Desalination

Drops to drink
Jul 19th 2011, 17:00 by The Economist onlinehttp://www.economist.com/blogs/babbage/2011/07/desalination
 
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.
 
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,
though, is put through the same process twice more and eventually has
its salt concentration reduced to 1%.
 
That is not bad, but is still double what is potable. There is
therefore one further step in the process. 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
to bring the salt concentration down below 0.5%, which makes the water
potable.
 
A demonstration plant has been operating since December, and a
full-scale pilot plant is now under construction and 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, but not a drop to drink.