MIT researchers develop a new approach for water desalination
A new approach to desalination being developed by researchers at MIT and in Korea could lead to small, portable units that could be powered by solar cells or batteries and could deliver enough fresh water to supply the needs of a family or small village. As an added bonus, the system would also remove many contaminants, viruses and bacteria at the same time.
One of the leading desalination methods, called reverse osmosis, uses membranes that filter out the salt, but these require strong pumps to maintain the high pressure needed to push the water through the membrane, and are subject to fouling and blockage of the pores in the membrane by salt and contaminants. The new approach, called ion concentration polarization, separates salts and microbes from the water with the help of electrostatics – repelling them away from the ion-selective membrane in the system.
The system works at a microscopic scale, using fabrication methods developed for micro-fluidic devices (similar to the manufacture of microchips) for materials such as silicone (synthetic rubber). Each individual device would only process minute amounts of water, but a large number of them. The researchers expect an array with 1,600 units, fabricated on an 8-inch-diameter wafer, could produce about 15 liters of water per hour. Salt water would be poured in at the top, and fresh water and concentrated brine collected from two outlets at the bottom, so the whole unit could be self-contained and driven by gravity.
So far, the researchers have successfully tested a single unit, using seawater they collected from a Massachusetts beach. The water was then deliberately contaminated with small plastic particles, protein and human blood. The unit removed more than 99 percent of the salt and other contaminants. “We clearly demonstrated that we can do it at the unit chip level,” said Sung Jae Kim.
While the amount of electricity required by this method is actually slightly greater than for present large-scale methods such as reverse osmosis, the researchers claim there is no other method that can produce small-scale desalination with anywhere near this level of efficiency. If the proposed system is properly engineered, it would use about as much power as a conventional lightbulb.
Since the separation occurs electrostatically, it doesn’t work for removing contaminants that have no electric charge. To take care of these remaining particles (mostly industrial pollutants) the researchers suggest the unit could be combined with a conventional charcoal filter system, thus achieving pure, safe drinking water through a single simple device.
Having proved the principle in a single-unit device, Kim and his associates plan to produce a 100-unit device to demonstrate the scaling-up of the process, followed by a 10,000-unit system. They expect it will take about two years before the system will be ready to develop as a product.
“After that,” says Kim, “we’ll know if it’s possible” for this to work as a robust, portable system, “and what problems might need to be worked on.”