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 its 5m inhabitants with water.

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 turn 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 0.05% 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. Brine is heated, and the resulting water vapour is condensed. Other plants employ reverse osmosis. This uses molecular sieves that pass 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. The Siemens system, by contrast, consumes 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 and 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.

The result is that the ions concentrate in half of the channels, creating a strong brine, while fresher water accumulates in the other half. As the brine emerges, it is thrown away. The fresher water is put through the same process twice more and eventually has its salt concentration reduced to 1%. There is then one further step. This is to employ an ion-exchange resin in addition to the membranes. Such resins increase the electrical conductivity of the system and allow one more passage, bringing the salt concentration below 0.05%.

A demonstration plant has been operating since December, and a full-scale pilot plant should be completed by 2013. If all goes well, then, Singapore's inhabitants will soon no longer feel like Coleridge's ancient mariner—that there is water, water, everywhere, nor any drop to drink.

Correction: The original version of this article wrongly stated that to make seawater potable, its salt content must be reduced to below 0.5%. The correct figure is 0.05%.