Reverse Osmosis
How reverse osmosis works
Reverse osmosis forces water under pressure through a semi-permeable membrane with pores approximately 0.0001 microns in diameter. This is smaller than most dissolved contaminants — including lead ions, PFAS compounds, fluoride, arsenic, and nitrates — which are rejected by the membrane and flushed down a drain line.
The process happens in multiple stages in a typical under-sink RO system:
- Pre-filtration (sediment): removes particles that would clog the RO membrane
- Carbon pre-filter: removes chlorine, which degrades the membrane
- RO membrane: the core stage — removes 95–99% of dissolved contaminants
- Post-filter (carbon polish): removes residual taste and odour from the storage tank
- (Optional) Remineralisation: reintroduces calcium and magnesium
What RO removes — and what it does not
Removes with high efficiency (NSF 58 certified):
- PFOA, PFOS, and other PFAS compounds
- Lead (95%+)
- Fluoride (90–95%)
- Arsenic (95%+)
- Nitrates (75–90%)
- Chromium, cadmium, barium
- Bacteria and viruses (via size exclusion — membrane pore is 0.0001 micron)
- TDS (total dissolved solids) at 95–99%
Does NOT remove effectively:
- Chlorine (handled by carbon pre-filter stage, not the RO membrane)
- Dissolved gases (radon, hydrogen sulphide)
- Some pesticides (depends on molecular size)
The mineral removal tradeoff
RO membranes do not distinguish between dissolved lead (harmful) and dissolved calcium or magnesium (beneficial). Both are rejected. Output TDS from an RO system is typically 5–20 ppm — near zero — which means near-zero mineral content.
In hard-water areas, tap water contributes roughly 10–20% of daily calcium and magnesium intake. RO-filtered water without remineralisation removes that contribution. A remineralisation stage (Stage 5 or 6 cartridge in most RO kits) reintroduces calcium and magnesium carbonate, bringing output TDS to 30–60 ppm and significantly improving taste.
Most buyers in hard-water areas should add the remineralisation stage.
NSF 58 certification — what it means
NSF/ANSI 58 is the standard for reverse osmosis systems. Certified products have been independently tested to verify:
- TDS reduction (minimum threshold)
- Specific contaminant reductions (lead, fluoride, arsenic, PFAS)
- Material safety (no harmful substances leaching from system components)
Verify any RO system at info.nsf.org/Certified/dwtu/ before buying. Do not rely on manufacturer pages for certification confirmation.
Water waste
Standard RO systems produce 3 gallons of wastewater (drain line) for every 1 gallon of filtered output — a 3:1 waste ratio. Modern tankless RO systems (Waterdrop G3, Frizzlife PD600) have improved this to 1.5:1 or 2:1. For households on metered water supply, this is worth factoring into running costs.
RO vs gravity filtration vs activated carbon — the one-line summary
| Method | PFAS | Lead | Fluoride | Bacteria | Minerals | Install |
|---|---|---|---|---|---|---|
| RO (NSF 58 certified) | Yes | Yes | Yes | Yes (via membrane) | Removes (remineralise) | Under-sink |
| Gravity (Berkey) | Unverified (2026) | Claimed | No | Yes | Retains | Countertop, no install |
| Activated carbon (Brita) | No (Elite: partial) | Partial (Elite) | No | No | Retains | Pitcher, no install |