Researchers at the Massachusetts Institute of Technology (MIT), U.S., have invented a portable desalination unit, weighing less than 10 kilograms, that can remove salts and impurities from the seawater to generate clean and clear drinking water.
The portable desalination unit is a suitcase-sized, user-friendly device that requires less power to operate, can be driven by a small portable solar panel and runs with the push of one button. This device generates clean drinking water without using any filters or high-pressure pumps. The water obtained exceeds the World Health Organisation (WHO) quality standards.
Unlike other desalination devices, it consumes less energy to discard impurities from water and reduces the long term maintenance requirements. This could enable the device to be deployed in remote areas such as communities on small islands. It could also be used to aid refugees or by soldiers carrying out long-term military operations.
“This is really the culmination of a 10-year journey that I and my group have been on. We worked for years on the physics behind individual desalination processes, but pushing all those advances into a box, building a system, and demonstrating it in the ocean, that was a really meaningful and rewarding experience for me,” said Jongyoon Han, a Professor of Electrical Engineering and Computer Science and Biological Engineering, and a member of the Research Laboratory of Electronics (RLE), MIT.
According to Junghyo Yoon, a Research Scientist at RLE, commercially available desalination units require high-pressure pumps to transport water through filters. Moreover, they are difficult to maintain and consume more energy.
However, their desalination unit uses a technique called ion concentration polarization (ICP) to filter the water. In this process, an electrical field is applied to membranes placed below and above a channel of water. The membranes repel negatively or positively charged particles as they flow over. These include salt molecules, viruses and bacteria. The particles then move to the second stream of water that is eventually discharged.
The ICP process removes suspended and dissolved particles and allows water to pass through channels. This process uses less energy as it requires a low-pressure pump.
As the process does not always remove all the particles floating in the middle of the channel, therefore, researchers used another process called electrodialysis to remove the remaining contaminants. Electrodialysis is a process in which electric current is used to remove salt molecules from an aqueous solution.
Researchers used machine learning to discover the ideal combination of electrodialysis and ICP modules. The setup involved a two-staged ICP process. In the first stage, water flowed through six modules and then through three modules in the second stage, followed by a single electrodialysis process. The modules minimized the energy usage and ensured that the process remains self-cleaning.
“While it is true that some charged particles could be captured on the ion exchange membrane if they get trapped, we just reverse the polarity of the electric field, and the charged particles can be easily removed,” explained Yoon.
The researchers stacked and shrunk the electrodialysis and ICP modules to improve energy efficiency and enable them to fit inside a portable device. They designed the device in such a way that the purification and desalination process could be done with just one button. The device notifies the users that the water is drinkable by decreasing the salinity level and the number of particles to specific thresholds.
They also created a smartphone app that can control the device and report on power consumption and water salinity levels to the users.
After performing lab experiments using water with different turbidity (haziness) and salinity levels, researchers decided to test the device at Boston’s Carson Beach.
They placed the unit near the shore and put the feed tube in the water. They observed that the device had filled the plastic drinking cup with drinkable water in about half an hour.
“It was successful even in its first run, which was quite exciting and surprising. But I think the main reason we were successful is the accumulation of all these little advances that we made along the way,” said Han.
The water collected after the experiment outperformed WHO quality checks and the unit decreased the number of impurities by at least a factor of 10. The desalination unit generated drinking water at a rate of 0.3 litres per hour and needed only 20 watts of power per litre.
“One of the biggest challenges of designing the portable system was engineering an intuitive device that could be used by anyone,” said Han.
Currently, Yoon is working on increasing the production rate of the device and energy efficiency. While Han is trying to create a technology that could rapidly detect impurities in drinking water.
“This is definitely an exciting project, and I am proud of the progress we have made so far, but there is still a lot of work to do,” said Han.
The detailed research has been published in the journal Environmental Science and Technology.
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