Science - November 3, 2005

Cheap drinking water from salt water

Is a cheap way of turning seawater into fresh water an impossible dream? According to Wageningen environmental technologists no. It can be done by passing an electric current through water using special electrodes. The first results are very promising.

‘We want to develop a simple and cheap system for the Third World,’ says chair of Environmental Technology, Professor Wim Rulkens, ‘A system in which you can convert salt water into drinking water or irrigation water.’ Methods for converting salt water into fresh water exist already, such as evaporation or reverse osmosis. But these techniques are expensive, often complex and require large amounts of energy.

Rulkens is working together with a PhD student on a new relatively simple technique called capacitive deionisation. The charge in the salt ions causes them to move to the electrodes, and then the salt-free water can be drawn off. Rulkens: ‘The beauty of this method is that you can discharge the electrodes after a while, and regain some of the electricity you put in to start with.

The first tests show clearly that a high return is possible: ‘If you put in 10 litres of brackish water, you get back 9.5 litres of fresh water,’ says PhD researcher Bart van Limpt, of the sub-department of Environmental Technology. He tested the technique and compared electrodes made of different materials. Carbon came out best. Woven carbon fibres and carbon plastic, a mixture of carbon powder and a polymer, gave the best results. Van Limpt: ‘Woven carbon fibres have the largest internal surface area and therefore attract more salt ions. A woven carbon electrode can attract more than a thousand times as much as conventional graphite electrodes.

An additional advantage of the technique is that it uses little energy. By discharging the electrodes you can win back more than half of the electricity you used in the first place. The principle of reusing the electricity is similar to that used in an electric car, explains Van Limpt. ‘When the car brakes, the energy that is released is stored in a condenser. When the car starts to go you start to use the stored electricity. We are still in the experimental phase, but it is clear that it is low on electricity to use. We hope that the system can run on a simple accumulator or solar cell, and therefore will be useful for developing countries.’ The system now needs testing on a bigger scale.

There are still practical obstacles to be overcome and costs remain an issue. The strongly concentrated salt water that remains has to be disposed of. Despite the technique itself being cheap, other costs may make the use of seawater for drinking or irrigation water unfeasible. Professor Gerard Bot of the Systems and Control Group comments: ‘You can come up with all sorts of wonderful ideas for getting water from deep under the ground, desalinating sea water or building seawater greenhouses in the desert. But it might be unfeasible if you have to transport water over long distances, and therefore have to lay expensive pipelines. In this kind of projects the crux of the matter is often the transport costs.’ / Hugo Bouter

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