desalination
Singapore is a lush, steamy spot, regularly
soaked by torrential tropical downpours. The
island nation lies just off the tip of the Malay
peninsula, barely 100km north of the equator,
right at the heart of the tropics. Over half of
the country is covered by parks and forest,
thriving in the rainforest climate.
And yet potable water in Singapore is in
short supply. Singapore’s population density
exceeds 7000 people per square kilometre
of land – a figure almost 30 times higher than
that of the UK – and supply can’t keep up with
demand. Despite receiving some 2400mm of
rain each year, the country is only 700km 2 in
size, and can’t collect and store enough water
for all these people. Singapore already relies
Keeping
the tap on
James Mitchell Crow investigates routes to
quenching our thirst without costing the Earth
44 | Chemistry World | February 2012 | www.chemistryworld.org
on imports from neighbouring Malaysia for
a significant proportion of its water. And the
country’s water authority, the Public Utilities
Board (PUB), predicts that within the next 50
years demand for water will double.
So Singapore has turned to technology,
becoming one of the world’s leaders in
tapping unconventional water sources and
declaring itself a ‘global hydrohub’. PUB chief
executive Khoo Teng Chye says that ‘Singapore
has turned its water vulnerability into its
strength.’ The organisation already has plans
in place for how it will meet the predicted
growth in water demand over the next half
century. Remarkably, it will try to do so while
ending its reliance on imported water.
desalination
(Right) Reverse osmosis
schematic
(Below) Forward osmosis
schematic
ben jeeves
Singapore’s focus on securing its water
supplies has already netted some notable
achievements. The country now meets 30%
of its water needs by recycling wastewater.
By 2060, PUB plans to meet half of its water
demand this way. However, recycling every
drop of water is impossible, and new supplies
will always be needed. For that, Singapore is
turning to the sea. 10% of the country’s water
currently comes from desalinated seawater,
and the 2060 target is for 30% of its water
needs to be met this way.
But at what cost? Desalination can come
with a number of potential environmental
penalties. Many of these can be at least
partially overcome (see box p47), but the
real stumbling block is that desalination is a
highly energy intensive process, with a carbon
footprint to match. Countries around the
world, from Spain to the US to Australia, are
increasingly turning to desalination plants
to supply their water, at the same time as
pressure grows to cut carbon emissions. Can
the research taking place in Singapore, and in
many other parts of the world, drive down the
energy demand of desalination?
drive it. But Chung is working on a new
generation of draw solution that requires
much less energy again, which should make
draw solution regeneration simply a case of
warming in the sun.
Chung’s team are developing stimuliresponsive nanoparticles for use as the solute
in the draw solution. Thanks to their high
surface area, hydrophilic nanoparticles can
generate a high osmotic pressure. Chung’s
original idea for recapturing the nanoparticles
– to leave behind pure water – was to use
particles with a magnetite core that could
be trapped with a magnetic field. However,
after several cycles the particles increasingly
clumped together, reducing the osmotic
pressure of the draw solution and so reducing
the yield of fresh water. 3
The team’s latest generation of
nanoparticles add temperatureresponsiveness to their repertoire. The
nanoparticles are functionalised with
an amphiphilic polymer coating poly(Nisopropylacrylamide) (PNIPAM), which
consists of a hydrophobic hydrocarbon
backbone decorated with hydrophobic
pendant isopropyl groups, each attached via
ben jeeves
Going with the flow
The vast majority of desalination plants being
built today are based on reverse osmosis
(RO). This technology relies on selectively
permeable membranes that allow water
through but not salt. When salt water is
placed under high pressure on one side of the
membrane, water molecules are forced across,
producing a flow of fresh water and leaving
behind a concentrated brine. It is this need
to force water through a membrane, against
the osmotic gradient, that makes RO such an
energy intensive process.1
Reducing this energy demand is the
ultimate aim of Neal Tai-Shung Chung and his
team at the National University of Singapore.
And there is one fledgling desalination
technology that they are particularly focused
on. ‘The major effort in my group is forward
osmosis, because it can lower the energy
consumption of desalination,’ Chung says.
Rather than pushing against the osmotic
gradient like RO, forward osmosis (FO)
exploits it instead. Like RO, FO relies on
a selectively permeable membrane, but
switches the osmotic gradient around.
On the far side of the membrane, a highly
concentrated ‘draw solution’ generates an
osmotic pressure that sucks water out of
seawater across the membrane.
The trick then is to get whatever solute was
used in the draw solution out, to leave behind
drinking water. This is where the main energy
demand of the FO process takes place, and is
driving some innovative thinking to make this
process as low energy as possible.
The first generation draw solution was
developed by desalination pioneer Menachem
Elimelech and his colleagues at Yale University
in New Haven, US. Elimelech’s answer was
to use ammonium bicarbonate as the draw
solution. Heat this solution up to around 60°C
and the salt decomposes – ammonia and
carbon dioxide bubble off, leaving behind
pure water. The gases can be captured and the
ammonium bicarbonate regenerated, forming
a closed loop system.2
The temperature required for Elimelech’s
process is low enough that waste heat from
factories or power plants could potentially
www.chemistryworld.org | February 2012 | Chemistry World | 45
desalination
sydney water/wdr
A desalination plant in
Sydney, Australia
hydrophilic amide groups. At temperatures
below 34°C, hydrogen bonding interactions
between the hydrophilic groups and water
molecules dominate, and the polymer
extends out into the water, forming a
powerful draw solution.
Once water has been drawn across the
membrane, the next step is to recapture the
nanoparticles to release fresh water. This
process involves heating the solution to 35°C,
a temperature readily achieved by solar
warming. At this critical temperature, the
hydrogen bond network between water and
PNIPAM becomes sufficiently destabilised that
polymer–polymer hydrophobic interactions
start to dominate. The polymer collapses into
a globule formation, and the nanoparticle’s
hydrophobicity jumps up.
The result is that the nanoparticles
clump together, making them much easier
to recapture. These larger particles can
be trapped using a weaker magnetic field,
avoiding the problem of gradual particle
agglomeration. Alternatively, the particles can
be captured by simple nanofiltration.
Once separated from the bulk of fresh
water, the original draw solution can be
regenerated by allowing it to cool below 34°C,
at which temperature the nanoparticles return
to their original hydrophilic state and disperse
throughout the solution again.4
Chung is very optimistic that his latest draw
solution could offer a genuine alternative
to RO, and he says he has an even higher
performance iteration in the process of
patenting. But RO is far from dead, according
to desalination industry expert Tom Pankratz,
the editor of the weekly Water Desalination
Report who is based in Houston, US. The RO
process has evolved to become much more
efficient, he explains.
The theoretical minimum energy for
desalinating seawater using RO with 50%
recovery – producing 50 litres of fresh water
from every 100 litres of seawater – is
1.06kWh/m3. The early large scale RO plants
built in the 1970s came nowhere near that
figure,5 requiring more like 12kWh/m 3. Largely
thanks to the dramatic
in
‘When you try to improvements
membrane technology
desalinate the
that have taken place
since, the latest plants
ocean, there’s a
consume less than
whole bunch of
2kWh/m3 for the RO
other junk in there process.1
besides salt ’
Today’s membranes
consist of an ‘active’
polyamide layer just 0.2μm thick, attached
to a polymer support which provides the
mechanical strength required to withstand
the high operating pressures of RO. Pores
in the polyamide layer are less than 0.6nm
across, which prevents 99% of the salt from
crossing the membrane while allowing a high
flow of water through, reducing the pumping
energy required.
Foul play
However, the overall desalination process
uses more than just the energy required for RO
itself. Overall, a desalination plant consumes
46 | Chemistry World | February 2012 | www.chemistryworld.org
more like 3–4kWh/m 3. Pumping feed water out
of the sea, and then pumping the brine back
again, accounts for some of that extra energy.
But the biggest single energy cost after the RO
itself is the energy required to pre-treat the
water so that it doesn’t foul the membrane.2
‘When you try to desalinate the ocean,
there’s a whole bunch of other junk in there
beside the sodium chloride – decomposing
seaweed, bacteria and other organic
matter – and that really complicates things
because that matter really likes to stick to
the surface of the membrane,’ says William
Phillip, who researches nanoscale structures
of membranes at the University of Notre
Dame, US. Current desalination plants try
to minimise the problem by pre-treating the
water to strip out these organics before the
water meets the membrane. But a much more
efficient solution – whether for RO or FO –
would be to come up with a membrane surface
where pre-treatment wasn’t required because
the organic matter simply didn’t stick.
‘Biofouling is probably the single
biggest problem for RO,’ says Pankratz. ‘It
reduces membrane life, reduces membrane
performance and increases energy
requirements. If you eliminate biofouling,
you’ve gone a long way toward eliminating
some of the inherent problems with RO.
‘There’s a lot of interesting stuff being
done,’ he adds. ‘There are people working
on coatings that can be applied to the
membrane, there are people who are doing a
lot of stuff on computational fluid dynamics
trying to see if they can come up with a
desalination
times the energy needed to treat water that is
already salt-free.
But this is exactly where FO could soon find
its niche. The technology can be used to draw
clean water out of wastewater in just the same
way that it can be used to pull fresh water out
of seawater, the membrane blocking the flow
of contaminants rather than salt. The trouble
with wastewater is that it is particularly rich in
organic material, so has a high propensity for
membrane fouling, but this is where FO could
have its key advantage over RO.‘The way that FO
works, drawing rather than forcing water across
a membrane, you’ll form a looser fouling layer
that is easier to rinse off,’ says Phillip.
Chung agrees that the technology isn’t too
far away. ‘I think FO membranes will be used for
water recycling in the very near future.’
That’s not to say that there won’t always
be a demand for turning seawater into fresh
water. ‘I don’t think anything will ever
replace RO,’ says Phillip. ‘I think what’s
going to happen is that there’s going to be a
wide variety of water treatment and water
conservation technologies that will be
deployed to try to make sure there’s enough
drinking water for the world’s population.’
tom pankratz
Reverse osmosis
membrane racks in a
desalination plant
membrane material that is less likely to
biofoul. There’s a company here in Texas that
says it has found a way to bind selenium to an
RO membrane, preventing growth.’
Phillip agrees. ‘People now have a good
idea of what makes a surface antifouling. The
real difficulty is that you need to combine that
antifouling surface with a membrane that is
as energy-efficient as the current technology.’
However, that’s not to say that it can’t be
done, he adds. ‘I think that combinatorial
and computational techniques will be very
valuable tools for figuring out just what
material properties allow a surface to be high
flux and antifouling.’
Last resort
Whatever progress is made in making
desalination less energy hungry, it should
always be seen as the water source of last
resort, Phillip affirms. ‘Finding ways to
treat contaminated water sources, reuse
wastewater and conserve water – those
should all be things viewed as a solution prior
to desalination,’ he says. Stripping salt from
seawater will almost always require several
James Mitchell Crow is a science writer based in
Melbourne, Australia
REFERENCES
1 M Elimelech and W A Phillip, Science, 2011, 333, 712
(DOI: 10.1126/science.1200488)
2 J R McCutcheon, R L McGinnis and M Elimelech,
Desalination, 2005, 174, 1 (DOI: 10.1016/
j.desal.2004.11.002)
3 M M Ling and T-S Chung, Desalination, 2011, 278, 194
(DOI: 10.1016/j.desal.2011.05.019)
4 M M Ling, T-S Chung and X Lu, Chem. Commun., 2011, 47,
10788 (DOI: 10.1039/c1cc13944d)
5 K P Lee, T C Arnot and D Mattia, J. Membr. Sci., 2011, 370, 1
(DOI: 10.1016/j.memsci.2010.12.036)
Environmental impact
desalination, identified about 150
potential impacts.1 The most serious
of these were problems that arise
when sucking water out of the sea,
and when pumping concentrated
science photo library
Desalination’s energy demand is
far from its only environmental
impact. One of the most exhaustive
studies yet completed on the wider
environmental effects of seawater
Discharge pipes from desalination plants pump concentrated brine into the ocean
brine back into the ocean at the other
end of the process.2
However, the study found that with
careful plant design and location, such
impacts can be minimised. This work
was carried out by Sabine Lattemann,
working at Delft University of
Technology in the Netherlands and
Oldenburg University in Germany, as
part of a European commission funded
project to address the drawbacks of
reverse osmosis desalination plants.
When water is sucked from the
sea there is the potential for marine
life to be drawn in with it. According
to Lattemann, this issue can be
avoided by building the inlet below
the water surface, several hundred
metres offshore, and using screens
to prevent fish from approaching
the mouth of the inlet. The added
advantage of this setup is that it
draws higher quality water than is
found closer to the beach.
At the other end of the process,
the local impact of concentrated
brine discharge can be much more
significant, and impossible to entirely
avoid. Just what level of salinity is
‘safe’ seems to vary from species to
species. Ideally the brine would be
mixed with another water source such
as treated wastewater or power plant
cooling water before being released.
By ejecting the water using multi-port
diffusers, and choosing to do so in
seawater with strong currents for
rapid mixing, dilution to background
levels can be achieved within a very
short distance of the outfall.
REFERENCES
1 S Lattemann, PhD thesis: Development of
an Environmental Impact Assessment and
Decision Support System for Seawater Desalination Plants. CRC Press, 2010 (ISBN:
9780415583268)
2 S Lattemann and T Höpner, Desalination,
2008, 220, 1 (DOI: 10.1016/
j.desal.2007.03.009)
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