Qatar sits on one of the driest stretches of land on Earth. Rain falls for only a handful of days each year, and the rivers that supply most countries simply do not exist here. Yet water flows from every tap. The reason is desalination: the large-scale removal of dissolved salts from seawater.
Two ways to separate salt from water
The older method is thermal distillation. Seawater is heated until it evaporates. Because salt is an ionic compound with very strong attractions between its ions, it does not evaporate with the water. The pure water vapour is then cooled on a cold surface and condenses back into liquid, leaving the salt behind. Multi-stage flash distillation repeats this trick in a series of chambers, each at lower pressure, so the water boils again and again without adding much extra heat.
The newer method is reverse osmosis. Osmosis is the natural movement of water through a partially permeable membrane, from a dilute solution to a concentrated one. Reverse osmosis pushes water the other way by applying pressure of around 60 to 80 atmospheres to the salty side. Water molecules squeeze through pores far too narrow for hydrated sodium and chloride ions, and freshwater collects on the far side.
The cost of a clean glass
Neither method is free. Thermal plants need enormous amounts of heat, which is why they are usually built next to power stations and share their waste steam. Reverse osmosis needs less energy overall but demands high-quality membranes that clog with algae, dust and biological films.
Both processes also produce brine: water left behind with roughly twice the normal salt concentration, often warmer than the sea and mixed with anti-scaling chemicals. Released carelessly, brine sinks and spreads across the seabed, where it can stress seagrass beds and the small animals that live in the sediment. Engineers now design diffusers that mix brine rapidly with a large volume of seawater so the concentration returns to near normal within a short distance.
An engineering problem worth solving
More than half the world's desalination capacity sits around the Arabian Gulf, and the Gulf is a difficult place to do it. It is shallow, semi-enclosed, unusually salty and unusually warm. Every neighbour's brine is every other neighbour's intake water.
That is why the research now focuses on three questions: how to cut the energy per cubic metre, how to run plants on solar electricity instead of fossil fuels, and how to recover useful materials such as magnesium and sodium from the brine instead of discarding it. Solve those, and a technology that keeps a country alive becomes one that a planet can afford.