Solar-Powered Desalination for Off-Grid VillagesSeptember 15, 2014
Around the world, there is more salty groundwater than fresh, drinkable groundwater. For example, 60% of India is underlain by salty water, and much of that area is not served by an electric grid that could run conventional reverse-osmosis desalination plants.
Analysis by MIT researchers shows that a desalination technology called electrodialysis, powered by solar panels, could provide enough clean, palatable drinking water to supply the needs of a typical village. The study, by MIT graduate student Natasha Wright and Amos Winter, appears in the journal Desalination.
After weeks of field research in India, and reviews of various established technologies, Winter says, "when we put all these pieces of the puzzle together, it pointed very strongly to electrodialysis," which is not what is commonly used in developing nations.
The factors that point to the choice of electrodialysis in India include both relatively low levels of salinity — ranging from 500 to 3,000 milligrams per liter, compared with seawater at about 35,000 mg/L — as well as the region's lack of electrical power. (For on-grid locations, the team found, reverse-osmosis plants can be economically viable.)
Such moderately salty water is not directly toxic, but it can have long-term effects on health, and its unpleasant taste can cause people to turn to other, dirtier water sources.
Organizations working to improve clean-water access focus their attention on controlling known pathogens and toxins such as arsenic, Wright says. But her analysis showed the importance of "what the water tastes like, smells like, and looks like." Even if the water is technically safe to drink, that doesn't solve the problem if people refuse to drink it.
At the salinity levels seen in India's groundwater, the researchers found, an electrodialysis system can provide fresh water for about half the energy required by a reverse-osmosis system. That means the solar panels and battery storage system can be half as big, more than offsetting the higher initial cost of the electrodialysis system itself.
Electrodialysis works by passing a stream of water between two electrodes with opposite charges. Because the salt dissolved in water consists of positive and negative ions, the electrodes pull the ions out of the water, , leaving fresher water at the center of the flow. A series of membranes separate the freshwater stream from increasingly salty ones.
Both electrodialysis and reverse osmosis require the use of membranes, but those in an electrodialysis system are exposed to lower pressures and can be cleared of salt buildup simply by reversing the electrical polarity. That means the expensive membranes should last much longer and require less maintenance, Winter says. In addition, electrodialysis systems recover a much higher percentage of the water — more than 90%, compared with about 40-60% from reverse-osmosis systems, an advantage in areas where water is scarce.