(a) Schematic diagram of possible optimized unit design. (b) (Left) Gravity-fed ICP desalination system. μCP stack has many parallel microfluidic ICP-device for salt/pathogen removal. Prefilter can eliminate larger particles and neutral organic compounds. (Right) Perspective of the unit device.
A new approach to desalination being developed by researchers at MIT and in Korea could lead to small, portable units that could be powered by solar cells or batteries and could deliver enough fresh water to supply the needs of a family or small village. As an added bonus, the system would also remove many contaminants, viruses and bacteria at the same time. The new approach is called ion concentration polarization.
The new system achieves “perhaps the lowest energy ever for desalinating microliters of water,” and when many of these micro-units are combined in parallel, as Kim and his co-authors propose, “it could be used to supply liters of water per hour using only a battery and gravity flow of water. Having proved the principle in a single-unit device, Kim and Han plan to produce a 100-unit device to demonstrate the scaling-up of the process, followed by a 10,000-unit system. They expect it will take about two years before the system will be ready to develop as a product.
Affordable clean water is needed to prevent millions of people from getting sick from water borne diseases and drinking contaminated water. Solving and deploying a solution to the clean water problem will save millions of lives every year.
A single unit of the new desalination device, fabricated on a layer of silicone. In the Y-shaped channel (in red), seawater enters from the right, and fresh water leaves through the lower channel at left, while concentrated brine leaves through the upper channel.
Photo: Patrick Gillooly
One of the leading desalination methods, called reverse osmosis, uses membranes that filter out the salt, but these require strong pumps to maintain the high pressure needed to push the water through the membrane, and are subject to fouling and blockage of the pores in the membrane by salt and contaminants. The new system separates salts and microbes from the water by electrostatically repelling them away from the ion-selective membrane in the system — so the flowing water never needs to pass through a membrane. That should eliminate the need for high pressure and the problems of fouling, the researchers say.
The system works at a microscopic scale, using fabrication methods developed for microfluidics devices — similar to the manufacture of microchips, but using materials such as silicone (synthetic rubber). Each individual device would only process minute amounts of water, but a large number of them — the researchers envision an array with 1,600 units fabricated on an 8-inch-diameter wafer — could produce about 15 liters of water per hour, enough to provide drinking water for several people. The whole unit could be self-contained and driven by gravity — salt water would be poured in at the top, and fresh water and concentrated brine collected from two outlets at the bottom.
So far, the researchers have successfully tested a single unit, using seawater they collected from a Massachusetts beach. The water was then deliberately contaminated with small plastic particles, protein and human blood. The unit removed more than 99 percent of the salt and other contaminants. “We clearly demonstrated that we can do it at the unit chip level,” says Kim. The work was primarily funded by a grant from the National Science Foundation, as well as a SMART Innovation Centre grant
While the amount of electricity required by this method is actually slightly more than for present large-scale methods such as reverse osmosis, there is no other method that can produce small-scale desalination with anywhere near this level of efficiency, the researchers say. If properly engineered, the proposed system would only use about as much power as a conventional lightbulb.
A shortage of fresh water is one of the acute challenges facing the world today. An energy-efficient approach to converting sea water into fresh water could be of substantial benefit, but current desalination methods require high power consumption and operating costs or large-scale infrastructures, which make them difficult to implement in resource-limited settings or in disaster scenarios. Here, we report a process for converting sea water (salinity ~500 mM or ~30,000 mg l−1) to fresh water (salinity < 10 mM or < 600 mg l−1) in which a continuous stream of sea water is divided into desalted and concentrated streams by ion concentration polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes. During operation, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding two problems that plague other membrane filtration methods. To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of sea water (~99% salt rejection at 50% recovery rate) at a power consumption of less than 3.5 Wh l−1, which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.
Salt and other charged particles are diverted off, leaving desalted water to flow down a separate channel
Han’s team developed a microchip-sized device that funnels a stream of water down to a fork and splits into two channels. The entrance to one channel is covered with a charged Nafion membrane, which shields the water flowing down it and pushes any salt down the other channel. Crucially, the shield also repels other charged particles, both positive and negative, which includes most organic matter and microorganisms, such as bacteria, viruses and other contaminants.
But to function effectively the process requires very small water channels and these can only produce tiny amounts of water on their own. ‘Our future direction is similar to how the semiconductor industry makes microchips,’ Han explains. ‘We can envision thousands of water channels on a single chip – the goal is to make systems that can produce around a litre of purified water over ten minutes.’
Although Han admits this is a relatively small amount, it may be possible to run the device continually for a long time using solar power, which could be extremely valuable in areas of critical water shortage.
Adel Sharif, an expert in water treatment and desalination at the University of Surrey, UK, is interested by the new process, but thinks that more work is needed. ‘There are some problems to overcome,’ he told Chemistry World. ‘Gold and titanium electrodes are currently used, so finding cheaper or alternative materials is needed to scale the technology up into devices. Also, some non-charged particles may cause fouling of the membrane – so a system of pre-treatment may be required.’