"Reverse Osmosis?" You'll never learn ANYTHING that way!

Part one in a series on water filtration.

As a teenager, I remember laying my head on a stack of books when I should have been studying.

When questioned how that might improve my grades, I said I was learning through osmosis.

All that "studying" did not prepare me to understand how Reverse Osmosis works, though...

In osmosis,  a solvent passes through a membrane to try to achieve an equilibrium on both sides of the membrane.  This happens naturally when a red blood cell is submerged in water.  These contain a high concentration of solutes including salts and protein.  Water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell).  The cell membrane is selectively permeable, so only necessary materials are let into the cell.  

Osmotic pressure is the catalyst, and can be increased on the high-solute side of the membrane to force the equilibrium and stop osmosis. This is, apparently, represented in the following formula:

δPV = -RT ln(1-x₂) (2)

Presumably, if you continued to increase the pressure on that side, you will reverse that process, and that is how Reverse Osmosis works:  It actually pulls the water away from the solutes, through a semipermeable membrane, leaving pure, "distilled" water: H2O.  This is usually the last stage in a 2 or 3 stage filtration system, and because it takes a long time to pull water molecules through the membrane, small storage tanks are usually part of the system.  A typical home system will fit in a kitchen cabinet and provide about 10-15 gallons per day, delivered to a tap by the kitchen sink.   

Now, since it has been stripped of all the salts (electrolytes) and minerals the water has no flavor- and some folks say that "no flavor" tastes an awful lot like plastic.  But, if one is really concerned about the quality of your drinking water and is willing to pay the $250-$700 for the system, plus another $150 or so in yearly filter replacements, it is by far the best way to be assured of the quality of your drinking water.