Osmosis is a phenomenon that people often encounter in nature. If a box of pure water is vertically divided into two parts by a semi-permeable membrane, only the solvent can pass through the membrane, while the solute cannot, so it is called “semi-permeable membrane”. Pure water is in contact with both sides of the semi-permeable membrane at the same temperature and pressure, and under these conditions no flow of water across the semi-permeable membrane is generated because the chemical potentials on both sides of the membrane are exactly equal. If dissolved salt is added to one of the sides, the chemical potential on the salt solution side decreases. Pure water then permeates to the salt solution side, creating an osmotic flow until the equilibrium of the chemical potential is re-established.
Semi-permeable are films with selective permeability properties. When a liquid or gas passes through a semi-permeable membrane, some components are permeated while others are trapped. In fact, semi-permeable membranes are permeable to any component, but the rate of permeation varies greatly. In reverse osmosis, the transmission rate of solvents is much greater than that of solutes dissolved in water. The separation of solvent and solute is achieved through the semi-permeable membrane, resulting in pure water as well as a concentrated salt solution. The difference in height or pressure between the surface of the pure water and the surface of the salt solution is called the osmotic pressure of the salt solution.
When an external pressure is applied to the brine side, the original equilibrium is affected. The applied pressure will increase the chemical potential on the salt solution side, causing the water in the brine to flow through the semi-permeable membrane to the pure water side. This phenomenon is called reverse osmosis.
The driving force of the reverse osmosis process is the applied pressure, and the energy required for reverse osmosis separation is directly related to the concentration of the brine. Therefore, the higher the salt concentration, the higher the energy required to reverse osmosis pure water from brine. Reverse osmosis is the use of reverse osmosis membrane selective only through the solvent and intercept the nature of the ionic material, the static pressure difference between the two sides of the membrane as the driving force, to overcome the osmotic pressure of the solvent, so that the solvent through the reverse osmosis membrane to achieve the separation of liquid mixtures of membrane processes. It belongs to the pressure-driven membrane separation technology. Reverse osmosis technology is the salt water to external driving force to overcome the osmotic pressure and make the water molecules through the membrane of the reverse osmosis process. China began to use reverse osmosis technology to prepare demineralized water in the 1970s, but because of the intrinsic quality of the membrane and the low rate of desalination and other problems, it has not been popularized. In recent years, reverse osmosis technology has been rapid development, after reverse osmosis treatment, can remove 99% of the salt and silica in the water, and can remove 99.5% of the calcium and magnesium components in the water.
Reverse osmosis water treatment process is basically a physical method, it has many aspects of traditional water treatment methods do not have the advantages of.
Reverse osmosis is at room temperature, the use of physical methods without phase change can make the water desalination, purification.
Water treatment only by the pressure of water as the driving force, its energy consumption in many physical treatment method is the lowest;;.
No large amount of chemicals and acid and alkali regeneration treatment.
No chemical waste liquid and waste acid, alkali emissions, no environmental pollution;.
The equipment occupies a small area and takes up little space.
Operation and maintenance and equipment repair work is minimal.
With the development of membrane technology, compared with the traditional ion exchange desalination, its more economical salt content cut-off point is also gradually reduced. Reverse osmosis desalination way more and more shows its superiority in performance and advanced technology.
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