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Salt Water
And Desalination
How
Desalination Works
BY LAURIE L. DOVE
Reducing
salt water to its basic elements -- salt and water -- is so simple that it's
become a science lesson for first-graders.
In
fact, a "solar still" can turn salt water into fresh water in just a
few days. Simply fill a large bowl with salt water and set an empty glass at
the center.
Then
cover the bowl -- empty glass and all -- with plastic wrap that has a small
hole poked in the middle.
Place
the contraption in direct sunlight, and watch the water cycle at work: The salt
water evaporates, leaves salt crystals behind, and creates condensation that
rises, gathers on the plastic membrane and drips into the empty glass. The
resulting fresh water is good enough to drink [source: Williams].
But
why remove salt in the first place? Turns out, drinking salt water can kill
you. Ingesting salt signals your cells to flush water molecules to dilute
the mineral.
Too
much salt, and this process can cause a really bad chain reaction: Your cells
will be depleted of moisture, your kidneys will shut down and your brain will
become damaged.
The
only way to offset this internal chaos is to urinate with greater frequency to
expel all that salt, a remedy that could work only if you have access to lots
of fresh drinking water [source: Thompson].
People
-- especially those in water-starved parts of the world -- have been searching
for fresh water solutions for centuries. Turns out the same folks who built
giant sphinxes and drove horse-drawn chariots also thirsted for clean, pure
water [source: Jesperson].
Even
in modern times, entire populations struggle with a cruel irony; they are
surrounded by salt water, but lack drinking water. The scarcity sometimes spurs
deadly conflicts.
In
2009, onlookers killed a family in drought-ridden India for collecting water
from a municipal well before it ran dry [source: Pacific
Institute].
But
what if an abundant supply of fresh water could be created from salt water? A
large-scale desalination operation -- using principles similar to a simple
classroom project -- could change the world.
On
the next page, we'll explore why it's not always so easy to turn salt water
into drinking water.
Desalination at Work
There's
more than one way to separate salt from water, but nearly 90 percent
of the time, only one of two methods are used: multistage flash and reverse
osmosis [source: WorldPumps.com].
Remember why it's so bad to drink salt water? When
your cells pass water through the outer membrane to keep you from dehydrating,
osmosis is occurring.
By
moving the water through the membrane, the cell is attempting to equalize
its high internal salt concentration with a low external salt concentration.
That's called osmosis.
Reverse
osmosis occurs when, for example,
you put salt water on one side of a semi-permeable membrane and pressure moves
the water molecules through the filtering membrane as the larger molecules --
including salt molecules -- stay trapped behind.
For
salty sea or ocean water, a considerable amount of pressure is required to move
the water through a filter, where each pore is just a fraction of the size of a
human hair [source: American Chemical Society].
This
means a series of pumps are usually in play, all exerting pressure on the water
[source: WorldPumps.com].
Unlike reverse osmosis, which relies on a membrane to
filter out salt molecules, the multistage
flash method uses heat to convert salt water into fresh water.
Why
such an unusual name? "Flash" refers to rapidly bringing the water to
a boil, and this happens multiple times, or in stages.
As
the salt water enters each stage of the conversion unit, it is subjected to
externally supplied steam heat and pressure.
During
each stage, water vapor forms and is collected. This water vapor is fresh water
and the left-behind salty concentrate is known as brine.
In
multistage flash distillation -- as with reverse osmosis -- chemicals or water
softening agents are not usually added [source: Organization of
American States].
So if desalination is possible, why aren't large-scale
plants quenching the world's thirst for fresh water? Only about 15 billion
gallons -- two-tenths of a percent of the fresh water consumed around the globe
each day -- is desalinated salt water [source: Schirber].
On
the next page, we'll explore where the newest generation of salty water
converters are cropping up.
DESALINATION FROM ARISTOTLE TO THE U.S. NAVY
As early as 350 B.C., the Greek philosopher Aristotle
put a lot of thought into removing salt from water using a series of filters [source: Aristotle].
By
the 1700s, the U.S. Navy made regular use of solar stills to create fresh
water, and the ensuing decades saw the advent of stills built into shipboard
stoves.
During
World Wa II, desalination took another leap: the U.S. Navy constructed a
land-based distiller on a Pacific island that churned out about 55,000 gallons
of fresh water a day -- more than doubling the output of any existing
distillery in the world [source: U.S. Congress].
Water Solutions
Water covers
at least 70 percent of the world's surface. But 97 percent of it is too salty
to drink [source: Frederick].
This,
coupled with inequalities in water distribution and geographic availability,
means water scarcity is a reality for many people. In fact, a lack of water
affects four out of 10 people in the world [source: World Health
Organization].
There
can be serious health consequences from not having enough water. Sometimes it
means people get their water from contaminated sources. Poor-quality water can
spread diseases like cholera, typhoid fever or salmonella [source: World Health
Organization].
Turning brackish or salty water
into fresh water could impact both the meager rations of water and the
ever-increasing demand. This is especially true for some coastal communities in
the United States struggling with a fresh water shortage [source: California Ocean Resources Program].
In
addition, desalination plants can provide a reliable water source even when a
drought is afoot.
In 2009, there were more than 1,400 desalination
plants operating in the world, producing more than 15 billion gallons of water
per day. Another 244 plants are under construction, many of them in the Middle
East [source: International
Desalination Association].
However,
the world's largest reverse osmosis desalination plant, which opened May
2010, is located on the Mediterranean coast of Israel [source: Dow].
And,
construction is underway in Carlsbad, Calif., to create the Western
hemisphere's largest desalination plant, estimated to produce 50 million
gallons of drinking water every day [source: Poseidon].
Still, there's often a public perception that
desalinated water doesn't taste good and isn't good for you.
In
Israel, for example, many people are increasingly reluctant to drink
desalinated tap water because of health concerns [source: Mizroch].
But
desalinated water -- straight from the tap -- is generally safe to drink.
A
study in Saudi Arabia, for example, found no significant differences between
desalinated water served on tap and bottled water -- except for the fact that
desalinated tap water doesn't leave empty plastic bottles behind [source: Ahmad].
But what if it were possible to carry a portable
desalination device to produce your own personal supply of fresh drinking
water? The idea may not be as far-fetched as you think.
SALT AND SO MUCH MORE
Sure, you might expect the desalination process to
remove salt from ocean water.
But
did you know the clean-up doesn't stop there? Desalination -- whatever the
method -- also removes organic or biological chemical compounds, in the end
producing high-quality drinking water that doesn't transmit diarrheal or other
diseases.
This
is important because nearly 4,000 children die each day in developing countries
from diseases linked to contaminated drinking water [source: Hull].
Future of Desalination
As
the number of desalination plants worldwide continue to grow, so do concerns
about developing new technology to power the plants.
Currently,
large-scale desalination efforts require a lot of energy to operate and often
are high-maintenance affairs, thanks to lots of working parts like membranes
that tend to foul frequently [source: Schirber].
Costs are another concern: During the past five
decades, public and private investment in developing desalination technology
has reached more than a billion dollars worldwide.
And
even with the progress that's been made, the idea that desalination would do
away with water scarcity is far from reality. And that's because it's still
really, really expensive to plan, build and manage desalination plants [source: Water Science and Technology Board].
In
fact, the average cost to turn one acre-foot -- about 325,000 gallons -- of
salt water into fresh water ranges from $800 to $1,400 and requires a
significant amount of energy [source: American Chemical Society].
Producing fresh water using reverse osmosis costs
about one-third less than multistage flash, largely because of the costs of the
thermal energy used by the latter method in the boiling process [source: Water Science and Technology Board].
Unfortunately,
both processes -- as with all desalination techniques -- create brine.
This
by-product of desalinated water contains high concentrations of salt and, when
released back into a natural body of water, can cause damage to marine life.
That's
because brine, which is usually denser than the water into which it's released,
settles atop low-lying sediment where it depletes surrounding waters of oxygen [source: University of Texas
at Austin].
These cost and environmental concerns are all
part-and-parcel of the next round of improvements in desalination technology
and processes.
Light-weight
portable desalination devices are being developed by researchers in Korea and
at the Massachusetts Institute of Technology.
The
units produce enough fresh water to support several people. The process uses
gravity -- simply pour saltwater into the top of the device -- to remove salt
and other sediments using 1,600 filters [source: MIT].
Like these solar-powered portable units, large-scale
reverse osmosis plants, although still in the planning stages, could also
greatly decrease desalination plants' reliance on fossil fuels.
In
fact, the National Science Foundation granted $2.5 million to a team of experts
at the University of Michigan studying solar energy's impact on desalination
technology [source: Richard].
In
addition to government funding in the United States and abroad, private funding
sources are beginning to pay attention to developing more efficient
desalination efforts.
One
thing is certain: If the desalination process improves, it could have the
potential to change entire water-poor regions for the better.
Laurie Dove
Laurie L. Dove
is an award-winning journalist who covers timely topics for HowStuffWorks. She
is the author of six books and the former owner of a newspaper and magazine.
When not reporting on the latest tech breakthrough, health advance or economic
development, Dove is tracking down hidden history, science innovations and
biologic discoveries. As the Honorable Laurie Dove, Mayor, she has brought
multi-million-dollar improvements to the small Midwest town where she lives
with her husband, five children and two Akitas.
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