PURIFYING YOUR WATER
OSMOSIS VOCAB
Diffusion – The movement of particles from a
region of high concentration to a region of low
concentration.
Concentration – The amount of a substance per
unit volume. There are a lot of ways to express
concentrations. The most useful units are grams
per liter (g/L) and milligrams per liter (mg/L).
Mg/L is also known as parts per million (ppm).
You might also see things like, “a 10% solution
of x” but unless more information is known, you
can’t always be certain what that means exactly
(mass/volume, volume/volume, or…?).
Entropy – Entropy is one of the most difficult
and most misunderstood thermodynamic state
functions. For simplicity’s sake, just refer to it
as the tendency of the universe to become less
ordered over time. Without an energy input,
things naturally get more random; they do not
become more organized.
Osmosis – The diffusion of water across a
selectively permeable membrane from a region
of low-solute concentration to a region of highsolute concentration.
direction opposite to the natural osmotic pressure and
force the water through a selectively permeable membrane, we have created a reverse osmosis (RO) system. A
system must also be implemented for removing the solute
as it tries to move across with the water. A selectively permeable membrane does just that.
For this next example, let’s assume the input water is
city water and we want to remove dissolved ions. Typically,
city water contains an ionized disinfectant (a chlorinated
compound), dissolved metals from pipes and salts that
naturally exist in the water supply. Here is the basic idea
behind an RO system:
• A pump pressurizes water to force it through a selectively
permeable membrane. The membrane allows the water
to pass, but blocks the passage of undesired molecules.
• A secondary flow parallel to the membrane is applied
to flush away the solute molecules that get trapped.
In this system, the membrane doesn’t get loaded with
solute molecules, allowing it to keep passing water without clogging. This differs from bulk filtration, which
entrains particles that are filtered from the flow.
• The higher the solute concentration in the feed water,
the higher the pressure must be to overcome the
osmotic potential.
Selectively Permeable Membrane – A barrier
that allows some things to cross it while blocking
the passage of others. A thin sheet of cellophane
or a cell membrane are typical examples.
• Multiple stages can be used to perform repeated RO
filtrations to increase the purity of the water to any
desired level up to the maximum achievable. Water
with an electrical resistance of 18.2 megohms per cm is
the best you can get.
Solvent – The part of a solution that exists in
the greatest quantity. For example, when a bit of
sugar is dissolved in water, water is the solvent.
The first RO membranes were made of cellulose acetate,
but virtually all of today’s RO membranes are the thin
film composite (TFC) type. These membranes consist of
a thin layer of polyamide over a thicker, highly porous,
polysulfone layer. The water that passes through the
membrane is called the