seawater-to-drinking water

MIT researchers have built portable desalination equipment that can remove particles and salts to produce potable water and weighs less than 10 kg.

A small, portable solar panel, which can be purchased online for roughly $50, may also power the suitcase-sized device, which uses less power than a cell phone charger. It automatically produces drinking water that meets or surpasses WHO quality criteria. The technology is housed in a user-friendly device that operates with a single button press.

Unlike previous portable desalination devices that require water to pass through filters, this gadget removes particulates from drinking water using electrical power. Long-term maintenance requirements are considerably reduced by eliminating the need for replacement filters.

This could enable the unit to be deployed in remote and severely resource-limited areas, such as communities on small islands or aboard seafaring cargo ships. It could also be used to aid refugees fleeing natural disasters 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,”

says senior author Jongyoon Han, a professor of electrical engineering and computer science and of biological engineering, and a member of the Research Laboratory of Electronics (RLE).

Joining Han on the paper are first author Junghyo Yoon, a research scientist in RLE; Hyukjin J. Kwon, a former postdoc; SungKu Kang, a postdoc at Northeastern University; and Eric Brack of the U.S. Army Combat Capabilities Development Command (DEVCOM). The research has been published online in Environmental Science and Technology.

Filter-free technology

According to Yoon, most commercially available portable desalination systems use high-pressure pumps to drive water through filters, which are challenging to miniaturize without affecting the device’s energy efficiency.

Instead, they use a technique known as ion concentration polarization (ICP), which was developed by Han’s group over a decade ago. Instead of filtering water, the ICP technique uses an electrical field to apply to membranes above and below a water channel. As salt molecules, bacteria, and viruses pass across the membranes, they resist positively or negatively charged particles. The charged particles are directed into the second stream of water, which is then expelled.

Both dissolved and suspended particles are removed during the process, allowing clean water to travel through the channel. ICP employs less energy than other procedures because it simply requires a low-pressure pump.

However, ICP does not always remove all of the salts floating in the channel. To eliminate any residual salt ions, the researchers used a second method known as electrodialysis.

Yoon and Kang used machine learning to discover the best ICP and electrodialysis module combination. A two-stage ICP process, with water running through six modules in the first stage and three in the second, followed by a single electrodialysis procedure, is the best setup. This reduced energy consumption while guaranteeing that the process was 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,”

Yoon explains.

The ICP and electrodialysis components were reduced and stacked to increase energy efficiency and fit inside a portable device. The device was created with non-experts in mind, using only one button to start the desalination and purification process. The device alerts the user when the salinity level and particle count fall below certain criteria, indicating that the water is safe to drink.

The researchers also developed a smartphone app that can remotely manage the device and provide real-time statistics on power consumption and water salinity.

Beach tests

After running lab experiments using water with different salinity and turbidity (cloudiness) levels, they field-tested the device at Boston’s Carson Beach.

Yoon and Kwon set the box near the shore and tossed the feed tube into the water. In about half an hour, the device had filled a plastic drinking cup with clear, drinkable water.

“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,”

Han says.

The resulting water exceeded World Health Organization quality guidelines, and the unit reduced the number of suspended solids by at least a factor of 10. Their prototype generates drinking water at a rate of 0.3 liters per hour and requires only 20 watts of power per liter.

“Right now, we are pushing our research to scale up that production rate,” Yoon says.

One of the biggest challenges of designing the portable system was engineering an intuitive device that could be used by anyone, Han says.

Through a firm he aims to form to market the idea, Yoon hopes to make the gadget more user-friendly while also improving its energy efficiency and production rate.

Han wants to use the lessons he’s learned in the lab to address water-quality challenges other than desalination, such as quickly detecting pollutants 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,”

he says.

For example, while the “development of portable systems using electro-membrane processes is an original and exciting direction in off-grid, small-scale desalination,” the effects of fouling, especially if the water has high turbidity, could significantly increase maintenance requirements and energy costs, notes Nidal Hilal, professor of engineering and director of the New York University Abu Dhabi Water research center, who was not involved with this research.

“Another limitation is the use of expensive materials,” he adds. “It would be interesting to see similar systems with low-cost materials in place.”

Source: Massachusetts Institute of Technology

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