Water desalination, for sweeter seas

All of the IMOCA Open 60s are fitted with two on board water purifiers. There's a simple reason for that: each of the two crew on the yachts needs to drink between two and three litres of water a day, and for a non-stop round the world race such as this without these devices would mean an on board water supply of at least 600 litres. As well as adding an extra 600 kg to the weight of the yacht, and not counting the tank itself, the skippers would also have to carry out  specific procedures to avoid the contamination of the storage tanks during the three months of competition. An added setback, this amount of liquid would                  also mean using sea water for cooking, personal hygiene, cleaning etc. or it would mean collecting rain water for the same use.

Everything is much easier and simpler with these on board desalination plants. The most modern type are able to convert up to 30 litres of water an hour, with 15 minutes function time daily producing 7.5 litres of drinking water. That also happens using relatively low levels of energy, compared, for example, with the energy required to move the keel

Inverse osmosis function

Desalination devices take in sea water and via an 'inverse osmosis' process they convert it into drinking water. It is made up of three basic parts: a pump the sucks in salty sea water, a pressure pump which pushes the saline solution through a membrane with 60 bars of pressure and then through a separating membrane which is made up of  an inverse osmosis process with the fresh water being sent to the storage tanks on the yacht and the salted water going back into the sea.

Natural osmosis is the transfer of a non-concentrated solution to a concentrated solution via a semipermeable membrane, with the aim of equalling them out. This action takes place due to the difference in saline levels and stops at a certain pressure, called the osmotic pressure. It is the same process employed by plants to draw in moisture from the ground. Inverse osmosis is the opposite phenomenon, allowing a concentrated solution to become a less concentrated solution via a special separator. For this to take place, a very high pressure must be applied to the salt water (45-60 bars). Once this pressure level is reached, salt water begins to flow through the special membrane towards becoming drinking water, which is the simple basis of the on board desalination plant.

Almost totally pure water

It's necessary to emphasise here that the water produced by these devices is almost completely void of mineral salts. The desalination equipment filters around 99 per cent of the salts dissolved in the water, allowing only one per cent to pass through. Therefore, the purified water obtained from a desalination device is perfectly drinkable and has between 0.2 and 0.5 grams of salt per litre of water.

It is water with an extremely low saline concentration, so it is also advisable to treat the water (with special salt and mineral tablets) to make it more suitable for long term use. That doesn't mean that it is harmful to those who drink it , but just that those drinking it won't get the same levels of salts found in normal tap or bottled water.