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How does reverse osmosis work?

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Quick answer

Reverse osmosis forces water through a semi-permeable membrane at a pressure higher than the solution's natural osmotic pressure. The membrane passes water molecules but rejects most dissolved salts and larger molecules, leaving a low-salt permeate and a concentrated brine that is flushed away.

Reverse osmosis works by pushing water at high pressure through a semi-permeable membrane whose tiny pores block dissolved salts and most other molecules. The pressure must exceed the natural osmotic pressure that would otherwise pull pure water toward the salty side.

Step by step

  1. Seawater or brackish water is pre-treated to remove particles that could clog the membrane.
  2. A high-pressure pump pushes the feed water against one side of the semi-permeable membrane.
  3. Water molecules squeeze through the membrane's tiny pores while dissolved salts are rejected.
  4. The purified water, called permeate, collects on the low-pressure side of the membrane.
  5. The remaining concentrated brine is discharged or further processed to reduce its impact.

Key things to understand

  1. Osmosis is the natural flow of water across a membrane from a dilute solution toward a concentrated one.
  2. The membrane allows water molecules through but rejects most dissolved ions and larger molecules.
  3. Applying pressure above the solution's osmotic pressure reverses the natural direction of water flow.
  4. Rejected salts stay behind and are flushed away as a concentrated brine stream.
  5. Only water that passes the membrane emerges as low-salt permeate, which may need mineral adjustment.

Labelled diagram

  1. Feed water

    Salty water enters at high pressure on one side of the membrane.

  2. High-pressure pump

    Pushes the feed above the solution's natural osmotic pressure.

  3. Semi-permeable membrane

    Pores let water molecules pass but block dissolved salts.

  4. Permeate

    Low-salt purified water collects on the other side.

  5. Brine

    Concentrated rejected salts are flushed away.

Reverse osmosis across a membrane

Real-world examples

  • Desalination plants in the Middle East and California use reverse osmosis to turn seawater into drinking water.
  • Many household under-sink water purifiers use a reverse osmosis membrane to reduce dissolved salts and contaminants.
  • Dialysis machines rely on related membrane separation to clean a patient's blood outside the body.
  • Bottled water companies sometimes use reverse osmosis to produce very low-mineral water before adding minerals back.

Common mistake

✗ Myth: Reverse osmosis is just a very fine filter that catches salt particles.

✓ Fact: It is a pressure-driven separation process, not simple straining. The applied pressure must overcome the solution's osmotic pressure, and water molecules actually pass through the membrane while dissolved ions are rejected.

People also ask

How does reverse osmosis remove salt from water?
A high-pressure pump forces salty water against a semi-permeable membrane. Water molecules pass through the membrane's tiny pores, while dissolved salt ions are too large or too strongly charged to follow. The salt stays on the feed side and is flushed out as brine, leaving low-salt water on the other side.
Why does reverse osmosis need high pressure?
Water naturally moves across a membrane toward the more concentrated solution, a flow called osmosis. To reverse that flow, the applied pressure must exceed the solution's osmotic pressure, which rises with salt concentration. Seawater therefore needs far more pressure than brackish water.
Is reverse osmosis water safe to drink?
Reverse osmosis water is generally safe because the process removes most salts, particles and microorganisms. However, it also strips out minerals, so some systems add them back. Storage and maintenance matter, because a fouled or damaged membrane lets more contaminants through.
What is osmotic pressure?
Osmotic pressure is the minimum pressure that must be applied to a solution to stop water flowing into it across a membrane. It rises with the concentration of dissolved particles, so seawater has a much higher osmotic pressure than fresh water.
What happens to the salty waste stream?
The rejected salts, along with other contaminants, leave the system as a concentrated brine. In desalination plants this brine must be diluted or dispersed carefully, because dumping it directly can harm marine life.
Does reverse osmosis remove everything from water?
It removes most dissolved salts, many organic molecules and particles, and most microorganisms. However, small uncharged molecules such as some gases can pass through, and the product water is often remineralised before drinking.
Why is the membrane so important?
The membrane is the barrier that makes separation possible, so its pore size and chemical stability set the limit on what can be removed. Fouling by particles, microbes or scale gradually blocks it, so membranes need cleaning or replacement.

Part of the guide: Everyday Science: Light, Heat & Matter →