Fresh water, one droplet at a time
Safe drinking water is usually in short supply in the wake of natural disasters like the earthquake in Haiti or Hurricane Katrina. In disaster zones near the ocean, converting salty seawater to potable fresh water seems like a no-brainer, but this usually requires large-scale desalination plants and plenty of dependable electrical power–neither of which was available in Haiti or New Orleans.
Now researchers at MIT and in Korea are developing a technology that could be used in small, portable desalination units powered by solar cells or batteries instead of diesel generators. Such devices, which would remove viruses, bacteria, and other contaminants as well as salt, could produce enough fresh water for a family or small village. Carrying them to places where water is needed would be more efficient than trying to transport water, especially if roads aren’t passable. While designed for emergency use, the devices could also be used in remote seaside locations in developing countries.
The new technique exploits a phenomenon called ion concentration polarization to separate salts and microbes from the water. A stream of salt water flows across a microchip-size device in a microchannel that splits into two branches. At the split, the channel is connected to a nanochannel covered with an ion-selective membrane made of Nafion, a synthetic polymer. When a current is passed through the membrane, an electrostatic barrier is created that repels charged particles such as salt, viruses, and microbes, pushing them into one branch of the water-bearing channel and allowing fresh, potable water to flow through the other. Because no water passes through the membrane, the particles don’t clog it–a problem that can plague desalination technologies based on reverse osmosis.
The process works at a microscopic scale, so each device would process only minute amounts of water. But an array with 1,600 units, fabricated on a wafer about 20 centimeters in diameter, could purify about 18 liters per hour. The whole array could fit into a container about the size of a coffee urn. Unlike reverse-osmosis systems, which require pumps to push water through a membrane, it would be driven by gravity. Salt water would be poured in at the top of the container, and fresh water and concentrated brine would flow from two spigots at the bottom.
The new approach is described in a paper in Nature Nanotechnology by postdoctoral associate Sung Jae Kim and associate professor Jongyoon Han, both in the Department of Electrical Engineering and Computer Science, and colleagues in Korea.
In a test with a single-unit cell, using seawater with contaminants added, the unit removed more than 99 percent of the salt and other contaminants. The researchers plan to produce a 100-unit array to demonstrate the scalability of the process.