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| "Water, water, everywhere / nor any drop to drink" . |
Reverse Osmosis Desalinators
How Reverse Osmosis
Desalinators Work
BY SARAH WINKLER
You're
setting out for a backpacking expedition and packing up the important gear
you'll need for your trip.
You've
got your compass, a map, some comfortable hiking boots, some snacks and an army
knife. Seems like you have everything you'll need, right?
Well,
you're missing one important item that could save your life in a pinch: a way
to purify water.
Without
water, you're susceptible to dehydration, hypothermia or altitude
sickness.
A
water purification system like a filter or charcoal tablets could provide
you with the purified water you'll need to survive in the outdoors.
But what if you need to do more than purify the water?
What if your only available water sources are
saltwater? (Or as Samuel Taylor Coleridge put it in his poem "The Rime of
the Ancient Mariner": "Water,
water, everywhere / nor any drop to drink.")
Although
seawater might look tempting, its high level of salt makes it unsuitable for
human consumption.
Average
ocean seawater contains three times the salt content found in a person's
bloodstream. If you drink seawater, you'll become even more dehydrated, which
could lead to seizures, kidney failure or even brain damage and death [source: Seawater Facts].
If you're in the outdoors and the only available water
is seawater, then you'll need to desalinate the water; that is, you'll need to
reduce the salt content of the water.
One
way to desalinate water is through reverse osmosis with a reverse osmosis desalinator. This filtration
process uses pressure to force water through a membrane.
The
solute (salt) remains on one side of the membrane, while the pure solvent
(freshwater) passes to the other side.
The
solvent (in this case, water) moves from an area of high solute concentration
to an area of low solute concentration.
While
osmosis was discovered as early as the 1700s, it wasn't until the 1960’s that
scientists were able to use the process to desalinate water [source: Water and Waste Digest].
As
its name indicates, this process is the reverse of normal osmosis, in which a
solvent moves with no added pressure from an area of low solute concentration
to an area of high solute concentration.
Not
only does a reverse osmosis desalinator remove salt from water, but it
also eliminates harmful bacteria and microorganisms.
The Science of Reverse Osmosis Desalinators
To understand the science of reverse osmosis
desalinators, you should first become acquainted with a few key terms:
Desalination:
Desalination is simply the process of removing salt content from water.
During
this separation process, the dissolved salt in water is reduced to make the
water usable.
Although
seawater is the largest source of water on our planet, it can't be used for
drinking due to its high salt content. Desalination makes seawater fit for
human consumption.
Osmosis:
Osmosis is a natural phenomenon that affects a variety of biological functions
in all forms of life.
Osmosis
does everything from allow plants to absorb nutrients from the soil to help the
kidneys purify blood.
An
osmotic membrane, a membrane that allows water to pass through at higher levels
than it does salt, allows for osmosis to occur.
An
osmotic membrane is semipermeable; that is, it allows some substances to pass
through while others do not.
Although
pure water can flow freely in both directions, salt and other impurities
can't pass through.
During osmosis, solvent water passes through a
semi-permeable membrane toward a concentrated substance on the other side until
the osmotic pressure across the membrane is equal (usually 350 pounds per
square inch gauge, or psig., freshwater/seawater) [source: Water and Waste
Digest].
Reverse osmosis:
Reverse osmosis is just like it sounds -- the exact opposite of osmosis.
While
in osmosis, solvent water passes through a membrane until the pressure across
the membrane is equal, during reverse osmosis, a force with pressure greater
than the osmotic pressure is needed to allow freshwater to pass through the
membrane while salt is held back.
The
higher the pressure is above osmotic pressure, the more
quickly freshwater will move across the membrane.
So, a reverse osmosis desalinator combines these
processes to make saltwater drinkable. On the next page, we'll take a closer
look at the reverse osmosis desalination process.
Desalination
Process
During reverse osmosis, saltwater is forced through a
semipermeable membrane that allows water molecules to pass through while all
other impurities, including salt, are held back.
Here's
a look at the step-by-step process of reverse osmosis desalination:
1. To set up a reverse
osmosis desalinator, you first need an intake pump at the source of the seawater.
2. Next, you need to create
flow through the membrane. This will cause water to pass through the salted
side of the membrane to the unsalted side.
Pressure
comes from a water column on the salted side of the membrane. This will both
remove the natural osmotic pressure and create additional pressure on the water
column, which will push the water through the membrane.
Generally,
to desalinate saltwater, you need to get the pressure up to about 50 to 60 bars
[source: Lentech].
3.
Feed
water is then pumped into a closed container. As the water passes through the
membrane, the remaining feed water and salt solution become more concentrated.
To reduce the concentration of the remaining dissolved
salts, some of the feed water and salt solution is taken out of the container
because the dissolved salts in the feed water would continue to increase and
thus require more energy to overwhelm the natural osmotic pressure.
4.
Once
water is flowing through the membrane, and the pressure is equal on both
sides, the desalination process begins.
After
reverse osmosis has occurred, the water level will be higher on the side where
salt was added.
The difference in water level is caused by the
addition of the salt and is called osmotic pressure; generally, the osmotic
pressure of seawater is 26 bars.
The quality of water is determined by the pressure,
the concentration of salts in the feed water, and the salt permeation constant
of the semi-permeable membrane.
To improve the quality of the water, you can do a
second pass of membrane.
Once the freshwater and saltwater are separated, the
freshwater should be stabilized; that is, the pH should be tested to make sure
it's fit for consumption.
Using Reverse Osmosis Desalinators
Reverse osmosis desalinators can operate on both large
and small scales.
A
backpacker or boating enthusiasts can purchase a reverse osmosis desalination
system for personal use, or they can be acquired by larger industrial or
community groups in need of freshwater.
Many
communities in equatorial zones, arid environments and coastal areas are good
candidates for reverse osmosis desalination systems because they generally have
available seawater but lack freshwater. For example, places like California,
Florida, the Caribbean, Central and South America, the Mediterranean, the
Middle East and the Pacific Rim are areas in which reverse osmosis desalination
could be a viable option for the production of freshwater on a large scale.
In
comparison to two other desalination processes, distillation and
freeze-thawing, reverse osmosis is the most cost effective and energy
efficient.
For
example, while distillation require 30-186 horsepower of mechanical energy to
remove one gallon (3.7 liters) of water from saline solution, reverse osmosis
desalination only needs about 0.5-1.4 horsepower [source: Desalination:
FAQ].
In
addition to being energy efficient, reverse osmosis desalinators are also
smaller in size than other desalination units.
On
a larger scale, they are also less costly to purchase and operate.
Most
desalinators are run by electricity; however, if electricity is not available
or too expensive, you can also use a diesel or solar-powered desalinator.
Although
solar powered desalinators are initially more expensive to purchase, the energy
savings may pay off in the end.
To
take care of your reverse osmosis desalinator, make sure to keep an eye on the
day-to-day operation of the system.
Make
sure to adjust the calibration and pumps for leaks or structural damage.
The
main problem that you can run into with reverse osmosis desalinators is
fouling, when membrane pours become clogged.
To
prevent fouling, clean the unit every four months or so and replace filter
elements about once every eight weeks.
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