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Water, water, everywhere Sean
Woods, Popular
Mechanics
It doesn't take a rocket scientist to figure
out why. Basically, there's just not enough of it to go
around. Although
most of our planet's surface is
covered by H2O, most of it is in the form of seawater, and
useless in terms of human survival (that is, if you discount
the fish we eat). Freshwater sources such as lakes, rivers,
aquifers and dams provide enough water for most of us, but
storing, delivering and purifying it costs money — and if we
live far from the source, we're talking lots of money. Things are going to get
nasty Many socio-political gurus
expect water to overhaul oil as the most critical, and
certainly the most emotive, commodity before the end of this
century. It's even possible that countries will go to war over
it. As economies grow, so industry and agriculture will demand
a bigger slice of the water pie. The general consensus is that
we have about 30 years before population growth and global
warming make us nasty. The answer? GrahamTek Systems of Somerset
West reckon they've got the answer. It's a system called
reverse osmosis desalination, and if all goes according to
plan, it's about to shake up the desalination industry.
Company founder This
company's revolutionary approach to desalination has
dramatically reduced the cost, both financial and
environmental, to a point where it has become a genuinely
practical alternative for thirsty regions such as ours. Its
plants are already churning out fresh water at the coast,
purifying brackish borehole water in the interior, and
filtering out all manner of toxic industrial waste in between.
Here's the interesting bit: no chemicals are used in the
process. Knowing what he does about
desalination, and his company's cost-effective process in
particular, He has a point. The salty sea
accounts for about 97 percent of our planet's water and
another two percent is locked in ice caps and glaciers,
leaving just one percent (fresh water) for the billions of
humans and animals who occupy terra firma. Bigger equals better
Reverse osmosis caught Graham's
attention while he was building a desalination plant on the
Cape West Coast for what was then the Department of Water
Affairs. He developed an interest in
membranes, and began to explore ways in which the plants could
achieve higher flux rates (fresh water yields). A strong
magnetic field was one option, but there was another concept
with even more potential. His next question: "Why doesn't
the world look at bigger membranes?" How it works It made perfect sense. A bigger
membrane (filter) would produce a higher flux, bringing costs
down. The conventional membrane diameter was 20cm; But the big membrane is only
one component in a system with many variables. Other
innovations, including an integrated flow distributor, the use
of electromagnetic fields, an energy recovery device and a
modular skid platform, contribute to the overall efficiency of
the system. Each pressure vessel houses two
membranes in series, encapsulating a membrane area of 316m²
in total. Their podgy appearance belies the fact that they
take up considerably less space than desalination plans of
conventional design. In a conventional vessel, you’d need up
to seven membranes to achieve an equivalent membrane area. To
put the GrahamTek design into perspective, a plant that can
produce one million litres of drinking water a day fits into a
6m container. Each membrane is made up
specifically to match its raw water source. For example, a
synthetic plastic membrane with a pore size of 0.0002 microns
is used to process seawater. It takes 46 leaves (individual
membrane segments), each 1.4m long, to construct one membrane.
It's real hard work!
Making these leaves is labour-intensive.
The feed (raw water) spacer, membrane and permeate (fresh
water) spacer are fed through rollers under tension before
being folded in half and glued along the sides, making them
look like open envelopes. The spacers keep the membranes from
collapsing under pressure and blocking the flow of water. The
open ends are then glued to the perforated permeate pipe
running through the centre of the membrane before being wound
around it under pressure and taped securely in place. The end
result is something that resembles a very large and very
thinly layered Swiss roll. Both the flow distributor (on
the inlet side) and anti-telescoping device fitted to the rear
are attached before the complete membrane is placed inside its
pressure vessel. The anti-telescoping device simply prevents
the membrane from popping out of the pressure vessel when
water is forced through it at 60 bar. The flow distributor, on
the other hand, changes the laminar flow of the feed water
under pressure, causing it to become highly turbulent and
create micro-bubbles (called cavitation). The feed water is
also directed towards the centre of the spirally rolled
membrane, where lower velocities are traditionally found. This
is achieved by placing many small-angled inlet holes,
positioned in concentric circle, on the flow distributor's
surface. These micro-bubbles actively
scour the membranes to prevent fouling, and increase membrane
flux by carrying away ions (salt molecules and so on) from the
membrane’s surface more rapidly. The result is a lower
osmotic pressure at the membrane surface, allowing a greater
permeate flux with less pressure. There is another advantage
to this system: intervals between maintenance backwashes are
lengthened, which translates to 60 per cent less backwashing
than in conventional plants. An electromagnetic device
virtually eliminates fouling from the equation, dramatically
increasing the life of the membranes. The principle is simple
and effective: a conductor is wound into the pressure vessel
at strategic points to establish an electromagnetic field,
which is then tweaked (by controlling the electric current)
for optimum effect. The calibrated harmonic field
effectively disorientates the steric formation of active
crystal growth by separating the chemical bonds. Organic
foulants are affected in such a way that normal attraction to
the charged membrane surface is diminished to the point that
fouling simply doesn’t occur. So efficient is the system
that plants have been known to operate for more than a year
without the membranes needing cleaning. The big plus here is
that the need for chemicals is eliminated, both in the pre-
and post-treatment phases, contributing further to lower
operation costs. Pumping out energy
Two energy recovery devices are
available, depending on the size of the plant. Whereas both
fulfil the same function, the pelkon wheel design (the older
and noisier of the two) is best suited to smaller units.
Connected to the brine (concentrated saltwater effluent)
outlet, and in turn attached to the high-pressure pump shaft,
it reduces the power requirement to between 2.0 and 2.9
kilowatts per hour for every 1000 litres of water produced.
That’s a power saving of about 37 percent. Alternatively, it
can be connected to a generator to supply about 50 homes with
electricity. According to Up to nine pressure vessels,
producing 1 million litres a day, can be mounted on to a
single pre-assembled, fully operational skid platform. The
modular design allows them to be linked together to provide
plants of varying capacities. Need another million litres?
Just add another skid – it’s as simple as that. “We have
the capability to build plants that produce 160 megalitres a
day,” says Not surprisingly, it took
serious capital to develop this system, which explains why On the beach When setting up a desalination
plant, the first consideration is the water source (no
surprise there). If it’s seawater, says A feeder pump primes the
pressurised turbine pump and ensures its bearings are
lubricated. Capable of producing pressures of up to 60 bar, it
forces the water into the pressure vessels – connected in
parallel – through the membrane, allowing only the water
molecules to pass through and trapping all the attached salts.
As South African seawater has an ionic constituency of 34 000 The recovery rate of drinking
water from seawater is about 45 percent, and with brackish
water about 80 per cent. The brine (reject water) then passes
the energy recovery device before being pumped harmlessly back
into the sea. In 1995, fresh water derived from seawater by
reverse osmosis cost R18.50 a kilolitre. GrahamTek now
produces it for R4.85 a kilolitre (off a 5 megalitre per day
plant). At R2.50 a kilolitre, brackish water costs even less
to process. This is an all-in figure, including the recouping
of costs, maintenance, electricity and even staff salaries.
All of which sounds like good news for housing developers and
golfing estates that invariably pay hefty bulk tariffs to
municipalities. On a smaller scale, the
“Aquadelfi” unit is aimed at farmers. Producing up to 25
500 litres a day, it can treat a wide range of brackish
waters. It’s a so-called “plug-‘n-play” unit, so all
you need do is load one on to your bakkie, drive home and
connect the pipes. Smaller still is the “Seawater Aquadelfi”,
designed for more modest applications such as holiday homes at
the coast or seagoing boats, and capable of producing 2 000
litres a day. Water, water, everywhere. And
you can drink it. For more information,
contact Ocean Mineral Water on 021-854 7676.
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