Desalination removes salt from seawater or brackish water to produce fresh drinking water. The technology has been in commercial use for decades but has scaled dramatically as water scarcity intensifies. This guide covers reverse osmosis, thermal distillation, and emerging technologies, plus their energy, cost, and environmental characteristics.
The basic problem
Seawater contains roughly 35,000 mg per litre of dissolved solids, primarily sodium chloride. Drinking water needs to be below 500 mg per litre total dissolved solids. Removing that salt is fundamentally an energy question: physics dictates a minimum energy of about 1 kWh per m3 to separate seawater; real plants use 3 to 5 kWh per m3 including losses.
The main technologies
| Technology | How it works | Energy (kWh/m3) |
|---|---|---|
| Reverse osmosis (RO) | Pressure driven separation through membrane | 3 to 5 |
| Multi stage flash (MSF) | Multiple evaporation stages at reducing pressure | 10 to 20 including thermal |
| Multi effect distillation (MED) | Multiple evaporation with heat recovery | 7 to 15 including thermal |
| Electrodialysis | Electric field drives ion migration | Brackish only, low |
| Forward osmosis | Osmotic pressure driven, emerging | Emerging |
Reverse osmosis
RO is the dominant technology globally. Seawater is pumped at high pressure (55 to 80 bar) through semi permeable membranes that let water pass but reject salt. Pressure energy is recovered from the reject stream by energy recovery devices, dramatically improving efficiency. Modern systems achieve 40 to 50 percent water recovery from seawater.
Key components
- Intake (open ocean or beach well).
- Pretreatment (coagulation, filtration, microfiltration).
- High pressure pump.
- Membrane bank (multiple parallel trains).
- Energy recovery device.
- Post treatment (remineralisation, disinfection).
- Brine discharge system.
Thermal distillation
MSF and MED plants use heat to evaporate seawater and then condense the vapour. Common in Middle East where waste heat from co located power plants provides thermal energy at low cost. MSF and MED share the top rank at large Saudi plants; RO dominates newer installations.
Feed water types
| Feed water | Salinity (mg/L) | Typical technology |
|---|---|---|
| Seawater | 30,000 to 40,000 | RO, MSF, MED |
| Brackish groundwater | 1,000 to 10,000 | RO (lower pressure) |
| River saline | Under 1,000 | Nanofiltration |
| Wastewater reuse | Variable | UF plus RO for potable reuse |
Energy efficiency
Modern RO has closed much of the gap to theoretical minimum energy. Further reductions expected from membrane improvements, energy recovery advances, and low pressure operation.
Cost trajectory
Desalination cost per cubic metre has fallen from over USD 5 in the 1990s to USD 0.50 to 1.50 today for large seawater plants. Continued reductions expected. See our companion article on 15 largest desalination plants.
Environmental considerations
| Consideration | Mitigation |
|---|---|
| Brine discharge | Diffuser design, brine dilution, ZLD in specific cases |
| Intake mortality | Slow intake velocities, screens, subsurface intakes |
| Energy carbon footprint | Renewables powered desalination |
| Chemical use | Optimised pretreatment, alternative chemistry |
Brine discharge
Renewable powered desalination
Solar and wind powered desalination is scaling. Chile, Australia, and Morocco have solar and wind coupled projects. Cost economics work well when renewable electricity is cheap and load matching is designed in.
Regulatory context
Desalination is regulated at national and state levels. Water quality standards must meet drinking water regulations. Environmental permits for intake and discharge. The EPA drinking water regulations apply to US desalination plants.
Global scale
Roughly 22,000 desalination plants operate globally producing 100 million m3 per day. See our companion article on how many desalination plants globally.
Where desalination is going
- Continued cost reduction through membrane improvements.
- Renewable powered plants scaling.
- Small modular units for isolated communities.
- Zero liquid discharge for inland brackish water plants.
- Membrane technology improvements including aquaporin biomimetic.
- Integration with wastewater reuse.
Frequently asked questions
Is desalinated water safe?
Yes when properly post treated. Meets drinking water standards.
Does desalination taste different?
Post treatment adds minerals for taste. Most people cannot distinguish.
How much energy?
3 to 5 kWh per m3 for modern RO seawater. About 0.5 percent of typical household electricity per m3.
Is it expensive?
USD 0.50 to 1.50 per m3. More than fresh water where available; cheaper than most alternatives in arid regions.
Where is desalination used?
Middle East, Israel, Australia, coastal US, Chile, Spain, and increasingly globally.
What about environmental impact?
Managed impacts on intake mortality and brine discharge. Cannot be ignored but manageable.
Can we desalinate at small scale?
Yes. Solar powered small units serve remote communities.
Are RO and MSF competitive?
RO dominates new build. MSF and MED remain competitive with cheap waste heat.
Is desalination sustainable?
Renewable powered can be. Fossil fuel powered has significant carbon footprint.
Where can I see plants?
The UtilityRadar directory lists desalination plants.
Summary
Desalination is a mature and increasingly cost effective water supply technology. Reverse osmosis dominates new deployment; thermal distillation persists where waste heat is available. Modern RO uses 3 to 5 kWh per m3 producing water at USD 0.50 to 1.50 per m3. Environmental impacts on intake and brine discharge require careful management. Growing global deployment is transforming water supply for arid regions.
Next reading
- 15 largest desalination plants
- Global desalination plant count
- How a water treatment plant works
- Browse the UtilityRadar directory
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