The planet’s biggest water resource, seawater, is in no danger of running out, making up 97% of Earth’s water. Why not harness it for drinking?
The most basic technology to do so has been in use for nearly as long as fire: distillation, the process of boiling water and catching the steam, condensing it into liquid. In small quantities, this can be done easily, and cleanses water of other impurities as well as salt. But at large scale, such as providing the drinking water needs for a city, the process is fuel-intensive, even using modern methods such as low-pressure vessels to lower the boiling point.
Alternative technologies use electrical currents, which when passed through the water can separate out salt and other minerals, and reverse osmosis, by which saline water is passed at high pressure through membranes that exclude salt and impurities. Both these methods also have high energy requirements, which makes them costly, and adds to global greenhouse gas emissions. Sucking in seawater can also suck in fish and damage coastal ecosystems. Waste from the plants is another issue: the salty residue is usually released back into the sea, but this must be carefully managed because at the concentrations produced it is toxic to marine life.
Energy costs have proved prohibitively high for most countries, so the main users of desalination to date have been among the fuel-rich and arid countries of the Middle East. However, the water crisis has gripped so hard in some areas of the world that some cities see little alternative. Cape Town’s first desalination plant has just started operating, after some severe budget woes. China, Pakistan and India are exploring new desalination plants. If renewable energy can be used to power the plants, this should reduce the impact on climate change.