Date: Thu, 9 Jan 1997 09:14:41 -0500 (EST) From: Perry Malouf <pmalouf@access.digex.net> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg132$foo@default> Subject: Re: Reverse Osmosis
Steve wrote:
> As a person with a strong curiosity about how things work,
> I have been intrigued by the term 'Reverse Osmosis'...
In order to describe reverse osmosis, I should first describe
an example of osmosis.
Consider a beaker filled with pure water. Take a sausage skin
(selectively permeable membrane), fill it with brine and seal both ends.
Put the brine-filled sausage skin in the beaker of water. After
some time, you will notice that the sausage skin begins to
balloon so that the skin becomes tight. This is because water
diffuses through the skin into the brine. Diffusion is driven
by concentration gradients, in this case there is a higher
concentration of water outside the skin as compared to inside
the skin. The membrane permits water to go across, but
not NaCl. So water diffuses across the membrane in an
attempt to lower the brine (NaCl) concentration. The process
continues and pressure builds up in the sausage skin because
the volume of water in the skin is increasing. The
pressure tends to act against the diffusion of water from
outside the skin to inside.
This pressure is known as osmotic pressure. Eventually an
equilibrium is reached where the pressure inside the sausage
skin stops the net diffusion of water across the membrane. In
other words, the diffusion rate of water from outside to inside is
equal to the rate from inside to outside.
Now we move to reverse osmosis. Imagine that I squeeze one end
of the sausage skin with my hand, thus increasing the pressure
inside. Whereas before we had an equilibrium where the net
diffusion of water across the membrane was zero, now the
increased pressure inside the skin acts to increase the rate of
diffusion from inside the skin to outside. So now there is a
net diffusion of water out of the sausage skin.
With water now diffusing out of the sausage skin, the concentration
of brine increases and the volume of pure water outside the
skin increases. We are purifying water! This continues as
long as adequate pressure is maintained inside the sausage skin.
With reverse osmosis water purifiers, a selectively permeable
membrane is used. Water is forced against this membrane
at high pressure, whereas water on the other side of the
membrane is at a low (ambient) pressure. As in the example
above, water molecules diffuse across the membrane leaving
the contaminants (salts and others) behind.
Regards,
Perry Malouf
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