Jul 252011
Clean drinking water...not self-evident for ev...

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SINGAPORE’S average annual rainfall is more than double that of notoriously soggy Britain, so the casual observer might be surprised to learn that the place has a shortage of drinking water.

Yet with around 7,000 people per square kilometre, Singapore is the third most densely populated country in the world. Its land mass is not large enough to supply the thirst of its 5m inhabitants.

One answer is to desalinate seawater. That, though, is expensive, so the Singaporean government is keen to find cheaper ways of doing it. And, in collaboration with Siemens, a German engineering conglomerate, it may have done so, for Siemens says its demonstration electrochemical desalination plant on the island can transform seawater into drinking water using less than half the energy required by the most efficient previous method.

To make seawater fit for human consumption its salt content of approximately 3.5% must be cut to 0.5% or less. Existing desalination plants do this in one of two ways. Some employ distillation, which needs about 10 kilowatt-hours (kWh) of energy per cubic metre of seawater processed. The energy is used to heat the brine, partially evaporating it, and to condense the resulting water vapour. Other plants employ reverse osmosis. This uses special membranes which act as molecular sieves by passing water molecules while holding back the ions, such as sodium and chloride, that make water salty. Generating the pressure needed to do this sieving consumes about 4kWh per cubic metre of water. The Siemens system, by contrast, consumes only 1.8kWh per cubic metre, and the firm hopes to get that down to 1.5kWh.

It works using a process called electrodialysis, in which the seawater is pumped into a series of channels walled by membranes that have slightly different properties from those used in reverse osmosis. Instead of passing water molecules, these membranes pass ions. Moreover, the membranes employed in electrodialysis are of two types. One passes positively charged ions. The other passes negatively charged ones. The two types alternate, so that each channel has one wall of each type. Two electrodes flanking the system of channels then create a voltage that pulls positively charged ions such as sodium in one direction and negatively charged ions such as chloride in the other.

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