New Technology Could Make Desalination More Accessible
Sonia Kolesnikov-Jessop
New York Times
03/21/2011
SINGAPORE — In the aftermath of earthquakes and tsunamis, one of the most desperate needs for survivors is often to have access to clean drinking water.
Desalination plants have long been essential to providing fresh water in places where natural supplies are lacking. Traditionally, two methods have been used to remove salt from water: thermal distillation, in which salt water is boiled and the purified vapor is collected; and membrane processes, like reverse osmosis and electrodialysis, in which water is forced through a semi-permeable membrane that acts as a microscopic strainer to filter out the salt crystals and other impurities.
More than 60 percent of all desalination plants worldwide use a membrane process, which typically consumes less energy than thermal distillation. Still, reverse-osmosis in particular remains very energy-intensive and usually requires huge plants to be economical.
Now, a Singapore company, memsys clearwater says it has developed a better technology that could revolutionize the economics of desalination. Called vacuum multieffect membrane distillation, it uses a relatively new process that combines thermal and membrane technologies, working in a vacuum so that the water boils at a much lower temperatures of 50 to 80 degrees Celsius (122 to 176 Fahrenheit) instead of the usual 100 degrees Celsius. It also comes in small, modular units suitable for use in disaster zones and remote communities.
In membrane distillation, steam from boiled seawater passes though a one-way permeable membrane, to be condensed on a cold surface and collected. The main advantage of membrane technology over reverse osmosis is that it does not require high pressure, said Adrian Yeo Piah Song, a researcher at the Nanyang Technological University in Singapore. Reverse osmosis needs large pumps and heavy-duty piping operating at high pressure. The supporting infrastructure makes it hard to install outside developed countries.
The new technology enhances efficiency by putting water through a cascade of membrane distillation processes working at progressively lower temperatures and pressures, recovering energy at each step to power the next.
“We have the first modular thermal separation process,” Götz Lange, managing director of memsys, said during an interview. “We didn’t change the thermal technology itself — you can’t change physics — we are just the first to put this advanced technology of thermal separation into a very tiny, cheap and reliable modular concept.”
After seven years of development, a small demonstration unit, powered by solar energy for extra sustainability, was installed last year at Marina Barrage, a dam completed in 2008 across the mouth of Marina Bay that has converted what is left of the old Singapore harbor after massive land reclamation, into a freshwater reservoir.
The demonstration unit has been designed for disaster relief. Able to fit in a 20-foot, or 6-meter, shipping container, it is completely mobile and runs on solar power so that it can operate without external infrastructure support. It can produce 1,000 liters, or 265 gallons, of fresh water a day. A key advantage of the technology is that it can use waste heat or solar-thermal energy to boil the water, Mr. Lange said.
“So far,” said Godart van Gendt, head of international distribution at memsys, “we already have 25 companies who have bought the technologies and are now doing research to apply the technology for their own purpose. They’re buying our unit and then they’re doing their research.
