How do desalination plants remove salt from seawater?

1. The sea holds most of the water on Earth, but it is too salty to drink. Salt water makes you thirstier, because your body uses up water to wash the salt out.
2. A pipe pulls seawater into the plant. Filters catch sand, seaweed and tiny bits, so the water is clear. It is still just as salty, because salt is dissolved and slips through an ordinary filter.
3. A powerful pump squeezes the seawater into a long tube. A gentle push does nothing. Only when the push is very strong do drops of fresh water begin to come through.
4. Zoom in on the membrane, a thin plastic skin. Its gaps are so small that single water bits squeeze through. Each salt bit wears a coat of water, so it is too big and bounces back.
5. Two streams leave the tube. One is fresh water. The other is the leftover seawater, called brine. It holds all the salt in less water, so it is about twice as salty as the sea.
6. A few healthy minerals are added to the fresh water, and pipes carry it to homes and schools. Now even a dry town beside the sea has plenty to drink.

Tap each part. Follow the seawater from the pipe on the left to the two tanks on the right.

• Intake pipe. The intake pipe brings seawater into the plant. Its mouth sits out in the sea, with a screen so fish and seaweed stay outside.
• Pre-filter. The pre-filter traps sand and tiny floating bits. The water comes out clear, but it is exactly as salty as before.
• High-pressure pump. The high-pressure pump gives the big push. Without a push stronger than the pull of the salt, no fresh water comes through at all.
• Membrane tube. Inside the tube is the membrane, a thin plastic skin. Salty water runs along the top. Water squeezes down through the skin and the salt stays behind.
• Fresh-water tank. The water that got through the membrane collects here. It is fresh, with almost no salt left in it.
• Brine outlet. The leftover water is called brine. It holds all the salt in less water, so it is saltier than the sea. It is mixed back into the sea with care.
• Energy recovery turbine. The brine is still under a huge push. It spins this turbine on its way out, and the turbine helps the pump. That saves a lot of electricity.

Turn the pump wheel or move the slider. Watch the tube and the two tanks. When does fresh water start to come out?

Experiment: What comes out if the pump only pushes gently on the seawater?

1. Start with a gentle push and look at the fresh-water tank.
2. Turn the push up a little at a time.
3. Find the push where the first drops come through. Then push harder.

What we found: A gentle push makes no fresh water at all. The salt pulls on the water and holds it back. Only when the pump pushes harder than that pull does water squeeze through the membrane, and the harder you push, the faster it comes. The leftover water gets saltier and saltier.

Turn on JavaScript to try it yourself.

Fill the fresh-water tank to the line before time runs out. Do not let the leftover water get too salty.

When salt dissolves in water it does not vanish. It breaks into bits far too small to see, and they spread out among the bits of water. That is why a paper filter cannot catch salt: the holes in paper are thousands of times too big.

A desalination membrane is a thin plastic skin with gaps so tiny that only water can work its way through. Each bit of salt holds a little coat of water around itself, and that makes it too big to fit.

But there is a catch. Salt pulls on water. If you just leave fresh water and salt water on two sides of a membrane, water moves toward the salty side. To make water go the other way, a pump has to push harder than the salt pulls. For seawater that takes a very strong push, as strong as the sea presses 600 meters down. So a desalination plant needs a lot of electricity.

About half of the seawater comes out as fresh water. The other half, called brine, holds all the salt, so it is about twice as salty as the sea. Plants mix it back into the sea carefully. Here the strainer idea stops working: a strainer needs no push at all, and its holes are easy to see.

Key words

Desalination

Taking the salt out of seawater to make fresh water.

Membrane

A thin skin that lets water through but holds salt back.

Brine

The extra-salty water left over after fresh water is taken out.

Osmotic pressure

How hard salt pulls on water. The pump must push harder than this.

Reverse osmosis

Pushing water through a membrane from the salty side to the fresh side.

Permeate

The fresh water that has passed through the membrane.

Check your thinking

1. Why can water get through the membrane when salt cannot?

   Show the answer

   Salt bits wear a coat of water, so they are too big for the tiny gaps. The gaps are just big enough for water. A salt bit with its coat of water does not fit.

2. The pump is running, but gently. How much fresh water comes out?

   Show the answer

   None, until the push is stronger than the pull of the salt. Salt pulls on water. Until the pump beats that pull, no water crosses the membrane.

3. A plant runs its pump at 20 bar on seawater. What happens?

   Show the answer

   No fresh water: 20 bar is below the osmotic pressure of about 27 bar. Only pressure above the osmotic pressure drives water through. Below it, nothing is produced.

4. A plant takes in 2 buckets of seawater and makes 1 bucket of fresh water. What is in the other bucket?

   Show the answer

   Water with all the salt in it, about twice as salty as the sea. All the salt from two buckets is now in one bucket of water, so it is twice as salty.

5. Why does a desalination plant use so much electricity?

   Show the answer

   The pump must push extremely hard. Beating the pull of the salt takes a huge push, and making that push takes electricity.

6. What does the energy recovery turbine do?

   Show the answer

   It uses the push left in the brine to help the pump. The brine comes out still squeezed hard. Its push spins the turbine, and that helps the pump.