Desalination.

Step 1 of 5

Water flows the wrong way

Almost all the water on Earth is in the sea, and every litre of it carries about 35 grams of dissolved salt, mostly sodium and chloride ions. In the Unit on solutions we learned that salt comes apart into ions in water, each ion wrapped in water molecules. Desalination is taking that salt out. Most new plants push seawater through a membrane that lets water molecules pass and stops the salt.

The Scene shows salt water on the left, fresh water on the right, and a membrane between them. Right now a piston presses on the salt water at Balance, and the levels hold still. Predict what happens when you drop Push on the salt water to None.

Step 2 of 5

Osmotic pressure

The flow from the fresh water into the salt water is called osmosis. Water molecules cross the membrane in both directions all the time, but on the salty side some of the space beside the membrane is taken by ions, which also hold water molecules close to them. Fewer molecules cross from the salty side than arrive from the fresh side, so on balance the water moves into the salt water.

Pressing on the salt water can stop the flow. The pressure that just stops it is the solution’s osmotic pressure. Find it with Push on the salt water.

Step 3 of 5

Saltier water

Osmotic pressure grows with the number of dissolved particles: each gram of salt in a litre adds about 0.78 bar. The Scene now holds Brine, 70 g/L, twice as salty as seawater. What push stops the flow now?

Step 4 of 5

Reverse osmosis

Push harder than the osmotic pressure and the flow turns round: water is squeezed out of the salt water, through the membrane and into the fresh side, while the ions stay behind. This is reverse osmosis, and it is how most of the world’s desalinated water is made today, Israel’s included.

The Scene is at Seawater plant, 60 bar. The water left under the piston loses water but keeps all its salt, so it grows saltier and its osmotic pressure rises. When it matches the pump’s push, the flow stops. What share of the seawater comes out fresh by then?

Step 5 of 5

The membrane

Show the molecules is On. A reverse-osmosis membrane has no holes you could see even with a good microscope. Its working layer is a film of polyamide, a polymer whose chains are bonded to each other, about a ten-thousandth of a millimetre thick. Watch how the water molecules wriggle through the polymer, and what happens to the ions.