As glaciers disappear, groundwater dries up, and Earth’s climate gets more and more extreme,
where will we get our drinking water?
What about from this gigantic reservoir that holds 97% of the Earth’s water?
Ocean water can be more than 60 times saltier than fresh water, which means it’s undrinkable.
That salt is mostly table salt -- sodium chloride.
Sodium ions are positively charged and attracted to negatively charged chloride ions.
Water is polar, meaning it’s positively charged on one end and negatively charged
on the other.
Those deltas represent partial charges.
This allows it to break sodium chloride crystals apart and then surrounds each ion, forming
ion-dipole bonds.
And that’s why it dissolves sodium chloride so well.
These bonds are so strong that it takes a lot of energy to pry the water and the ions
apart from each other.
Today, the most widely used large-scale way to create freshwater from seawater is by reverse
osmosis.
Unlike osmosis, where freshwater would, on its own, rush toward a solution with a higher
salt concentration, like seawater, reverse osmosis takes human intervention.
Water from the ocean is pushed through membranes, keeping out salt ions but allowing water molecules
to pass through.
Here’s a reverse osmosis desalination plant in San Diego county.
This facility is North America’s largest effort to turn seawater into freshwater.
Every day they pump in 100 million gallons of seawater, and after desalination, they
can pump 50 million gallons of freshwater into the city, reaching about 300,000 people.
OK so at this point you might be thinking -- wait a sec, why are we talking about reverse
osmosis?
I know how to get freshwater from the ocean -- just boil it and then collect the steam.
That’s called distillation, and yeah it’s a way to both purify and desalinate water.
You can heat seawater--using fossil fuels or the sun--until the water molecules begin
to evaporate.
As the water vapor rises and comes in contact with a cool surface, it condenses and what’s
collected is freshwater, leaving salts and other minerals behind
So why don’t we just distill the ocean water when we need some freshwater?
Well, the energy to separate water from salt has to come from somewhere.
And it turns out it takes less energy to do reverse osmosis on millions of gallons of
water per day than to boil millions of gallons of water per day.
Hydrogen atoms in water molecules are not only covalently bonded to their oxygen, they’re
also attracted to the oxygens in other water molecules.
That’s a lot of attraction and a lot of bonding, so when you boil water and vaporize
those water molecules you’re using a lot of energy to break those interactions.
When you’re just forcing water through a membrane you don’t have to worry about any
of that.
So, understandably, reverse osmosis is the method of choice for bringing water to lots
of people.
In an ideal world, Elon Musk would whip up a solar array powerful and reliable enough
to make reverse osmosis plants the method of choice for coastal areas--but we’re just
not good enough yet at harnessing the Sun’s energy.
People are working on it though.
Maybe just not Elon.
Honestly who knows where that guy’s at.
Elon, are you listening?
But, the Sun can be useful -- if you’re stranded on a desert island and happen to
have one of these cute little stills.
Awww… look at this little guy.
But say Elon succeeds and we do have a money-saving, zero-emissions desalination plant on our hands,
that still leaves out one very important part of the equation: leftover salt.
So when we remove all that salt where do we put it?
One option is...back to the ocean!
But if it isn’t spread out across a large area it can be really bad for marine life.
Super-salty water is denser than your typical ocean water, so as it sinks it can negatively
affect creatures on the ocean floor who are sensitive to changes in salinity.
One option, although used less often because it requires more work and equipment, is to
harvest the remaining salt and sell it.
So at the end of the day, yes, we have SO MUCH seawater that we’d never fear running
out of it and that’s a huge plus, but the reality is that it often takes a lot of energy
and money and, in most places, there are still other ways of getting freshwater...for now.