A new low-energy method for removing salt from seawater has been developed by researchers from Germany and the US. Freshwater is needed all over the world for drinking and irrigation purposes, and desalinated ocean water is one obvious source for coastal communities with low rainfall and abundant energy resources. Conventional methods for desalinisation include vacuum distillation, in which freshwater is boiled off as steam and then condensed; and reverse osmosis, where seawater is passed through a membrane which filters out the salt. The problem with these techniques lies in their cost – their energy demands mean they are too expensive for use in a domestic setting.
The electrochemical desalination technique developed by the researchers is shaping up to be an affordable alternative to these methods. The team have created a prototype ‘water chip’ to demonstrate the process, known as electrochemically mediated seawater desalination. Water flows through a small channel in the chip, the path of which forks at an electrode. This electrode neutralizes some of the chloride ions in the water, altering the local electric field by forming an ‘ion depletion zone’. The change is enough to funnel salt into one branch of the chip, with purer water coming out of the other. The whole process doesn’t need much energy: the water chip can be powered by a standard, three volt store-bought battery.
The water chip is now being prepared for commercial application by a spin-off company, Okeanos Technologies. At present, however, the prototype’s capacity is limited – it only removes around 25% of the salts from seawater and outputs freshwater at a rate of 40 nanolitres per minute. At such speeds, it would take around 13.5 years to process enough water to fill a cup.
"While the results appear interesting, the low salt rejection and high water recoveries per stage of treatment remain a challenge for practical application”, says Prof. Stephen Gray, Director of the Institute for Sustainability and Innovation, Victoria University. “On the current data, multiple stages will be required to treat seawater to useful salinity values”.
Nevertheless, the researchers are happy with the prototype chip as a proof of concept. “The chip is currently small scale and requires little infrastructure so it could find applications…in resource limited settings”, says Kyle Knust, a researcher at the University of Texas at Austin, and one of the authors of a paper outlining the process. The researchers are now working to scale up the technique for larger applications, such as households or communities, and are confident it will eventually produce 99% desalinated water at practical speeds. – Ian Randall
Photo credit: The University of Texas at Austin