A briefcase-sized device makes seawater drinkable

As climate change exacerbates droughts in many parts of the world, people are turning increasingly to desalination as a means to have access to fresh water. The construction of a desalination plant depends on the water source, location, and many other factors, but by most measures it’s a costly endeavor that can easily demand tens of millions of dollars. An infrastructure investment of that magnitude isn’t possible in rural, remote locations and won’t be of immediate help during natural-disaster emergencies.

For those places and situations, researchers have been working on portable desalination units. The currently available options can pump about a liter of fresh water using 3–14 watt-hours of energy. But those reverse-osmosis-based units are heavy (>20 kg), and the pumps they use are power-intensive (100–400 W), which limits their utility.

A government–academia research team, led by Eric Brack of the US Army Combat Capabilities Development Command Soldier Center and Jongyoon Han of MIT, has now designed a more portable desalination device, shown in the picture above. It weighs 9.4 kg and appears particularly useful for low-power situations and for soldiers operating in environments where fresh drinking water cannot be easily and routinely resupplied.

In reverse-osmosis desalination, water is purified by passing through a semipermeable membrane that removes ions and larger particles. An applied pressure from various pumps overcomes the water’s osmotic pressure to keep the flow unidirectional. One challenge with reverse-osmosis systems, though, is the filters. When they are fouled with contaminants, they need to be replaced.

Brack, Han, and their colleagues sidestepped that issue by turning to a separation technique that was first developed in the mid-20th century: electrodialysis. The process works by a series of two stacked membranes and a spacer in between. Under an applied electric potential, one membrane allows anions to pass through, and another, cations. The membranes are self-cleaning too when the polarity is reversed. Electrodialysis wasn’t particularly energy efficient in its infancy, but that has steadily improved over the past several decades.

The main limitation to electrodialysis now, however, is its inability to remove oil, bacteria, and other suspended solids. To solve that problem, the researchers paired their ion-concentration-polarization device, in which an ion-exchange membrane interacts with dissolved and suspended solids to remove them from the water, with an electrodialysis instrument.

The new desalination prototype was tested in the field near MIT at Carson Beach in Massachusetts. It was controlled automatically through a smartphone app and used 9.4 W of power to desalinate 1/3 of a liter of water in one hour, or about 28 Wh/L. When designing desalination systems, researchers face a trade-off between the energy efficiency of the separation process and the speed of the separation. Although larger, industrial-scale desalination plants do have a higher energy efficiency than the small portable device from Brack and Han’s study, the new device can be brought to places where a large system isn’t feasible. (J. Yoon et al., Environ. Sci. Technol., 2022, doi:10.1021/acs.est.1c08466 .)