Water desalination is very important in areas where earthquakes, tsunamis or other natural catastrophes occurred. Usually, these areas are located near the sea or the ocean, and desalinizing the water could definitely save lives.
The problem is that, in such crisis situations as those mentioned above, classic desalination systems often don’t work, because they are big, heavy, and require a great amount of energy, which is hard to obtain at those times.
Researchers from the MIT and from Korea have devised a new, smaller, portable desalination system that can be carried by a person to provide clean water to his family. The device works by electrostatically repelling salts and microbes from water, instead of pushing them through a membrane, which requires the usage of pressure pumps. It uses an ion-selective membrane, fact that also eliminates the problems of fouling, met in the older reverse osmosis systems.
The basic principle that makes the system possible, called ion concentration polarization, is a ubiquitous phenomenon that occurs near ion-selective materials (such as Nafion, often used in fuel cells) or electrodes, and this team and other researchers have been applying the phenomenon for other applications such as biomolecule preconcentration. This application to water purification has not been attempted before, however.
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.
The system works at a microscopic scale, using fabrication methods developed for microfluidics devices – similar to the manufacture of microchips, but using materials such as silicone (synthetic rubber). Each individual device would only process minute amounts of water, but a large number of them – the researchers envision an array with 1,600 units fabricated on an 8-inch-diameter wafer – could produce about 15 liters of water per hour, enough to provide drinking water for several people.
The whole unit could be self-contained and driven by gravity – salt water would be poured in at the top, and fresh water and concentrated brine collected from two outlets at the bottom. Only a battery is used to provide the necessary electricity.
The researchers have so far tested only one unit, filtering seawater from a Massachusetts beach, which was purposely contaminated with small plastic particles, protein and human blood (I wonder who cut his finger to do this?). The results were more than satisfying: the system filtered 99% of the impurities and bacteria in the water, making it safe to drink. The team plans to produce a 100-unit device, followed by a 10,000-unit one, and hope that in two years they will be able to commercialize it.
