How desalination works
Separating salt from seawater takes energy — physics sets a floor of about 1 kWh per cubic metre, and modern plants get within a factor of three of it.
Desalination converts the 97% of Earth’s water that is salty into water that can be used. It is not a new idea — sailors were distilling seawater centuries ago — but it only became economically viable at scale when membranes replaced boiling.
The energy floor
Separating salt from seawater has an irreducible thermodynamic cost. The minimum work required to produce fresh water from seawater at typical recovery is around 1 kWh per cubic metre. No technology can go below it.
Modern seawater reverse osmosis plants use roughly 3 to 4 kWh per cubic metre — within a factor of three of the theoretical limit, which is a remarkable engineering achievement and also means the scope for further reduction is limited. Efficiency gains from here are incremental, not transformational.
| Technology | Specific energy (kWh/m³) | Note |
|---|---|---|
| Thermodynamic minimum, seawater | ~1.0 | Physics; unattainable in practice |
| Seawater RO, modern with energy recovery | 3–4 | The dominant technology for new capacity |
| Seawater RO, 1980s | 8–10 | Before pressure exchangers |
| Brackish water RO | 0.5–1.5 | Far cheaper — lower salinity means lower osmotic pressure |
| Multi-stage flash (thermal) | 10–25 equivalent | Viable where waste heat is available |
| Multi-effect distillation | 6–15 equivalent | More efficient than MSF, still well above RO |
| Conventional surface water treatment, for comparison | 0.2–0.4 | An order of magnitude below desalination |
Thermal figures are converted to electrical equivalent and depend heavily on the assumed conversion. Ranges reflect plant design, feedwater salinity and recovery ratio.
The brine problem
Every desalination plant produces two streams: fresh water and concentrate. A seawater plant recovering 45% of its feed produces roughly 1.2 cubic metres of concentrate for every cubic metre of product, at roughly twice the feed salinity, often a few degrees warmer, and containing the antiscalants and cleaning chemicals used in the process.
Discharged badly, it sinks and forms a dense hypersaline layer on the seabed that suppresses benthic life. Discharged well — through a diffuser, into an area with strong currents, or blended with power station cooling water — it disperses to near-background salinity within a short distance. Brine management is a design problem with known solutions rather than an unsolved obstacle, but it is a real cost and a real constraint on siting.
When desalination makes sense
- Where there is no adequate freshwater alternative — the Gulf states, Malta, many small islands
- Where drought resilience justifies the cost of standby capacity, as in Australian coastal cities
- Where energy is cheap relative to water, which is the underlying economics of the Gulf
- For brackish groundwater, where the energy cost is a fraction of seawater and the case is much easier
Sources
- International Renewable Energy Agency / academic literature — Desalination capacity and energy intensity assessments. Open — attribution required · CC BY 4.0
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted
- International Energy Agency — Water–energy nexus analysis. Public but restricted · Publisher terms — public access, reuse not clearly granted