One sixth of the world's population lacks access to clean water. About 40 per cent suffer from water shortages. This seems absurd on a planet whose surface is 70 per cent water. We even have the desalination technology to turn seawater into fresh, clean drinking water. Why, then, is it confined to a few rich nations?
The simple answer is cost. Desalination requires the building of expensive treatment plants, which are beyond the means of the countries that most need the technology: Africa, central and south America, India, China and large stretches of the middle east.
Desalinated water costs about 50 US cents per cubic metre, according to John Tonner of World Wide Water. He says that the coming water shortages, caused by population increases and growing industrialisation, will force many countries, rich and poor, to consider desalination. "People will have to pay more for water, and that will make desalination more attractive," he argues.
For one thing, he adds, desalination produces high-quality water suitable for use in modern industries like chip production. For another, the technology is proven: one desalination plant in Aruba recently celebrated its 70th birthday.
But his optimism about desalination is rejected by others. Ray Haslop, an adviser to WaterAid, counters: "Desalination is no answer for developing countries, they need practical, cheap solutions." The charity looks to provide fresh water for a cost of ?10-?15 per head, giving people 45 litres of water a day if possible. That level of aid would provide only a few months' worth of desalinated water per person. WaterAid runs no desalination projects as a result.
Will the technology come down in price? Some of its basic characteristics make this unlikely. Both of the most popular forms of desalination, reverse osmosis and distillation, involve complicated machinery that guzzles power.
Normal osmosis sees water with a lower concentration of salt drawn through a semipermeable membrane into a neighbouring compartment with a higher concentration of salt, until they achieve equilibrium. In order to reverse such a process, salty water must be passed under great pressure through a very fine membrane. It is a slow and complex technique, and the energy demand remains stubbornly high.
Distillation and condensation use steam to heat salt water and pass it through several evaporation chambers, each at a lower temperature and pressure than the last. As the water vapour passes through each chamber, some is collected by a condensation cooler. But heating, cooling and pressurising the water mean high energy costs. What's more, desalination plants require a high degree of maintenance, as salt water can cause corrosion and an accumulation of chemical deposits. Seawater often has to be chemically treated before it can be put through a desalination plant.
Small-scale solutions to desalination have also been proposed, but so far have had little impact. Michael Pleass in the US led a team that developed the "Delbuoy," a device that uses wave power to desalinate seawater. A buoy on the surface, rising and falling with the waves, connects a piston that drives a pump on the sea's floor. The pressure created by the piston forces the seawater through a reverse osmosis filter, then sends the fresh water through a pipe to the shoreline. However, such systems can only produce a small amount of desalinated water at a time.
In Britain, Seawater Greenhouse has devised a new, low-cost technique for producing irrigation water from the sea in hot countries. The method condenses fresh water from air that has been made humid by seawater. It uses a greenhouse design: as the humid air hits the glass walls of the greenhouse, its water content condenses and runs down to be collected. As water leaves the salt behind when it evaporates, the water is pure.
The key is to develop an efficient heat exchange system that will ensure that the humid air is warm but the greenhouse glass remains cold, encouraging the water to condense. Conventional systems of this kind use tubes of copper, nickel and aluminium, but these can be corroded by seawater. So the new system uses plastic. Tests have proved successful and the system should cost only a quarter the price of conventional systems. A prototype will be tried in Oman.
A larger-scale solution is to combine desalination with energy production. Nuclear desalination, using a nuclear reactor as the source of energy holds great promise. Yet asking poorer countries to build and maintain nuclear plants may be unrealistic. Besides, the water produced remains expensive, at close to $1 per cubic metre.
What about solar power? Botswana has invested in solar-powered desalination with some success, bringing water to remote areas. So why has it not taken off? It is still too expensive: solar cells cost a great deal to produce and a large quantity are needed. New manufacturing techniques are bringing solar cells down in price, making their use increasingly practicable, but the short-term prospects for desalination as an answer to the water needs of poor nations look bleak. The alternatives, from digging wells to building pumps and preventing water becoming polluted by animals and human waste can still yield much more practical, immediate and sustainable results for poor countries. These more prosaic techniques will have to suffice while we wait for desalination to fulfil its long-delayed promise.
