Desalination by stove distillation: principle and limits

Salt is dissolved in seawater, but it does not evaporate with it. When you heat salt water, only the water rises as steam, and the salt stays behind in the pot, becoming ever more concentrated. When the steam meets a cooler surface, it gives up its heat and turns liquid again – it condenses. The condensed water is practically free of salt.

In the simple stovetop set-up you place the pot lid upside down, with the dome pointing downwards. The drops forming on the underside run to the lowest point in the middle and drip from there into a small, heat-resistant vessel standing in the pot. Cold water or snow on top of the lid keeps it cool and increases condensation.

The catch is energy. Evaporating water takes much more energy than just heating it to the boil. That is why every sip of distilled water costs a lot of fuel and time, and part of the steam escapes unused at the edges. It is not enough for a group over a longer period.

The pot is filled only partly with salt water, and the collecting vessel must stand so that it cannot tip over and no salt water splashes into it. That is why you heat gently, so the water simmers but does not boil vigorously. A vigorous boil throws salty drops into the collecting vessel and ruins the work.

Let the condensate cool and taste a tiny amount. If it tastes salty, salt water has splashed in, and you distil it again. Remove the salt residue from the pot regularly and top up with fresh salt water before the pot runs dry.

Pour the distillate into a clean container. The collecting vessel and the lid must be clean, otherwise germs get into the finished water. Distilled water tastes flat because it lacks minerals and dissolved air; pouring it between clean vessels makes it a little more pleasant.

Never drink seawater, not even in small amounts or mixed with fresh water. The body has to get rid of the excess salt using water and loses more water doing so than it took in. The deficit gets worse, not better.

Steam, hot lids and boiling water cause severe scalds. Use pot holders, lift the lid away from your body and keep children away. Run stoves and fires only outdoors or with very good ventilation, because in enclosed spaces there is a risk of carbon monoxide – a gas you can neither see nor smell and that can be deadly. With an open fire, check local rules and fire bans.

Volatile chemicals such as fuels or solvents can carry over with the steam and end up in the distillate. If oil or chemicals are suspected, for example in a harbour or after an accident, do not distil – look for another source.

Stove distillation is an emergency solution for coastal situations without fresh water, or for brackish water when no other source can be reached. It bridges the time until fresh water is found or help is reached. Collecting rainwater or looking for a freshwater source near the coast is often far less effort and should be tried in parallel.

For boats and life rafts there are hand-pump desalinators that use reverse osmosis: the water is pressed through a very fine membrane that holds back salt. They are expensive and need maintenance, but deliver water without fuel. They are used only according to the manufacturer's instructions.

Distillation is the oldest method of making salt water drinkable. Large desalination plants today mostly use reverse osmosis or optimised evaporation processes.

The same principle – water evaporates, salt and dirt stay behind, steam condenses on a cool surface – is also behind the solar still. There the sun provides the heat instead of the stove, but the yield is even smaller and depends heavily on the weather.