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How does reverse osmosis work?
To understand "reverse osmosis," it is probably best to start with normal osmosis.
According to Merriam-Webster's Collegiate Dictionary, osmosis is the "movement of
a solvent through a semipermeable membrane (as of a living cell) into a solution
of higher solute concentration that tends to equalize the concentrations of solute
on the two sides of the membrane." That's a mouthful. To understand what it means,
this picture is helpful:
On the left is a beaker filled with water, and a tube has been half-submerged in
the water. As you would expect, the water level in the tube is the same as the water
level in the beaker. In the middle figure, the end of the tube has been sealed with
a "semipermeable membrane" and the tube has been half-filled with a salty solution
and submerged. Initially, the level of the salt solution and the water are equal,
but over time, something unexpected happens -- the water in the tube actually rises.
The rise is attributed to "osmotic pressure."
A semipermeable membrane is a membrane that will pass some atoms or molecules but
not others. Saran wrap is a membrane, but it is impermeable to almost everything
we commonly throw at it. The best common example of a semipermeable membrane would
be the lining of your intestines, or a cell wall. Gore-tex is another common semipermeable
membrane. Gore-tex fabric contains an extremely thin plastic film into which billions
of small pores have been cut. The pores are big enough to let water vapor through,
but small enough to prevent liquid water from passing.
In the figure above, the membrane allows passage of water molecules but not salt
molecules. One way to understand osmotic pressure would be to think of the water
molecules on both sides of the membrane. They are in constant Brownian motion. On
the salty side, some of the pores get plugged with salt atoms, but on the pure-water
side that does not happen. Therefore, more water passes from the pure-water side
to the salty side, as there are more pores on the pure-water side for the water
molecules to pass through. The water on the salty side rises until one of two things
occurs:
- The salt concentration becomes the same on both sides of the membrane (which isn't
going to happen in this case since there is pure water on one side and salty water
on the other).
- The water pressure rises as the height of the column of salty water rises, until
it is equal to the osmotic pressure. At that point, osmosis will stop.
Osmosis, by the way, is why drinking salty water (like ocean water) will kill you.
When you put salty water in your stomach, osmotic pressure begins drawing water
out of your body to try to dilute the salt in your stomach. Eventually, you dehydrate
and die.
In reverse osmosis, the idea is to use the membrane to act like an extremely fine
filter to create drinkable water from salty (or otherwise contaminated) water. The
salty water is put on one side of the membrane and pressure is applied to stop,
and then reverse, the osmotic process. It generally takes a lot of pressure and
is fairly slow, but it works.
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