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Desalination: A Solution for Our Water Needs


Female narrator: The National Library of Medicine
is proud to present Desalination. A solution for our water needs. [recycle logo appears] Female narrator: [female cartoon character
speaks] Did you know that 97% of Earth’s water is saltwater from oceans? [Cartoon boy appears. image of Earth appears
with three red arrows pointing at water in the oceans] Cartoon boy: Hmm! That’s a lot of water!
If there is so much ocean water, why don’t we just drink the water? I don’t get what everyone’s saying about all
these water shortages and stuff. Female: It’s not that simple. We can’t drink
saltwater. Cartoon boy: Why not? What would happen if
we did drink it? Female: Well, the salts in our food and drinks
ends up in our blood, (images of pizza, burger, coffee, cake, and drink appear] but we can’t
have too much salt in our blood. [cross-section kidney appears] Our kidneys use water to flush out the extra
salt, generating urine that exits the body. This is one of the reasons we need to drink
water. [human trunk, kidneys labeled] Cartoon boy: But if urine leaves the body,
then getting rid of salt must cause us to lose water from our bodies! Female: That’s correct! And what is sea water?
Cartoon boy: I guess it’s just water with a lot of salt dissolved in it. Female: Exactly! So, say, you drink one cup
of sea water [cup appears], and, say, in that cup there is a tablespoon of salt [tablespoon
of salt appears]. The amount of salt in that cup of water is so high, that it would take
way more than the one cup of liquid you drink to remove the one tablespoon of salt from
your body [many cups appear]. Cartoon boy: So drinking sea water causes
you to lose even more water than you drink! Female: That’s right, and losing too much
water causes dehydration, which is very bad for you.
Cartoon boy: It’s such a shame, there’s so much salt water. If we could only get rid
of the salt, we wouldn’t have to worry about water shortages or anything like that.
Female: Well, there is actually a way to remove salt from sea water, it’s called desalination,
and it’s more familiar than you think. [Flowchart of water cycle appears: precipitation,
evaporation, transpiration, condensation.] Female: During the water cycle, water gets
desalinated. After rainfall the water flows toward the ocean. Energy from the sun causes
the salty water to evaporate, leaving the minerals on the ground. The pure water travels
upward as a gas until it hits cold air and condenses, forming clouds. Then fresh water
falls as rain. Cartoon boy: Hold on, condenses? What’s that?
Female: What is condensing? It’s when water gas cools and turns into a liquid. Have you
ever drawn pictures on the bathroom mirror after you took a shower?
The water on the mirror came from your shower. When the water is hot, part of it turns into
gas, which then rises in the air. When the gas comes into contact with a cold mirror,
it cools, or condenses, and turns into water. Again, clouds are just condensed water in
the air, and those clouds produce rain. Although nature produces fresh water, it is a precious
but limited resource. Desalinating ocean water provides much needed freshwater for people
to drink, especially those in locations without much.
Cartoon boy: So how do they do it? Female: Well, there are two types of desalination.
The first is called thermal desalination. [Illustration of thermal desalination appears.]
Thermal desalination is similar to the water cycle. Saltwater is heated so that it evaporates,
leaving the salt behind. The evaporated steam is then cooled and condensed into pure water.
Cartoon boy: Like on the bathroom mirror! Female: Thermal desalination is a good method
because it works, and can handle large volumes [picture of water silo]. However, it does
use up a lot of energy [picture of lightning]. Another method of desalination is membrane
desalination, which uses semi-permeable membranes. A semi-permeable membrane is a thin sheet
of material with tiny holes that let some amount of liquids, such as water, but not
solids like salt, to pass through it. To desalinate, membranes are used in a process
called reverse osmosis. This process involves tanks divided by a membrane. Pressure is applied
to the saltwater side, forcing water through the membrane from the side where there is
more salt, to where there is no salt. Since the membrane only lets liquids in, the
salt gets left behind. Reverse osmosis is more efficient than thermal desalination,
meaning it produces more freshwater for less money. However, a reason not to use reverse
osmosis is that salt and other particles get stuck in the membrane holes, causing them
to stop working. Membranes also get old and break down. Because
of their short lifespan, membranes must be replaced frequently, which is expensive.
Cartoon boy: That makes sense. So if we have these technologies, why are there still water
shortages? Female: Industrial desalination is not yet
our main source of drinking water because it requires a lot of energy, making it expensive
and harmful to the environment. Furthermore, harmful leftover salt is hard
to dispose of safely. However, scientists have started combining technologies to make
desalination more efficient, such as using both membrane and thermal desalination technologies
together and finding better ways to power desalination plants.
Cartoon boy: So while we have a long way to go, desalination really is a solution for
our water needs. The End.

Bernard Jenkins

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