The sea holds 97% of the planet's water, and you can't drink a glass of it. Desalination means paying in energy for what nature does for free with sun and rain.
Why you can't drink seawater
Seawater contains about 35 grams of salt per litre. Drinking water contains less than 0.5 to 1 gram.
Kidneys can't make urine as salty as the sea. To get rid of the salt in one litre of seawater, they need more than one litre of water. Drinking seawater dehydrates you faster than drinking nothing. That's why castaways who drink it die before the others, and what makes desalination essential at sea. See Déshydratation.
The two big methods
Heating: distillation
You boil the water, the steam leaves without the salt, you condense it. That's what the sun does above the oceans.
Modern plants don't simply boil: they chain chambers at lower and lower pressure, where water boils at lower and lower temperatures, and recover the heat from each stage to warm the next. These are multi-stage flash (MSF) and multi-effect distillation (MED). They use a lot of energy, and are found mostly in the Gulf, often paired with power stations whose waste heat they recover.
Filtering: reverse osmosis
A very thin membrane lets water molecules through and holds back salts. Naturally, water would move towards the saltier side (that's osmosis). By pushing seawater against the membrane at 55 to 80 bar (800 to 1,200 psi), you force it through the other way.
This is now the dominant method: nearly 70% of the world's capacity. A modern plant uses about 3 to 4 kWh per cubic metre of fresh water, thanks to recovery devices that reuse the pressure of the outgoing brine.
The process needs:
- serious pre-treatment: filtration, sometimes flocculation, so that sand, algae and bacteria don't clog the membranes;
- post-treatment: reverse-osmosis water is too pure, aggressive to pipes and poor in minerals. Calcium is added back and the pH corrected.
The numbers
- The world desalinates about 95 million cubic metres a day (late 2010s), in more than 15,000 plants.
- The Middle East and North Africa produce nearly half. Saudi Arabia, the Emirates and Israel depend on it for a large share of their drinking water.
- The largest reverse-osmosis plant in the world, at Sorek in Israel, produces more than 600,000 m³ a day.
- From 100 litres of seawater, a reverse-osmosis plant returns about 40 to 50 of fresh water. The rest goes back as brine, twice as salty as the sea.
What doesn't work so well
Energy. Even efficient, a plant uses ten times more than pumping fresh water. A power cut stops it, and tap water with it.
Brine. Released into the sea, it sinks to the bottom and can smother local life if it isn't well diffused. It also carries pre-treatment chemicals.
Membranes foul and are replaced every five to ten years. They are made by a handful of manufacturers.
The target. A desalination plant is a single piece of infrastructure feeding an entire city. The Gulf countries protect them like power stations.
At small scale
- The manual watermaker in life rafts: a small hand-pumped reverse-osmosis unit that makes a few litres an hour. It's the one piece of equipment that really changes survival at sea.
- Boat watermakers: electric, from 20 to several hundred litres an hour. They live on the boat's batteries and solar panels.
- The solar still: a black tank under a sloping pane of glass. It makes around 2 to 5 litres per square metre per day in full sun. Slow, but with no moving parts.
- Old sea ice is almost fresh: as it freezes, water pushes the salt out. Bluish, rounded ice more than a year old can be drunk once melted. Young, grey, opaque ice stays salty.
Tips you learn at sea
- Never keep the first water from a watermaker after it has been stopped: it's salty. Taste it before filling the tank.
- A membrane that dries out dies. When stopped, it's kept wet with a storage solution, or run regularly.
- Near a port or a river mouth, seawater is polluted with hydrocarbons and discharges: they damage membranes and aren't all held back. Desalinate offshore.
- Rain is still the best fresh water at sea. A tarp stretched into a funnel gives more than a still. See Collecting and storing rainwater.