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 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 top 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 inyour 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.
