- →RO works by pressure — 60–80 PSI forces water through a 0.0001-micron membrane that only water molecules can pass. Everything dissolved is rejected to a waste stream.
- ✓What RO removes: fluoride (95–96%), lead (95–98%), PFAS (90–96%), nitrates (87–90%), TDS, bacteria and viruses. The most comprehensive household filtration available.
- !Waste water ratio: 3–5 litres waste per litre of clean water. For typical 5–15L daily drinking use this is a small fraction of total household consumption.
- →Four stages: sediment pre-filter → carbon pre-filter → membrane → post-carbon. The carbon pre-filter protects the membrane from chlorine damage.
- ✓Verify with NSF 58 certification. Test TDS rejection annually — below 80% means the membrane needs replacing.
The core idea — pressure through a tiny membrane
Reverse osmosis is simple in principle: apply enough pressure to force water through a membrane with pores so small that only water molecules can fit. Everything dissolved — salts, fluoride, lead, nitrates, PFAS — is too large to pass and gets diverted to a waste stream.
Normal osmosis is the natural process where water flows from dilute to concentrated through a membrane. Reverse osmosis applies pressure to force the opposite — pushing water from the contaminated side through the membrane, leaving contaminants behind.
The four stages
Stage 1 — Sediment pre-filter: Removes particles, rust and coarse sediment before they reach the membrane. Physical fouling is the main cause of premature membrane failure. Replace every 6 months.
Stage 2 — Carbon pre-filter: Removes chlorine and chloramine before the membrane. This is critical — chlorine chemically degrades the thin-film polyamide membrane rapidly. Replace every 6–12 months.
Stage 3 — RO membrane: The core. At 0.0001 micron pore size, it rejects dissolved ions, metals, fluoride, nitrates, PFAS, bacteria and viruses. Replace every 2–4 years.
Stage 4 — Post carbon polisher: Removes any residual taste from the storage tank before the tap. Replace every 12 months.
The waste water ratio
RO produces two streams: clean water (permeate) and waste water carrying the rejected contaminants. Typical systems produce 3–5 litres of waste per litre of clean water. For household drinking and cooking use of 5–15 litres per day, this is a small fraction of total water consumption. The waste water contains the concentrated contaminants in a small volume — it goes to drain.
What RO actually removes: fluoride (95–96%), lead (95–98%), PFAS (90–96%), nitrates (87–90%), TDS (85–96%), arsenic (93–96%), chloramine (93–97%), bacteria and viruses. It is the most comprehensive single water treatment technology available for household use.
What RO does not do well
- Speed: RO produces water slowly (0.5–2L per hour) and stores it in a tank. Not practical for high-flow whole-home use.
- Minerals: RO removes beneficial minerals alongside harmful ones. Output at very low TDS tastes flat. A remineralisation post-filter is optional but improves taste. RO water is safe for formula-fed infants — formula provides all required minerals.
- Espresso machines: Zero-mineral RO water corrodes metal boiler components. Always remineralise before using in an espresso machine.
NSF 58 certification
NSF 58 is the standard that covers reverse osmosis systems as a whole — pre-filters, membrane, and post-filter together. It verifies the complete system against claimed reduction percentages for TDS, lead, fluoride, nitrates, and other parameters. Verify any specific product at nsf.org before purchasing.
To check if your existing RO membrane is still performing: use a TDS meter before and after the membrane. A working RO system reduces TDS by 85–96%. Below 70% reduction means the membrane is degrading. Below 50% means replace immediately.
The case for RO is strongest in Adelaide (highest TDS and sodium of any capital), for households with pre-1980s plumbing (lead risk), near known PFAS contamination sites, and for families who want fluoride reduced.
RO is a point-of-use technology for the kitchen tap — not a whole-home solution. Pair it with a TAC or carbon whole-home system for complete protection.