To understand the science of reverse osmosis desalinators, you should first become acquainted with a few key terms:
Desalination: Desalination is simply the process of removing salt content from water. During this separation process, the dissolved salt in water is reduced to make the water usable. Although seawater is the largest source of water on our planet, it can't be used for drinking due to its high salt content. Desalination makes seawater fit for human consumption.
Osmosis: Osmosis is a natural phenomenon that affects a variety of biological functions in all forms of life. Osmosis does everything from allow plants to absorb nutrients from the soil to help the kidneys purify blood. An osmotic membrane, a membrane that allows water to pass through at higher levels than it does salt, allows for osmosis to occur. An osmotic membrane is semipermeable; that is, it allows some substances to pass through while others do not. Although pure water can flow freely in both directions, salt and other impurities can't pass through.
During osmosis, solvent water passes through a semi-permeable membrane toward a concentrated substance on the other side until the osmotic pressure across the membrane is equal (usually 350 pounds per square inch guage, or psig, freshwater/seawater) [source: Water and Waste Digest].
Reverse osmosis: Reverse osmosis is just like it sounds -- the exact opposite of osmosis. While in osmosis, solvent water passes through a membrane until the pressure across the membrane is equal, during reverse osmosis, a force with pressure greater than the osmotic pressure is needed to allow freshwater to pass through the membrane while salt is held back. The higher the pressure is above osmotic pressure, the more quickly freshwater will move across the membrane.
So, a reverse osmosis desalinator combines these processes to make saltwater drinkable. On the next page, we'll take a closer look at the reverse osmosis desalination process.