Permeation is a physical phenomenon. When two different concentrations of water, such as a semi-permeable membrane, are separated, it is found that the water with a small amount of salt permeates through the membrane to a high salt content. In the water, the salt does not penetrate, so gradually the salt concentration on both sides is fused to equal. However, it takes a long time to complete this process, which is also called infiltration.
 However, if a pressure is applied to the water side with a high salt content, the result can stop the above-mentioned permeation. If the pressure is increased, the water can be infiltrated in the opposite direction, and the salt remains, therefore, reverse osmosis. The principle of desalination is to apply a pressure greater than the natural osmotic pressure in the salty water (such as raw water), so that the penetration proceeds in the opposite direction, and the water molecules in the raw water are pressed to the other side of the membrane to become clean. Water, in order to achieve the purpose of removing impurities and salt from water.