Water has always been a volatile topic in Australia, the world's driest inhabited continent. Yet, protesters are complaining that a planned desalination facility outside Melbourne, Victoria, will generate too much freshwater. The $3-billion government-owned plant will produce more than 300,000 cubic meters of drinkable water a day when it opens in 2011, placing it among the world's biggest.
Environmental groups claim that the plant is unnecessary. Even if water consumption rose by 25 percent, there would be an excess of about 60 percent in supply over consumption by 2016!
"Desalination is the most energy-intensive form of water supply," says Peter Gleick, president of the Pacific Institute, an independent environmental think-tank in Oakland, California.
In California alone, proposals have been put forward for at least 20 new large desalination facilities, which together could ultimately supply some 6 percent of the state's urban water demand. Even the very energy-intensive thermal plants in the Gulf region, which purify seawater by boiling and condensing, can produce fresh water at less than $1 per cubic meter.
And the desalination plant at Ashkelon in Israel, once the world's largest, produces more than 300,000 cubic meters of freshwater per day at costs of around 50 cents per cubic meter. But on average, the technique is 3.5 times more expensive than using other sources of freshwater such as pumping from aquifers.
Technological future
Advances in chemical engineering promise to make desalination more affordable. Polyamide membranes are the basic components of reverse-osmosis plants, which produce more than half of the world's desalinated water and are replacing less-efficient thermal distillation facilities.
To remove dissolved organic matter and other impurities, brackish water or seawater is pre-filtered and then forced under pressure through bundles of these semi-permeable membranes, which separate salts from the water. Pretreatment cannot fully prevent the membranes from fouling and degrading, so they need to be cleaned chemically and replaced frequently — a major cost factor.
Every company has its own way to fight ‘bio-fouling’, salt deposition and other processes that reduce the flux of water through the membrane.
In a bid to tackle fouling, where geology allows it, some operators of coastal plants have begun to draw water from beach wells rather than from the open sea. The sand acts as a natural filter, pre treating the sea water. Beach wells also have the advantage of preventing fish and marine life from getting trapped and killed in the uptake pipes, a widespread problem with coastal desalination.
But although polymer membranes have become more permeable and durable since they were first developed, neither the basic technology used in reverse osmosis nor the membrane materials used in the desalination process, have changed much.
Scientists in Singapore — which has recently earmarked $250 million for developing desalination technologies — are testing alternative techniques such as membrane distillation, which combines both, membrane technology and evaporation processing, in one unit. This can then be coupled with solar energy, geothermal energy or waste heat.
New studies
Another promising method is the use of aligned carbon nanotubes - molecular-scale pipettes through which water can be forced frictionless, 1,000 times faster than through polymeric membranes. However, no one has as yet demonstrated the desalination ability of nanotubes, or suggested how to get around the fouling problem. Moreover, this technology, which requires hydraulic pressure, would reduce energy consumption by just 20 percent according to experts.
Prototypes now exist for a desalination technology based on ‘forward osmosis’, which works at very low pressure. Menachem Elimelech, an environmental engineer at Yale University in New Haven, Connecticut, leads a team that has constructed a pilot desalination plant that uses osmotic, rather than hydraulic, pressure.
The researchers position a concentrated solution of dissolved ammonia and carbon dioxide gases behind a membrane, creating osmotic pressure. This draws the saltwater on the other side through the membrane. Freshwater can then be recovered from the draw solution by heating it to 58 degrees Celsius, so that ammonia and carbon dioxide bubble out of solution and are captured.
"In absolute terms, the process is not quite as efficient as reverse osmosis, but the nice thing is that you can use waste heat to decompose salts from solution," says Elimelech.
Besides being less energy-intensive, forward osmosis would greatly reduce brine discharge. Residual brine from existing desalination processes must be watered down to concentrations that are harmless to marine life.
However, forward osmosis requires membranes that must be extremely thin and porous, and tolerant to strongly basic water, and such devices are not yet commercially available, Elimelech says.
Energy will always remain the crucial constraint. Twenty years ago, 510 kilowatt hours of electricity was needed to produce one cubic meter of desalinated water. Modern reverse-osmosis plants, such as that at Ashkelon, now need around 2 kilowatt hours to produce the same volume. The world record, achieved in a pilot plant in California, is 1.58 kilowatt hours.
The laws of thermodynamics impose a theoretical limit of around 0.7 kilowatt hours on the energy-efficiency of desalination.
Despite these limitations, well-designed desalination plants can still be more efficient and environmentally sound than large dams, pipelines or canals.
Nature News
