The system desalination rate of reverse osmosis equipment is generally 98-99%. Such a desalination rate can meet the requirements in most cases. In the electronics industry, ultra-high pressure boiler feed water, and individual pharmaceutical industries, the requirements for pure water It may be higher. At this time, single-stage reverse osmosis equipment cannot meet the requirements.
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 saline solution is an osmotic process. As shown, if a water pool is separated into two parts by a membrane that only water molecules can penetrate, pure water and brine are injected to the same height on both sides of the membrane. After a while, it can 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, at this time, water molecules will migrate from the salt water end to the pure water end. 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 The reverse osmosis semi-permeable membrane is separated from the water that passes through the reverse osmosis membrane. Among the many impurities in the 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 rate. The level of reverse osmosis salt removal rate 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%.