x Abu Dhabi, UAEMonday 22 January 2018

Chipping away at salinity

Even state-of-the-art desalination plants offer room for improvement. Scientists have developed a chip that uses an electrical charge to separate salt from water far more cheaply than current methods.

A chip that uses an electrical charge to separate out salt could be the answer to eliminating the drawbacks of reverse osmosis. Courtesy Kyle Knust
A chip that uses an electrical charge to separate out salt could be the answer to eliminating the drawbacks of reverse osmosis. Courtesy Kyle Knust

Few countries in the world depend as heavily as the UAE does on desalinating seawater.

Lacking the freshwater resources of many other nations, and with economic expansion and population growth fuelling demand for water, it must desalinate vast quantities of seawater – 1.7 billion cubic metres in 2011, the equivalent of almost 700,000 Olympic-sized swimming pools and second only to Saudi Arabia, which pumped 2.02 billion cubic metres.

No wonder, then, that more desalination plants are being built as quickly as possible. The country’s largest desalination plant, the Dh10 billion M Station in Jebel Ali, operated by Dubai Electricity and Water Authority, opened in April.

This 2,060-megawatt gas-fired facility can pump a staggering 530 million litres – more than 200 Olympic swimming pools – in a day.

Like other new or forthcoming plants, such as the extension to the Emirates Sembcorp Water & Power Company plant in Fujairah, where ground was broken in June, the Jebel Ali station houses the latest technology to improve efficiency and keep down costs.

But even state-of-the-art desalination plants offer room for improvement.

To remove the salt, the plants employ a process called reverse osmosis, in which pressure forces water through a membrane.

But to preserve these membranes, the microorganisms in seawater must first be destroyed with chlorine. This chlorine then needs to be removed, at further expense, as it too could damage the polyamide membranes.

A possible way of eliminating these drawbacks is being developed by scientists at the University of Texas, who have developed a chip that uses an electrical charge to separate out the salt. In doing so, it avoids the use of both chlorine and membranes.

Contained within the chip is a tiny water channel that separates into two. An electric current produced by a metal strip embedded in the chip causes heavily salted brine to travel up one path, while water with a much lower salt content flows through the other channel.

“The unique thing about it is that it doesn’t require a membrane,” said Prof Richard Crooks, who runs the laboratory where the chip was created. “That’s the main hook. Membranes are expensive, they’re fragile and you cannot pump seawater into them – you have to disinfect the seawater first and then remove the disinfectant.

“With our approach, all of these problems go away.”

Membranes wear out, too, adding to the cost.

All in all, reverse osmosis uses about twice as much energy as the theoretical minimum that could be used to do the job.

Prof Crooks believes his chip could use about a fifth less – just 60 per cent more than the theoretical minimum.

And once the energy normally used to disinfect the water and then remove the disinfectant is factored in, his method offers savings of about 30 per cent – or just 40 per cent more than the lowest possible energy use.

The chip has been developed by Kyle Knust, a fourth-year doctoral student in Prof Crooks’s lab. He is now focused on adjusting the chip to make it more effective.

Mr Knust is working to increase the amount of salt the electrical current sends up the brine channel of the chip, something that requires a fundamental understanding of the electric field created by that current, and how to the size and shape of that field is controlled.

“Once we have a firm, fundamental grasp of what’s going on, then we can tune the electric field to increase salt rejection,” he said.

As it stands now, the chip can only take a quarter of the salt out of seawater. That, clearly, is nowhere near enough – to produce water fit for drinking, it will need to remove as much as 99 per cent.

To get closer to that figure, the Texas group is working with others at the University of Marburg in Germany to model the whole process on computer. They believe that, with refinements, it should be able to remove half the salt.

That would be much better than nothing. Half as much salt means, if nothing else, half as much work to be done by traditional methods such as reverse osmosis to make the water potable. And that represents an enormous energy saving.

And semi-desalinated water has uses in its own right – as cooling water for power stations, for example.

“Our system would be fine for that application. That’s a huge market,” Prof Crooks said.

There is one other small snag. The system can produce about 0.04 microlitres of desalted water a minute – or about a teaspoon every three months.

“There is considerable effort going towards increasing that number,” Mr Knust said.

He admitted that low productivity puts the technology at risk of becoming little more than a “laboratory curiosity”.

Still, he pointed to the fact that a company, Okeanos Technologies, was already working at scaling it up as an indication that others see its commercial potential.

While industrial desalination using the chip remains “a long way from being reality”, the hope is that ultimately it could be used on the grand scale of the kind seen in the UAE and other Arabian Gulf countries.

And that, Mr Knust said, was only going to become more important as climate change cut into the amount of freshwater available.

“The demand for water in general is increasing because of population growth, climate change and economic drivers,” he said. “People want more water for economic growth.”