Treatment process
Reverse osmosis
A dense semi-permeable membrane that rejects almost everything dissolved. Reverse osmosis is the most comprehensive treatment available at household scale, and the basis of nearly all modern seawater desalination.
Key facts
- Scale
- Municipal and household
- Treatment stage
- Membrane separation
- Energy
- Seawater desalination by RO uses roughly 3–4 kWh per cubic metre in modern plants with energy recovery, against a thermodynamic minimum near 1 kWh/m³. Brackish RO uses roughly 0.5–1.5 kWh/m³.
- Certification
- Household systems are certified to NSF/ANSI 58.
How it works
Pressure exceeding the osmotic pressure of the feed drives water through a membrane that rejects ions and molecules above roughly 100 daltons. Rejected material leaves in a concentrate stream. Seawater requires 55–70 bar; brackish water and household units far less.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Dissolved salts, typically 95–99% | Dissolved gases including carbon dioxide, which passes through and lowers the pH of the product water |
| 2 | Nitrate, fluoride, arsenic, lead and other metals | Some very low molecular weight neutral organics |
| 3 | PFAS across the whole family, including short-chain | Nothing, if the membrane is damaged — and a household unit gives no indication that it has been |
| 4 | Pesticides and pharmaceuticals | — |
| 5 | Bacteria, viruses and protozoa | — |
| 6 | Hardness | — |
Trade-offs
- Removes beneficial minerals as well, so remineralisation is standard practice at municipal scale
- Produces a concentrate stream: household units typically waste two to four litres per litre produced, though modern permeate-pump designs do better
- Product water is slightly acidic and mildly aggressive to plumbing
- Slow flow rate at household scale, requiring a storage tank
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Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted