There is a common phenomenon that molecules move from a higher concentration to a lower concentration. This phenomenon is called as diffusion. In our word, this is very common. Osmosis is such one type of diffusion, but it is also very special. There is a membrane which is semipermeable and only allows H2O molecules to go through, while other larger molecules (like glucose, bacteria and virus) and ions( like Ca2+,Na+, Cl-, Fe2+ etc.) cannot allow to pass. Naturally, there is an equilibrium that will reach between the two sides. This means the number of water molecules goes through the membrane is equal to molecules from the other side come back. In this situation, the concentration gradient stops.
This is not the end of this story. If we apply sufficient pressure to the membrane from the concentrated side, the phenomenon is reversed. And we call this reverse osmosis. Water molecules are coerced to go through the semi-membrane from a lower concentration side to a higher concentration side. Naturally, this will not happen. The key point is the pressure applied, so the concentration gradient is reversed. We can take a good use of this principle to purify water. Reverse osmosis water filters rely on this principle to purify water. Water with contaminants is forced to pass the semi-membrane and this can separate water and other contaminants. These water molecules which have passed this semipermeable membrane will become filtered water. Filter water is pure and doesn’t contain most contaminants, because these contaminants are left on the other side of the membrane.
Reverse osmosis water filtration systems are ideal for both residential and commercial water purification. Nowadays, more and more people have accepted RO filters. If you just plan to buy a water filter recently, a RO system is your best choice.