Reverse osmosis technology is the most advanced, most energy-saving, and most efficient separation technology today. The principle is to separate the solute in the solution from the solvent under the pressure higher than the osmotic pressure of the solution by means of the selective retention effect of the reverse osmosis membrane that only allows water molecules to penetrate, so as to achieve the purpose of pure water. The reverse osmosis membrane is composed of polymeric cellulose with a highly ordered matrix structure. Its pore size is 0.1 nanometer -1 nanometer, which is one ten billionth of a meter (equivalent to one thousandth of the size of E. coli and one hundredth of a virus).
Osmosis phenomenon is common in nature. For example, if a cucumber is put in salt water, the cucumber will become smaller due to loss of water. The process by which water molecules in cucumber enter the brine solution is an osmotic process. As shown in the figure below, if a pool is divided into two parts with a membrane that only water molecules can penetrate, pure water and brine are injected to the same height on both sides of the diaphragm. After a while, it will be found that the level of pure water has decreased, while the level of brine has increased. We call the phenomenon that water molecules migrate into the salt water through this membrane as osmosis. The elevation of the salt water level is not endless, and an equilibrium point will be reached at a certain height. At this time, the pressure represented by the difference between the two ends of the diaphragm is called osmotic pressure. The osmotic pressure is directly related to the concentration of brine.
After the above devices reach equilibrium, if a certain pressure is applied to the liquid surface of the salt water end, water molecules will migrate from the salt water end to the pure water end at this time. The phenomenon in which liquid molecules migrate from dilute solution to concentrated solution under pressure is called reverse osmosis. If salt water is added to one end of the above facility and a pressure that exceeds the osmotic pressure of the salt water is applied to that end, we can get pure water at the other end. This is the principle of reverse osmosis water purification.
There are two keys to the production of pure water in reverse osmosis facilities. One is a selective membrane, which we call a semi-permeable membrane, and the other is a certain pressure. Simply put, the reverse osmosis semi-permeable membrane has many pores, the size of these pores is equivalent to the size of water molecules. Because bacteria, viruses, most organic pollutants and hydrated ions are much larger than water molecules, they cannot be penetrated. The reverse osmosis semi-permeable membrane is separated from the water that passes through the reverse osmosis membrane. Among the many impurities in water, soluble salts are the most difficult to remove. Therefore, the water purification effect of reverse osmosis is often determined according to the level of salt removal. The level of reverse osmosis salt removal is mainly determined by the reverse osmosis semipermeable membrane The selectivity. At present, the salt removal rate of highly selective reverse osmosis membrane elements can be as high as 99.7%.