Using desalination to slake the world's thirst has been an uphill struggle, but now we're learning to go with the flow
STROLLING along Williamsons beach, a quiet strip of sand about 100 kilometres south-east of Melbourne, Australia, you would never guess that a monster lurks just behind the dunes. Nestled at the bottom of a 27-metre-deep pit is a 500-tonne mechanical giant that is about to begin burrowing under the beach and out to sea. In its wake the machine will leave a 4-metre-wide, 1.5-kilometre-long tunnel, the inlet for one of the world's largest plants to turn seawater into drinking water.
Australia is turning to desalination as fresh water in many parts of the country runs short following years of drought. It is not alone. Many countries are eyeing the oceans as a potential source of drinking water as populations grow and rainfall patterns change. Even the relatively rain-drenched UK now has its first large-scale desalination plant, opened earlier this year on the river Thames in east London.
Today's desalination plants are unlikely to solve our looming water crisis, however. That's because they have their own unquenchable thirst- for energy. It's needed to drive reverse osmosis (RO), the process in which salty water is forced at high pressure through a membrane that lets water molecules through but blocks the salt. But now a number of researchers and start-up companies think they have a more energy-efficient alternative, and it works by turning RO desalination on its head.
Any breakthrough would come not a moment too soon. A 2006 UN report estimates that by 2025, 2 out of 3 people could be living under conditions of water stress. Even the US may not be immune: the country is guzzling groundwater around 25 per cent faster than it can be replenished.
Modern RO desalination plants, like the one being built outside Melbourne, use a fraction of the energy required by the original facilities of this type, constructed in the 1960s. Still, the Melbourne plant will consume at least 90 megawatts of electrical power- roughly the peak output of 20 large off-shore wind turbines- to produce 150 billion litres of water per year. That is because RO is an inherently energy-intensive process: left to its own devices, water flows from a dilute solution into a salty one, whereas RO forces water to do the opposite.
Quick draw
So instead of fighting this energy gradient, why not try to harness it? That's the thinking behind the experimental "forward osmosis" plants that are starting to appear. Water can be sucked effortlessly out of seawater if you offer it a more concentrated "draw solution" to flow into. At first sight that might not appear to achieve anything, but if you are clever about what you use in the draw solution, you can get pure water out at the end.
One of the first companies to harness the power of forward osmosis is Hydration Technology Innovations (HTI), based in Albany, Oregon. In 2004 it released the X-pack, a portable water filter that incorporates a forward osmosis membrane into a small sealed plastic packet. Inside the packet is a powder containing sugar and flavourings, which acts as a seed for the draw solution. "It can be thrown into a muddy puddle and the sugar powder will draw the water molecules through the membrane to create a drink," says Walt Schultz, HTI's chief executive.
Many US soldiers now carry these packs, which can also be chucked over the side of a boat to pull a sweet drink out of the sea. The packs have also been supplied in relief aid following disasters such as the Haiti earthquake this year. But the X-pack is not going to solve the world's water crisis. "Our hydration products are intended for emergency use," says Schultz. "It is a relatively expensive way of producing a clean drink."
In the same year that HTI launched the X-pack, a team at Yale University hit on an idea that took the concept a step forward. Menachem Elimelech, Jeffrey McCutcheon and Robert McGinnis decided to use a draw solution based on ammonium bicarbonate (Desalination, vol 174, p 1). Just as HTI's sugary powder does, the ammonium and bicarbonate ions can pull water through the membrane. If you then heat the solution to around 40 °C, ammonia and carbon dioxide are given off, leaving behind pure water. The gases can be captured and reused, and the team says its method could produce fresh water while using only 20 per cent of the energy of today's desalination plants. That figure assumes, however, that waste heat from power stations is available to drive off the gases, which will limit where such plants can be sited.
Another challenge is finding a suitable membrane- one permeable to water but impermeable to salts. "It is the main hurdle for the forward osmosis industry," says Tom Pankratz, editor of the Water Desalination Report newsletter, based in Houston, Texas. The membrane needs to be as thin as possible to keep the salt water close to the draw solution and so maintain a high osmotic pressure, but robust enough to cope with the flow of water that results.
