Scientific Method / Science & Exploration

Small scale, energy-efficient desalination demoed

Combining a voltage difference with a selectively permeable membrane can …

Right now, estimates are that a few hundred million people suffer from limited access to fresh water, and population growth and climate change are expected to rapidly exacerbate that problem. Many of these people have access to brackish or salty water, but desalinization plants only work efficiently on the large scale, with an attendant infrastructure. A paper released by Nature Nanotechnology on Sunday describes a microfluidic approach to desalinization that is roughly as energy-efficient as a large-scale plant, but compact enough that it could operate as a small, battery-powered device.

The basic idea behind the device is extremely simple: send a flow of salty water down a channel with a Y-shaped junction, and convince all the charged items, whether they're salt ions or cells, to take the left turn. That leaves a flow of relatively pure water running down the right fork.

Convincing everything to make a left turn requires a technique termed ion concentration polarization. The right-hand fork of the device has a small channel covered with a membrane called Nafion that only admits small, positively charged ions (called cations). There's also a voltage difference across that channel that repels the cations. Instead of backing everything up right at the Nafion membrane, this setup actively repels all charged particles. Placing it at the mouth of the right-hand channel neatly forces all the charged materials to the far-left-hand wall of the device, where it will be carried down the left turn of the device. The water that flows down the right-hand side will be free of charged materials—meaning salt-free.

Membranes are currently used in desalinization processes, such as reverse osmosis, but those techniques require a great deal of care to be taken in order to avoid clogging the pores of the membranes. In the technique outlined here, most of the material is actively repelled before it gets anywhere near the membrane, which the authors suggest should greatly expand its usable life span.

The authors tested a small version of the device using sea water obtained from a beach in Ipswich, Massachusetts (they were based at MIT), which was passed through a filter to get rid of particulates. To monitor the flow and confirm that biological materials would go along for the ride (most biological materials carry a charge under voltage), they spiked the sea water with red blood cells and a fluorescent molecule.

Under a bit of pressure, the material would flow evenly down both the left and right channels. But, almost as soon as the voltage is applied, the active repulsion kicked in, and an ion depletion boundary formed just upstream of the right-hand channel. As a result, the fluid that flowed down the right-hand passage was clear (the process was so efficient that about half the material flowing down the left-hand passage was also clear.

The material collected from the desalinized channel had about 180mg/l of ionic material in it; that's a significant improvement from the 30,000mg/l in the source material, and it is less than a third of the upper limit for potable water. The power requirements are estimated at between four and five Watt-hours for each liter of water, which is in line with large commercial desalinization techniques. The water requires so little force to move through the channels in this new system that the hardware could simply be gravity fed.

Leaving the realm of implementation, the authors consider how to scale the device up by expanding the diameter of the channels and adding multiple channels to a single device. They come up with a six-to-eight inch chip (<20cm) that would be capable of producing over 200ml of fresh water every minute. By combining the Nafion membrane and one of the electrodes, it's also possible to cut down on the energy required, but in any case, it should require far less power than current commercial devices, which aren't nearly as portable and require in the neighborhood of 100W-hr/liter. (For context, it's possible to pick up 100W solar panels on Amazon for roughly $300; they cover roughly a square meter.)

There are a couple of major caveats to this work. For starters, the filtering technique doesn't get rid of an neutral organic materials, which means that, although the water that comes out is relatively salt-free, it may not be safe to drink until it's also passed through a material like activated carbon. The other issue is that concentrating calcium ions tends to cause them to precipitate, so the authors added sodium hydroxide to the sea water first, which precipitated out the calcium before it hit the filter. Neither of these steps are show-stoppers, but they would add to the complexity of the device.

In any case, the authors don't think that the hardware's ready to replace large-scale facilities. Instead, they suggest that it might make sense in "disaster- and poverty-stricken areas," where there's either a need for temporary desalinization, or access to the infrastructure needed to support industrial desalinization is unlikely to arrive any time soon.

(The paper's supplementary data contains diagrams of the device.)

Nature Nanotechnology, 2010. DOI: 10.1038/NNANO.2010.34  (About DOIs).

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