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.