |
| Pharmaceuticals / herbicides | 90–98% | NSF 58 (some) | Strong performance on most organic compounds. Specific certifications vary by contaminant. |
What RO does not effectively remove
RO is not a perfect filter for everything. It performs poorly on dissolved gases — CO₂, hydrogen sulphide, radon — because gas molecules are small enough to pass through the membrane. It also has limitations with certain volatile organic compounds (VOCs) depending on molecular weight. The carbon pre-filter stages address most of these.
The more important limitation is that RO removes virtually all minerals from water, including beneficial ones — calcium, magnesium, potassium. This produces slightly acidic, flat-tasting water. This is why remineralisation is strongly recommended as the final stage in a residential RO system: it restores a mineral profile similar to good spring water, raises pH to approximately 7.5–8.5, and significantly improves taste.
Waste water — the main practical concern
Every RO system produces some waste water: the concentrate stream that carries the rejected contaminants to drain. In older systems, the ratio could be 3–4 litres wasted per litre produced. Modern high-efficiency membranes and tankless systems achieve 1:1 to 1:2 — one to two litres wasted per litre produced.
For the average household producing 3–6 litres of RO drinking water per day, this means approximately 3–12 litres of waste water daily — roughly equivalent to half to one toilet flush. This is a manageable quantity, not an environmental catastrophe as some anti-RO marketing suggests. The waste water is not contaminated — it is simply more concentrated mains water — and can be collected for garden use if desired.
When evaluating an RO system, ask for the specific waste water ratio at normal operating pressure. Avoid systems that won't provide this figure.
Tankless vs tank-type RO systems
Traditional RO systems include a pressure storage tank (typically 3–8 litres) that stores pre-filtered water ready for use. Tankless systems filter on demand — water passes through the membrane when you open the tap. Both have their place:
- Tank-type: Higher flow rate at the tap (tank empties faster than the membrane can produce). Better for high-volume families. Tank requires periodic sanitisation — a maintenance item often overlooked.
- Tankless: No stagnant storage, therefore fresher water and lower bacteria risk. Slower flow (limited by membrane production rate). Better for smaller households and where bench space under the sink is limited.
Who RO is right for
RO is the right choice when:
- You want fluoride removal from drinking water
- You are concerned about PFAS in your local supply (Blue Mountains zone, near industrial/defence sites)
- You are on bore water or tank water with high TDS, salinity, or dissolved heavy metals
- You are in Adelaide and want to address Murray River mineral content and salinity
- You want the most comprehensive contaminant removal available in a residential system
RO is probably not necessary if:
- Your main concern is chlorine or chloramine taste — a good carbon block filter addresses this at much lower cost
- You are in Melbourne — very soft, low-TDS water where the main issue is just taste
- You want to treat the whole house — whole-home RO is impractical for most households (cost, waste water volume, flow rate limitations)
NSF 58 — the certification that matters for RO
NSF 58 is the certification standard for reverse osmosis drinking water systems. It covers membrane rejection rates, system construction materials, and contaminant reduction claims. An NSF 58-certified system has been independently tested and verified to meet specific performance benchmarks. Verify any system at nsf.org using the exact product model before purchasing.
Some suppliers sell "RO systems" that have not been certified under NSF 58 — they may use RO membranes but the complete assembled system has not been independently verified. Without NSF 58, the contaminant removal claims are unverified manufacturer assertions.
RO is genuinely the best residential technology for PFAS, fluoride, lead, nitrates, and dissolved mineral removal. It is not the right tool for every household — a good carbon filter is cheaper and sufficient for most city households whose main concern is taste. But where comprehensive purification is the goal, RO + remineralisation is the right answer.
If you're considering RO, verify NSF 58 certification for the complete system at nsf.org. Check whether the system includes a remineralisation stage. Compare waste water ratios. Use our comparison tool to find suppliers who stock verified RO systems.