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Seawater is not the only source for
desalination. According to Poseidon Water,
almost 50 per cent of desalination plants
use brackish groundwater, which is too
salty to drink without processing. It’s easier
to desalinate brackish groundwater than
seawater because it is less salty. In the United
States, desalination of brackish groundwater
is increasing in Florida, Texas, and California
in inland locations. Not far from IAPMO
headquarters, brackish water in the Chino
Basin area is being treated at a desalination
plant in Santa Ana, which produces 14 million
gallons a day of freshwater.
THE BIG BENEFIT
To state the obvious, desalination is a
method of producing drinkable water that is
not dependent on rainfall. This is important
in a world that has plenty of seawater, yet
many arid countries, dwindling traditional
water sources, long transports of imported
freshwater, and prolonged droughts.
In another example, California Govender Jerry
Brown recently declared the end to a five-year
drought, which had placed mandatory cuts
in urban water use on all state residents.
“This drought emergency is over, but the next
drought could be around the corner,” he said.
The World Health Organisation (WHO)
estimates that almost one-fifth of the world’s
population lives in areas where water is
scarce. WHO predicts that “this situation is
expected to worsen as competing needs for
water intensify along with population growth,
urbanisation, climate change impacts, and
increases in household and industrial uses.”
HOW DOES DESALINATION WORK?
Basically, there are thermal and reverse
osmosis (RO) technologies. You can use
heat to distil saltwater and turn it into
steam, leaving the salt behind. Or you can
use pressure to force saltwater through
membranes to filter out salt and other
large molecules, resulting in potable water
and brine. The 2008 Official article gave
a comprehensive description of the RO
technology used at a desalination plant in
Long Beach, California. A visual analogy
of the size of filtration particles is if water
molecules are tennis balls, then salt
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HEALTH AND SANITATION
35
molecules are soccer balls and softballs,
viruses are trucks, bacteria are buildings, and
protozoa are mountains.
Most desalination plants use RO. While the
technology is continually improving and
becoming more efficient, this method requires
lots of energy. To become potable, the parts
per million (ppm) of salt must be reduced from
35 000 to 1 000 or lower. Water to irrigate
crops may have 2 000ppm.
There are five stages in RO desalination.
First, incoming seawater is pre-treated to
filter out algae and sea debris. Secondary
pre-treatment removes more impurities.
Then RO removes salt and other minerals
by pushing the water with intense pressure
through semi-permeable membranes, which
are essentially microscopic strainers.
A new plant in Carlsbad, California, uses
more than 2 000 pressure vessels and
16 000 RO membranes. The resulting
freshwater undergoes post-treatment to add
some minerals and to disinfect with chlorine.
The water is then stored and distributed. The
brine is discharged.
The Claude ‘Bud’ Lewis Carlsbad
Desalination plant in San Diego County
opened in 2015 and is the largest in the
nation, producing about 50 million gallons
of desalinated water per day. That amount
is impressive, but it could be argued is a
‘drop in the bucket’. The plant provides
about seven per cent of San Diego County’s
water, which relieves pressure on imported
water supplies. It has won awards for design,
implementation, and energy efficiency. PA
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