IEEE Spectrum June, 2010 - 59

special report

The company calculated that the recycled
water's exceeding purity saves it more
than half a million dollars a year, in part
by cutting out steps in its internal waterpurification process.
Singapore also started priming the
public. The prime minister drank a bottle of NEWater at a national festival, and
the crowd cheered. The subtext was
clear-patriotic Singaporeans drink
wastewater. But the rest of Singapore
was slower to follow. A parody of a popular nationalist song, "Count on Me,
Singapore," cheerily urged residents to
"Drink Our Pee, Singapore."
T he que a sy r e ac t ion s of s om e
Singaporeans didn't deter the utility,
which built four more wastewater treatment plants and is about to increase its
NEWater production to 555 megaliters
a day. By the end of this year the plants
will treat enough sewage to cover nearly
a third of Singapore's water needs.
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For now, only a tiny fraction of the
drinking supply is NEWater. About
40 ML are blended in with the city-state's
potable water each day, equivalent to
2 percent of consumption. The utility also
built a seawater desalination plant that
produces 136 ML of water each day, or a bit
less than one-tenth of Singapore's supply.
The city-state has all the water it needs.
N at u r e ' S h y d r o l o g i c a l c yc l e r e l i e S

on sunlight to evaporate water, leaving behind salt and other impurities
and returning freshwater to Earth in
the form of rain and snow. But it can be
capricious: Most freshwater evaporates
from the oceans, where it rises through
the atmosphere and then cools to form
clouds. Winds push around the clouds,
driving a few of them over land, where
they deposit a mere tenth of the world's
precipitation, according to the U.S.
Geological Survey. Singapore decided to

n

water vs. e nergy

cut the atmosphere out of the transaction
and to replace evaporation with speedier
mechanisms-membranes.
Here's how it works: First, a treatment
plant gathers up the city's used water and
separates out the large, easily removable
impurities. About 60 percent of the water
is released into the sea. The rest gets sent
to a NEWater factory. The water enters
contaminated with bacteria, viruses, and
all sorts of carbon-based particles and
emerges cleaner than what flows from
just about any tap in the world.
The main tactic of the water trade is
to simply push water through tiny holes-
the smaller the holes, the fewer the undesirable molecules that can sneak through.
The art is to do so without sending the
electricity bill skyrocketing or clogging
up the tiny holes with grime. Here, the
first step is to force the water through a
membrane that blocks particles of up to
0.2 micrometers in size, catching most
bacteria and protozoa. The membrane
looks like a cylinder filled with skinny,
hollow tubes. The stream flows into the
porous straws. Water molecules push
through the pores and collect outside
the membranes, while the larger particles continue traveling inside the tubes,
to be disposed of separately.
The water still needs to be stripped
of any viruses that might be adrift in the
flow. For this the partially treated stream
needs a reverse-osmosis membrane. In
one configuration, paperlike sheets of
membrane more than a meter long are
sandwiched between sheets called spacers. The stack of membranes and spacers
is wound up into a cylinder, like a thick
roll of wrapping paper. The core of the
cylinder is an empty channel where the
clean water collects.
As the stream is pushed into one end
of the roll, the impurities slide along the
spacers and never penetrate the polymer
membrane. The water molecules, however, escape through the 0.0001-µm pores
and slip into the central channel.
These two steps remove just about
everything, but in a third and final step,
mercury lamps generate ultraviolet-light
rays that penetrate the water. The radiation scrambles the genetic material of any
bacteria and other microorganisms that
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59
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Table of Contents for the Digital Edition of IEEE Spectrum June, 2010
