Sun-powered desalination for villages in India
While many homes in India currently use individual, home-based filtration systems to treat their water, Wright says after consulting with nongovernmental organizations that work in the area, she and Winter concluded that village-scale systems would be more effective — both because fewer people would be left out of access to clean water, and because home-based systems are much harder to monitor to ensure effective water treatment.
Most 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 because of the unpleasant salty taste, she says.
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.
How it works
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, Winter says, 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 percent, compared with about 40 to 60 percent from reverse-osmosis systems, a big advantage in areas where water is scarce.
Having carried out this analysis, Wright and Winter plan to put together a working prototype for field evaluations in India in January. While this approach was initially conceived for village-scale, self-contained systems, Winter says the same technology could also be useful for applications such as disaster relief, and for military use in remote locations.
Susan Amrose, a lecturer in civil and environmental engineering at the University of California at Berkeley who was not involved in this work, says, “This paper raises the bar for the level and type of scientific rigor applied to the complex, nuanced, and extremely important problems of development engineering. … Solar-ED isn’t a new technology, but it is novel to suggest developing it for systems in rural India, and even more novel to provide this level of detailed engineering and economic analysis to back up the suggestion.”
Amrose adds, “The water scarcity challenges facing India in the near future cannot be overstated. India has a huge population living on top of brackish water sources in regions that are water-scarce or about to become water-scarce. A solution with the potential to double recoverable water in an environment where water is becoming more precious by the day could have a huge impact.”
The research was funded by Jain Irrigation Systems, an Indian company that builds and installs solar-power systems, and sponsored by the Tata Center for Technology and Design at MIT.
— Read more in Natasha C. Wright and Amos G. Winter, “Justification for community-scale photovoltaic-powered electrodialysis desalination systems for inland rural villages in India,” Desalination 352 (3 November 2014): 82–91 (DOI: 10.1016/j.desal.2014.07.035)
Reprinted with permission of MIT News
