Advanced drinking water treatment technology – reverse osmosis membrane (RO)

The pore size of the reverse osmosis membrane is the smallest, below 2-3 nm. Except for water molecules, all other impurity particles (including ions) cannot pass through the reverse osmosis membrane, so the water separated by the reverse osmosis membrane is pure water. Reverse osmosis technology has been widely used in seawater desalination, brackish water desalination, and preparation of high-purity water for industrial feed water (electronic industrial water, boiler feed water, etc.). The core technology of drinking pure water and high-quality direct drinking water that has developed rapidly in recent years is reverse osmosis. The operating pressure of reverse osmosis technology is relatively high. The osmotic pressure of the water being treated must be exceeded. For seawater desalination, the operating pressure is generally above 3MPa, and for the preparation of drinking pure water with tap water, the operating pressure is generally below 1MPa (determined according to the salt content of raw water, pure water recovery rate, and membrane characteristics, generally about 0.6MPa for household reverse osmosis membranes) , industrial reverse osmosis membrane about 1MPa).

Working principle of reverse osmosis membrane

A membrane that is selective for permeable substances is called a semipermeable membrane, and a membrane that can only permeate a solvent but cannot permeate a solute is generally called an ideal semipermeable membrane. When the same volume of dilute solution (such as fresh water) and concentrated solution (such as salt water) are placed on both sides of the semipermeable membrane, the solvent in the dilute solution will naturally pass through the semipermeable membrane and flow to the concentrated solution side spontaneously, This phenomenon is called penetration. When the osmosis reaches equilibrium, the liquid level on the side of the concentrated solution will be higher than the liquid level of the dilute solution by a certain height, that is, a pressure difference is formed, and this pressure difference is the osmotic pressure (see the figure for the working principle of the reverse osmosis membrane).
The size of the osmotic pressure depends on the inherent properties of the solution, that is, it is related to the type, concentration and temperature of the solution and has nothing to do with the properties of the semipermeable membrane. If a pressure greater than the osmotic pressure is applied on the solution side, the flow direction of the solvent will be opposite to the original osmotic direction, and it will begin to flow from the concentrated solution to the dilute solution side. This process is called reverse osmosis.
Reverse osmosis is a reverse migration movement of osmosis. It is a separation method that separates the solute and solvent in the solvent by means of the selective interception of the semipermeable membrane under the pressure drive. It has been widely used in the purification of various solutions. The most common application example is in the water treatment process, using reverse osmosis technology to remove impurities such as inorganic ions, bacteria, viruses, organics and colloids in raw water to obtain high-quality pure water.
Reverse osmosis membrane is a semi-permeable membrane artificially synthesized by a special process in order to realize the reverse osmosis phenomenon of aqueous solution. The pore size of the reverse osmosis membrane is 0.0001µm, and only the solvent in the aqueous solution, water, can pass through, while its solute cannot pass through the reverse osmosis membrane. The diameter of the virus is 0.02-0.4µm, and the diameter of the bacteria is 0.4-1.0µm. Therefore, the water passing through the reverse osmosis membrane is truly pure water.

Several issues that need to be paid attention to in the work of the reverse osmosis system:

  1. Influence of hardness Salts of calcium and magnesium ions, such as calcium carbonate and calcium sulfate, have only a small solubility in water. During the normal operation of the reverse osmosis membrane, because the working water is concentrated in the membrane, various salts will also When it is concentrated, when the saturated concentration of salts is reached, scale will be produced in the system, which will reduce the water flux of the membrane, reduce the salt rejection rate, and increase the pressure difference of the membrane system, which seriously affects the use of the membrane.
  2. Influence of colloidal substances Colloidal substances in water mainly include clay, colloidal silicon, metal oxides (iron oxide, aluminum oxide), etc., which will form colloidal fouling or scale in the working layer of the membrane, which will also seriously affect the performance of the element , resulting in a decrease in water production, a decrease in the desalination rate, and an increase in the pressure difference of the membrane system.
  3. Influence of organic matter The adsorption of organic matter in water on the membrane surface will form organic matter scale, which will cause the reduction of water flux, which cannot be cleaned in particular; and organic matter is not easy to be removed in pretreatment, so choose the water source of reverse osmosis should be given attention.
  4. Influence of oxidizing substances Most low-pressure reverse osmosis membranes are composite membranes, and the working surface of the membrane is easily oxidized by oxidizing substances and rapidly broken. Chlorine must be removed to ensure that free chlorine is less than 0.1ppm. Other oxidizing substances include chlorine dioxide, ozone, chlorine gas, bleaching powder, etc.
  5. The effect of pH value All kinds of reverse osmosis membranes have their own pH value range. Exceeding the range may cause problems such as degradation of the membrane, so pay close attention to the pH value of the influent water and the range of pH value used by the equipment during use.
  6. Influence of water temperature The reverse osmosis membrane reflects the temperature change very obviously. The higher the water temperature, the greater the water flux, but the desalination rate will decrease. The lower the water temperature, the lower the water flux. change, this is a normal change.
  7. Effect of Influent Salt Concentration The higher the influent salt concentration is, the greater the osmotic pressure, and the higher the pressure required to ensure the water flux during operation. When the osmotic pressure is equal to the outlet pressure of the high-pressure pump, the system will not work. At the same time, the higher the salt concentration in the influent water, the higher the salt content of the produced water.
  8. Influence of pressure The water production of the membrane system is based on the principle of reverse osmosis. When the influent pressure is higher, the water production of the system will be larger. Therefore, adjusting the pressure is an effective method to adjust the water production.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

Advanced drinking water treatment technology - reverse osmosis membrane (RO)

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