Desalination

Kim and Philip are in their dinghy in the middle of the ocean. Oh, no! Philip! We’re running out of drinking water! But there is plenty of water around us.

Let’s drink that! Unfortunately, seawater is not suitable for drinking. It contains too much salt and actually draws water from your body, which can cause dehydration. But if we remove salt from the seawater, would it be drinkable? Yes!

This is what people do in places where it doesn’t rain much, but there is plenty of saltwater instead - for example on tropical islands, in cities on the coast, and on ships. But how do you remove salt from saltwater? Let’s run an experiment. All you need is: a large bowl of saltwater, a cup smaller than the bowl, a piece of cling film or a plastic bag, and some weight, for example a couple of small rocks. Place the cup upright inside the bowl.

Stretch the plastic tightly over the top of the bowl. Secure it with a rubber band if needed. Place your weighted thing on the plastic, right above the cup. Place the covered bowl in a sunny spot, and wait a few hours. Now the cup in the middle contains some water, which isn’t salty at all!

How? Well, the sun heated up the saltwater in the bowl. Some of the liquid water turned into water vapour. Then, the water vapour touched the plastic covering the bowl. This surface was cooler, so the vapour turned back into liquid - it condensed into drops of water.

The weight on top of the plastic covering caused the drops to roll into the centre of the plastic, and drip straight into the cup. Salt and some seawater are left behind in the bowl, forming a concentrated, salty brine. This process of heating a substance, and cooling it so that it condenses is called thermal distillation. It’s the oldest method used to remove salt from seawater - to desalinate it. However, desalinating water using a bowl and sunny weather is not efficient.

It takes a long time, and you only get a small amount of freshwater. To provide many people with water for drinking, cooking, and farming, distillation needs to be done on a larger scale. For instance in a desalination plant, using more advanced equipment. Another common method of desalination uses a special membrane. This membrane allows some substances to pass through, and stops others.

Pressure is applied to saltwater on one side of the membrane. This pressure pushes the water molecules through the membrane, but salt can’t get through. The result is pure water on one side, and concentrated salty brine on the other. This method is called reverse osmosis. It is more advanced than distillation, and used in the majority of desalination plants around the world.

Thermal distillation and reverse osmosis require special equipment, which has to be maintained and replaced. Both methods require a lot of energy too - heat and electricity. The remaining brine is a pollutant and is problematic to dispose of. All these factors make desalination costly and harmful to the environment. However, as the world population is growing, desalination might become increasingly necessary.

Scientists and engineers are working hard to develop solutions that would make desalination cheaper, more efficient, and safer for the environment. Look! I distilled some water! Great, Philip! And I will build a desalination system for us, using solar panels!