With new water desalination method, potable water for disaster areas

Water, water, everywhere; nor any drop to drink.

Potable water is always in high demand and short supply following a natural disaster.

A new technique for water desalination could lead to small, portable units that could be sent to disaster sites or remote locations.

Converting salty seawater to potable fresh water usually requires two things: a large amount of reliable electrical power, and a large-scale desalination plant.

A popular desalination method is reverse osmosis, which uses membranes that filter out the salt. But it requires powerful pumps to maintain the high pressure needed to push the water through the membrane. Moreover, the pores of the membrane can be blocked by salt and contaminants.

But a new technique developed by researchers at MIT and in Korea could lead to small units that could deliver enough fresh water to supply the needs of a family or small village, powered by solar cells or batteries.

The system can also remove contaminants, viruses and bacteria.

The new technique is called ion concentration polarization, and was developed by postdoctoral fellow Sung Jae Kim, MIT professor Jongyoon Han and colleagues in Korea. The process separates salts and microbes from the water by electrostatically repelling them away from the ion-selective membrane in the system.

Because the flowing water never needs to pass through a membrane, the high pressure requirement and the blockage problems of a conventional membrane are avoided.

The system works at a microscopic scale and uses fabrication methods developed for microfluidics devices — think silicone microchips for water instead of silicon microchips for electricity.

At scale — say, an array of 1,600 units on a wafer that’s eight inches in diameter — the system could produce about 15 liters, or approximately four gallons, of water per hour.

That’s enough drinking water for several people.

The entire unit is self-contained and driven by gravity: pour salt water into the top, and freshwater and brine are collected in two outlets at the bottom.

The electricity required by this method is actually more than a large-scale desalination method, but no other solution has been demonstrated that can do so with such portability and efficiency — about as much power as a conventional light bulb.

The researchers so far have successfully tested a single unit, and were able to remove more than 99 percent of contaminants in seawater. It doesn’t work for contaminants without an electric charge, such as industrial pollutants, but can be combined with a conventional charcoal system to achieve purity.

While the amount of electricity required by this method is actually slightly more than for present large-scale methods such as reverse osmosis, there is no other method that can produce small-scale desalination with anywhere near this level of efficiency, the researchers say.

The researchers plan to produce a 100-unit system (and then a 10,000-unit system) to demonstrate the process at a higher scale. It will be at least two years before the innovation becomes commercialized.