Electrodialysis could provide the most economic solution to desalinating India's brackish groundwater [Credit: Christine Daniloff]

Small-scale desalination powered by renewables ideal for India

Small-scale desalination plants running on renewables could make the salty groundwater underlying 60 per cent of India palatable, according to a new study.

Groundwater is more commonly salty than drinkable across the globe, but in particular in India where many rural areas are not served by an electric grid that could run conventional reverse-osmosis desalination plants.

Now an analysis by MIT researchers shows that a different desalination technology called electrodialysis, powered by solar panels, could provide enough clean, palatable drinking water to supply the needs of a village of 2,000 to 5,000 people.

Susan Amrose, a lecturer in civil and environmental engineering at the University of California at Berkeley who was not involved in this work, said: “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.”

Groundwater in India has relatively low levels of salinity — ranging from 500 to 3,000 milligrams per liter, compared with seawater at about 35,000 – and while moderately salty water is not directly toxic it can have long-term effects on health, and its unpleasant taste can cause people to turn to other, dirtier water sources.

Finding optimal solutions to the problem involved “detective work to understand the full set of constraints imposed by the market,” according to mechanical engineering professor Amos Winter, who co-wrote a study appearing in the journal Desalination with graduate student Natasha Wright.

After weeks of field research in India and reviews of various established technologies, he says, “when we put all these pieces of the puzzle together, it pointed very strongly to electrodialysis” – a technology not commonly used in developing nations.

It works by passing a stream of water between two electrodes with opposite charges and 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 centre of the flow. A series of membranes separate the freshwater stream from increasingly salty ones.

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 meaning the solar panels and battery storage system can be half as big, more than offsetting the higher initial cost of the electrodialysis system itself.

For on-grid locations, the team found, reverse-osmosis plants can be economically viable, but while both electrodialysis and reverse osmosis require the use of membranes, those in an electrodialysis system are exposed to lower pressures and can be cleared of salt build-up 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 per cent, compared with about 40 to 60 per cent from reverse-osmosis systems, a big advantage in areas where water is scarce.

Having carried out their 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.

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