More than 200 years later, a modification of this process, known as “reverse osmosis,” allows people throughout the world to affordably convert undesirable water into water that is virtually free of aesthetic or health contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence, to installations used in manned spacecraft.
Here are the basics of how the process works: A semi-permeable membrane, whether it's a cell wall or pig bladder, is selective about what it allows to pass through. It generally allows water to pass through easily while limiting the passage of other contaminants. Water typically will be present on both sides of the membrane, with each side having a different concentration of dissolved minerals.