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If you have any doubts about the importance of desalination, talk to any Kuwaiti. The desalination plants in Kuwait were sabotaged as Saddam Hussein retreated during the closing stages of the Gulf War, leaving the country without an adequate supply of freshwater. As with many Gulf states unable to draw on vast underground aquifers, the sea is Kuwait’s major source of water and so its drinkable supplies come mainly from desalination plants. ‘These are similar to rivers, lakes and freshwater wells in other countries,’ says Mahmoud Abdel-Jawad, head of the Water Desalination Department of the Kuwait Institute for Scientific Research. ‘If you bomb them, it should be considered a war crime because if you cut water, you cut life.’

But the problem of inadequate freshwater supplies is no longer confined to the Gulf. The Western world has suddenly recognised that population growth and industrial development over the past decade have placed unprecedented demands on its own natural freshwater resources. Continuing drought in the western US, for instance, has already motivated many Californian communities to consider building desalination facilities both to meet current needs for freshwater and to avoid future mandatory water rationing. The city of Santa Barbara is spending $36 million ( £21 million) on a desalination plant that, from next year, will be turning 12 million cubic metres of ocean brine each year into drinking water.

Such concerns led to a review of desalination research last month by the committee on science, space and technology of the US House of Representatives. Existing techniques for converting saline and brackish water into usable water are largely based on research done before 1982, noted the committee. It said now was the time to reconsider the establishment of a federal desalination R&D programme.

Thousands of desalination plants exist throughout the world. At the end of 1989, there were 7536 plants producing 13 billion cubic metres of water daily, an increase in capacity of more than 40 per cent since 1986. According to Klaus Wangnick, a German consultant based at Gnarrenburg near Bremen, the Middle East has 60 per cent of the world’s desalination capacity, followed by North America with 13 per cent, Europe (including the Soviet Union) with 10 per cent and Africa with 7 per cent. The biggest plants, all in the Gulf region, can each produce more than 27 million litres of drinking water daily. Many islands around the world, from Jersey to the Virgin Islands, already use desalinated water, as do arid countries, such as Spain and Australia.

There are two ways of desalinating water. You can heat it and distil the vapour, or you can filter it with a membrane. Distillation techniques, which have been used for centuries, now account for around 70 per cent of the world’s desalination capacity, while filtering techniques, which are less than 20 years old, provide the rest.

As the demand for cleaner water increases, however, desalination companies are hoping to convince water utilities of the potential for raising water quality to the very highest European standards by treating sewage and industrial waste water with membrane techniques, which can remove troublesome contaminants including pesticides, nitrates, viruses and bacteria. But, for the moment, these techniques are too expensive for most potential users.

Meanwhile, practitioners of distillation or evaporative techniques are seeking new outlets for their technology, again driven by regulations requiring cleaner water. For instance, Licon, an engineering company based in Florida, has developed evaporative techniques for cleaning up both the corrosive effluents produced during electroplating, and the radioactive waste water in the 100 or so sites in the US where spent fuel rods are stored in ponds. ‘The desalination fraternity should think about some of these things, because we are at the beginning of an enormous market which will be worth hundreds of billions of dollars per year within the next three or four years,’ says Cecil Hughes, head of Licon. Some desalination techniques powered by renewable energies and aimed at rural and impoverished communities are also being developed .

Nature charges us nothing for performing the world’s biggest desalination services – the formation of clouds and rainwater from pure water vapour rising up from the salt-ridden oceans and the formation of ice at the poles. The key to separating water from brine artificially is the amount of energy needed to do it, says Richard Morris. He is chairman of the working party on desalination and water technologies of the European Federation of Chemical Engineering and head of the energy and environmental technologies group of Scottish Enterprise in Glasgow. ‘We need processes that are more energy efficient, but we are no nearer getting cheap water.’

According to Britain’s Water Services Association, tap water costs about 30 pence (50 cents) per cubic metre. This compares with between $1.6 and $2.2 per cubic metre for most desalinated drinking water, says Morris. ‘There have been no technological breakthroughs in the past 10 years to bring the cost down.’

Research being conducted by the International Atomic Energy Agency suggests that nuclear-powered desalina-tion plants could provide clean water for as little as 50 cents per cubic metre. According to Gunter Tusel, a German consultant based in Homburg in the Saar, who is working with the IAEA, only one nuclear-powered desalination plant exists, at Shevchenko in Kazakhstan in the Soviet Union. The plant produces 140 000 cubic metres daily at a cost of 0.5 roubles (50 pence) a cubic metre, he says. However, the desalination industry has not promoted the use of nuclear power because of the risk of contaminating drinking water with radionuclides. Morris also questions the economics of the Soviet plant and of nuclear power in general.

The earliest references to the concept of distillation appear in the Bible and the Koran, although Aristotle was the first to make any real pronouncement on the matter. In the 4th century BC, he wrote: ‘Salt water evaporated forms fresh, and the vapour does not when it condenses into sea water again.’ By 200 AD, sailors were known to be boiling up sea water and trapping the vapour in sponges. In An Introduction to Desalination Technology, published in 1989 by Porthan of Glasgow, Bill Hanbury points out that while the technique of boiling sea water to desalinate it was well known for centuries, the idea of assisting the process by deliberately cooling and thus condensing the vapour remained unexploited until the 1800s. By then, interest in desalination had been reawakened by the need to provide sailors on long voyages with the means of purifying water.

Most of the early stills were designed for the British navy but were very inefficient. Towards the end of the 19th century, however, Scottish engineers developed so-called ‘multiple effect distillation’ systems – sequences of interconnected stills that were, in effect, the first land-based desalination plants. The advantage of MED systems is that they recycle the energy supplied to turn water into steam in the first still. Subsequent stills scavenge the heat released when vapour condenses in order to evaporate water in the adjoining still, and so on until the condensing vapour can no longer trigger significant evaporation. Continuing refinements to the designs of MED systems have made them cheaper to build and more efficient to operate. Engineers working for Israel Desalination Engineering, an Israeli company based in Tel Aviv, have developed a system made of aluminium, instead of copper and nickel, and have lowered the maximum temperature at which it works from around 90 °C to 70 °C.

After the introduction of MED systems, there were no further significant breakthroughs in desalination technology until the 1950s when Weir of Scotland and Westinghouse of the US, working independently, developed ‘multi-stage flash’ distillation. MSF works by using a difference in pressure to draw hot brine from a boiler through a succession of connected chambers, typically between 18 and 24. The pressure in the first chamber is close to atmospheric pressure and, in the last one, is almost zero. Although the brine cools down as it passes from one chamber to the next, the successive drops in pressure ensure that some of it vaporises, or ‘flashes off’, in each chamber. As this vapour condenses on narrow tubes carrying cooler brine to the boiler, troughs collect the distillate and the heat released by condensation helps to warm up incoming brine before it enters the boiler.

The first two MSF plants were completed in Kuwait in 1960. Each produced 1 million gallons of drinking water daily and lasted 20 years. Nowadays most of the world’s biggest sea water desalination plants, such as the 40 plants that make up the 200 million gallon a day complex at Al Jubail, Saudi Arabia, rely on MSF technology. According to Morris, the main advantages of MSF systems are their simplicity, reliability and low maintenance cost. Though they are less energy efficient than modern MED systems, this is not a problem in the Gulf where there is plenty of waste heat from thermal power stations to use up.

Membrane desalination relies on an adaptation of a natural phenomenon, known as osmosis, to filter contaminants in wa-ter. If pure water and salty water are separated by a so-called ‘semipermeable membrane’ through which only water molecules can pass, then some pure water will flow through the membrane in an effort to equalise the concentrations of water on either side of it. The flow continues until the pressure on the salty side of the membrane is great enough to stop it. In desalination plants, engineers reverse this process. They pressurise brine on the salty side of the membrane so much that they drive water molecules, against their natural inclination, from the salty side to the pure side.

Since the development of reverse osmosis desalination techniques, two designs of membranes have come to dominate the technology. Both designs, which are sold as cylindrical cartridges between 1 and 4 metres long and between 5 and 30 centimetres across, operate at pressures of around 1000 pounds per square inch. A pinprick in a hose operating at this pressure would send a jet hundreds of metres into the air.

In ‘hollow-fine fibre’ modules, hollow semipermeable fibres no thicker than human hairs are bent round in a hairpin shape and laid parallel with a central shaft. Brine is forced into the shaft and through holes along its length into the surrounding packed fibres. As the brine passes to the outer shell of the module and back to the feed tank for retreatment, water permeates the fibres and flows through them to a collecting trough at the other end of the module.

In ‘spiral-wound’ modules, a sandwich of four sheets of membranes is wrapped round a perforated shaft. One membrane, known as the feed channel spacer, allows brine pumped into the sandwich to travel from one end of the module to the other. As this is happening, water passes through semipermeable membranes on either side of the spacer into a sponge-like membrane. This is sealed from the spacer and channels the water to a collecting trough.

Manufacturers of reverse osmosis membranes argue about which design of cartridge is more efficient. Du Pont, the American manufacturer that supplies both types of membrane, prefers hollow-fine fibre modules. In a study published in April, Seawater reverse osmosis: a study in use, the company concludes that these modules last longer, withstand higher pressures and are simpler to make than spiral-wound cartridges because they need no adhesives and fewer seals. They are also easier to install and maintain, says Du Pont.

Although the biggest reverse osmosis plant is in Saudi Arabia, producing 56 800 cubic metres a day in Jeddah, almost a third of those built are in the US where they are used mainly to treat brackish water from inland aquifers. Compared with distillation plants, which tend to be huge and consume large amounts of thermal energy, reverse osmosis plants are compact and more energy efficient, even though they run on electricity rather than on thermal energy directly. Also, their membranes can be designed to suit local water sources.

Such factors are less important in the Middle East where there is plenty of space, energy is cheap and the main requirement is to make sea water drinkable. Moreover, membranes do not produce as pure water as distillation plants. Unless passed twice through a reverse osmosis plant, the water fails to meet the standards of the World Health Organization. For distillation plants, the only usual requirement is to dose the feed with acid or polyphosphate to prevent salts such as magnesium hydroxide and calcium carbonate from forming scale on the heat transfer surfaces in the evaporation chambers.

More research needed

Though big distillation plants consume enormous amounts of energy and operate inefficiently, few nations have invested in research to improve them. Until recently, there has been little incentive. Most distillation plants are in the Gulf, where there is plenty of waste heat to run them. Also, according to Morris, research in the West throughout the 1960s and 1970s indicated that improvements in design would be hard-won. As a result, he says, most desalination research over the past few years has focused on reverse osmosis techniques because the components are small and scale up easily. Though these bring big savings in energy, the membranes most widely used today, made from either polyamide or cellulose acetate, are highly sensitive to contaminants in the feed water – particularly chlorine, which hardens the membranes, and microbes, which block them. Pre-treatment regimes must be extremely rigorous, says Morris, ‘as one tiny amount of contamination can wreck the whole membrane.’

At present, membranes last around 5 years, compared with around 6 months with the early systems. Most of the research is aimed at developing membranes with greater resistance to chlorine; one such membrane is made of polypiperazineamide by Separem, an Italian company based in Biella. Researchers are also trying to develop membranes that can cope more readily with the warmer waters of the Gulf or Pacific, where temperatures can reach 50 °C.

Enrico Drioli, professor of chemistry in the School of Engineering at the University of Calabria, foresees combinations of plants as the way ahead. Before long, he suggests, desalination plants could be mixing the water from distillation and reverse osmosis processes to produce an economical blend that matches WHO standards.

Further reading: Proceedings of the 12th International Symposium on Desalination and Water Re-Use, available from the Institution of Chemical Engineers, 165-171 Railway Terrace, Rugby CV21 3HQ, price £142.50. From 26-28 September, the Centre for Renewable Energy Sources in Athens is staging a conference, ‘Renewable Energy Sources in Water Desalination’, on behalf of the European Commission.

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Boiling water as cheaply and cleanly as possible

Researchers are trying to harness renewable energy from the sun and the sea to power desalination units, and not all of them on a grand scale.

Pierre le Goff of the Laboratory of Chemical Engineering Sciences in Nancy, which is run by France’s National Centre for Scientific Research, has devised a still for remote, arid areas that operates on solar energy. ‘This solar distiller is designed as a simple, rugged unit, easy to maintain and repair by any village craftsman with limited technical means,’ he says.

Le Goff claims that his device produces 20 litres of distilled water per square metre of solar collector. This consists of a transparent sheet of ethylene, which traps heat, overlying a movable mirror. Its output compares with just 2.5 to 3 litres in a conventional ‘single basin’ solar still, which is essentially a small greenhouse.

With his system, le Goff feeds brine down, by gravity, to fine gauze cloth draped over the faces of six parallel aluminium sheets placed vertically alongside one another, 4 centimetres apart. The solar collector’s mirror directs sunlight on to the back of the first aluminium sheet, heating it up to around 94 °C. Water vapour liberated from the gauze draped down the other side of the sheet swirls across the 4-centimetre divide to the back of the next parallel sheet. As it condenses, the vapour releases heat to the second sheet, which causes water to evaporate from the gauze on the other side. This process continues to the final sheet where the water condenses at around 45 °C. The condensate trickles down the uncovered faces of all the aluminium sheets and collects at the bottom in a storage vessel.

At the University of New South Wales in Sydney, Tony Fane of the Centre for Membrane and Separation Technology has designed a solar desalination unit for remote homesteads and farms in the Australian outback, where drinking-water supplies are often unreliable, but sunshine is abundant.

Fane claims that the pilot plant he has set up in the outback recovers between 60 and 80 per cent of the latent heat of vaporisation, the energy released when the vapour condenses, to produce up to 50 litres of water daily.

The device uses a semipermeable membrane that allows water vapour to pass through its pores when a temperature differential exists across it. Fane sets up the differential by having warm brackish water on one side of the membrane and cold water on the other. With commercial units expected to be priced at around A $3000, he estimates the cost of the distilled water at US $3 per cubic metre.

At the University of Edinburgh, Colin Pritchard of the Department of Chemical Engineering and Stephen Salter of the Department of Mechanical Engineering are developing a desalination unit driven by wave energy. The technique is based on the ‘nodding duck’ generator, devised by Salter to exploit wave energy. Partially submerged hollow booms, several metres long, are set rocking back and forth by waves that hit a ‘beak’ protruding above the water along the length of each boom.

In the ‘desalinating duck’, the rocking motion causes the water in a boom’s partially flooded core to act as a piston, alternately compressing and expanding vapour trapped in two chambers in the top half of the boom. The compressions force vapour into the tubes of a condenser unit, which connects the two chambers and collects the distillate.

Pritchard calculates that in coastal regions with plenty of wave power, a seagoing duck 6 metres in diameter and 20 metres long could produce around 1000 cubic metres of water daily. With a single boom priced at around £500 000,he estimates that the water would cost about $3 per cubic metre.