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How does desalination work?

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Quick answer

Desalination separates salt from water using either high-pressure membranes (reverse osmosis) or evaporation and condensation (distillation). Reverse osmosis forces seawater through a membrane that blocks dissolved ions; thermal plants boil water and collect the pure vapour. Both need substantial energy and produce concentrated brine that must be managed carefully.

Desalination removes dissolved salt from seawater or brackish water by forcing water through a salt-rejecting membrane (reverse osmosis) or by boiling and recondensing it (thermal distillation). The result is fresh water, but the process needs large amounts of energy and leaves behind a concentrated brine that must be disposed of.

Step by step

  1. Seawater is drawn from the ocean or a brackish well and screened to remove debris, sand and large organisms.
  2. The water is pre-treated with filters and chemicals to stop scale, corrosion and membrane fouling.
  3. In reverse osmosis, high-pressure pumps force the water through a semipermeable membrane that rejects dissolved salts.
  4. In thermal distillation, the water is heated until it evaporates, and the vapour is condensed into fresh liquid.
  5. The fresh water is remineralised and disinfected before it enters the drinking-water supply.
  6. The remaining brine is diluted or disposed of according to environmental regulations.

Key things to understand

  1. Seawater contains roughly 3.5% dissolved salts, mostly sodium and chloride, which must be separated from the water molecules.
  2. Reverse osmosis pushes water at high pressure through a semipermeable membrane whose pores block dissolved ions while letting water pass.
  3. Thermal distillation works by heating seawater until it evaporates, then condensing the pure vapour, leaving salts behind as brine.
  4. Both methods consume significant energy, which is why desalination is expensive compared with drawing water from rivers or aquifers.
  5. The leftover brine is saltier than the original seawater and is usually discharged back into the sea or injected underground.

Labelled diagram

  1. Intake

    Seawater is drawn from the ocean and screened for debris.

  2. Pre-treatment

    Filters and chemicals remove particles that would clog membranes.

  3. High-pressure pump

    Pressure forces water against a semipermeable membrane.

  4. Membrane

    Water molecules pass through; dissolved salt ions are rejected.

  5. Fresh water

    Permeate is remineralised and sent to the water supply.

  6. Brine outfall

    Concentrated salt solution is discharged or diluted.

How reverse osmosis desalination works

Real-world examples

  • The Sorek plant in Israel uses reverse osmosis and supplies a large share of the country's domestic water.
  • Saudi Arabia relies heavily on thermal distillation and reverse osmosis because it has little freshwater.
  • Perth, Australia, runs a reverse-osmosis plant whose electricity use is matched by power bought from a wind farm.
  • The Carlsbad plant in California turns seawater into drinking water for the San Diego region.

Common mistake

✗ Myth: Desalination creates new water that did not exist before.

✓ Fact: Desalination does not create water; it separates fresh water from salty water that already exists. The total amount of water on Earth stays the same, but the fresh portion becomes usable for drinking and irrigation.

People also ask

What is the difference between reverse osmosis and distillation desalination?
Reverse osmosis uses pressure to push water through a membrane that blocks salt ions, while distillation uses heat to evaporate water and leave salts behind. Reverse osmosis generally uses less energy and is now the dominant method worldwide. Distillation is more common where cheap heat or waste heat from power plants is available.
How much energy does desalination use?
Reverse osmosis typically uses about 3 to 4 kilowatt-hours of electricity per cubic metre of fresh water, though the figure varies with salinity and plant design. Thermal distillation uses several times more energy because it relies on boiling. Energy recovery devices and renewable power have reduced the electricity demand of modern membrane plants.
Does desalination harm the ocean?
Discharging concentrated brine can raise salinity and temperature near the outfall, which may stress bottom-dwelling marine life. Modern plants dilute brine with seawater or use diffusers to spread it quickly. Intake pipes can also trap fish and larvae, so many facilities use low-velocity screens to reduce the impact.
Why is desalination so expensive?
Desalination is expensive mainly because separating salt from water requires large amounts of energy, whether to drive high-pressure pumps or to boil water. Membranes and corrosion-resistant equipment also cost money to build and replace. The price per litre is falling as energy recovery devices and renewable power improve, but it remains higher than most conventional freshwater sources.
What happens to the salt left over after desalination?
The leftover concentrated salt solution, called brine, is typically piped back into the ocean, where it can sink and affect marine life if not diluted. Some plants spread it on land or inject it into deep wells. Researchers are testing ways to extract useful minerals like magnesium and table salt from brine instead of discarding it.
Can desalination run on solar or wind power?
Yes. Solar panels and wind turbines can generate the electricity that drives reverse-osmosis pumps, and solar thermal collectors can supply heat for distillation. Because desalination is energy-intensive, pairing it with renewable power reduces both fuel costs and carbon emissions. Several countries, including Saudi Arabia and Australia, already operate large renewable-powered plants.
Is desalinated water safe to drink?
Yes, desalinated water is safe to drink after treatment. Reverse osmosis and distillation remove salts and most contaminants, but the water is often remineralised with small amounts of calcium and magnesium to improve taste and protect pipes. Health authorities test the output regularly to ensure it meets drinking-water standards.

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