Nanofiltration is a new membrane separation technology between reverse osmosis and ultrafiltration. Because the original reverse osmosis separation process requires enormous energy consumption, nanofiltration membrane technology with relatively low operating pressure and high permeability was used. Although this technology also has some disadvantages, compared with other separation technologies, the advantages outweigh the disadvantages. Meanwhile, it also has some advantages that other separation technologies cannot replace. Under the action of pressure, because water flux and pressure are in direct proportion, that is, the greater the water flux, the greater the pressure, so as long as the reasonable control of pressure can effectively remove some harmful and toxic substances in the water, at the same time, still retain the more beneficial minerals and trace elements in the water for human body.
Technical analysis
Characteristic analysis
The technology is not a panacea, and its use is subject to certain conditions. The technology is suitable for the separation of the relative molecular mass of 200 to 1000, at the same time the size of 1 nanometer dissolved components, so according to this characteristic analysis can be found that it can be used in different molecular weight of organic matter separation; Second, the separation must be achieved in such a way that the pressure difference between membrane penetration is not high, normally between 0.5 mpa and 2.0 mpa.
Study on separation mechanism
At present, many scholars believe that the mass transfer mechanism of nanofiltration membrane is dissolution and diffusion mode, and its separation of inorganic salts is not only controlled by the chemical potential gradient, but also under the comprehensive influence of the electric potential gradient. At present stage, there is no comprehensive and rational explanation for the separation mechanism of nanofiltration membrane studied in our country. The main models used to analyze the separation mechanism of nanofiltration membrane are charge model, thermodynamic irreversible model, electrostatic repulsion model and solid resistance model. Charge model is mainly based on different assumptions about the distribution of charge and potential in the film. After this analysis, charge model can be divided into space charge model and fixed charge model. In fact, the separation process of nanofiltration membrane is almost exactly the same as that of microfiltration, ultrafiltration and reverse osmosis membrane. It is mainly driven by pressure difference. The flux of this model should be analyzed on a problem by problem basis, but it is generally characterized by phenomenological equation constructed by non-equilibrium thermodynamic model: JV is equal to LP (ΔP-δΔπ), and JS is equal to (1-δ) CSJV+ kp-δC, where, JV refers to the solvent migration flux; JS refers to solute transport flux; LP refers to the final hydraulic permeability coefficient of the membrane. In addition, ΔP refers to the pressure difference between the two sides of the membrane. δ refers to the final retention coefficient of the solute. Δπ refers to the osmotic pressure difference between the two sides of the membrane. KP refers to the solute permeability coefficient. CS refers to the concentration of solute in the membrane; Electrostatic repulsion and steric hindrance model. This model first assumes that the membrane separation layer is composed of micropores with uniform pore size and uniform surface charge distribution. The structural parameters of this model are relatively complex and cover a very wide range, but generally it only covers pore size, pore opening rate, volume charge density of the film and the thickness of the membrane separation layer. When the above parameters are fully known, the electrostatic steric hindrance model can be used to judge the separation characteristics of various solutes through membranes.
Technical application
Applications in sewage treatment
The nanofiltration membrane technology has been briefly described in the above article, and it can be found that its inherent characteristics determine its unique role in drinking water preparation and preparation. These effects can not be replaced by other technologies, mainly reflected in the removal of all kinds of toxic and harmful substances and non-electrode salts, if the water quality is not particularly poor, basically can reasonably intercept impurities, pathogens and bacteria, and effectively remove all kinds of trace organic matter in sewage, especially in the sewage part of the highly toxic and have the “three-cause” effect of substances, According to the understanding of charge effect, it can be believed that nanofiltration membrane can reduce the hardness of water quality. Although this hardness is not felt by ordinary people, it actually plays such an effect to remove nitrate, fluoride, heavy metal, arsenic and other inorganic pollutants in drinking water that are harmful to human body. From the point of view of screening effect, nanofiltration membrane can scientifically and reasonably remove trichloromethane, its intermediates, hormones and other pollutants. Nanofiltration technology has the characteristics of simple process, high separation efficiency and easy control in the treatment of micro-polluted water. At the same time, in the process of use, the analysis of the removal rate of inorganic substances is low, and the degree of membrane scaling and pollution is low. In this way, the membrane efficiency is greatly improved, the membrane flux is large, and the water production does not need mineralization.
Applications in industrial water treatment
Although today’s economy is developing rapidly, serious environmental problems are hidden behind the rapid economic development, which has a great impact on human health. Wastewater, especially industrial wastewater, is filled with fertilizer, compost and intermediate products. Such as suspended solids in power plants and metallurgical plants; There are a lot of acid, alkali and oil in the wastewater of some chemical enterprises. Nanotechnology, combined with primitive wastewater purification techniques, is now widely used in the treatment of industrial wastewater. Some foreign scholars have used nanofiltration membrane to treat tissue wastewater (high content of inorganic salts). Under the condition of operating pressure of 500 kpa, the flux can reach a high level and the interception rate of dye can reach 97%. However, in this case, the interception of sodium chloride is only 13%, so the recycling of wastewater can be completely realized. But the technology has to reach a certain level. It should be noted that because of the high concentration of salt and dye in the dye wastewater, it will directly lead to the loss of reverse osmosis membrane in the process of interception, but nanofiltration can effectively solve this problem. Moreover, nanofiltration is also widely used in the treatment of yeast wastewater, beverage wastewater, soybean wastewater and other food industry wastewater. After the treatment of this technology, it can be found that the water quality can fully meet the relevant requirements of chemical and biological treatment.
Application of water treatment in rural production
At present situation, pesticide use phenomenon exists in most rural areas of our country, which directly causes the water to be polluted from chemical drugs. At present, the original method of removing pesticides in water is adsorption filtration of activated carbon and oxidation of ozone or hydrogen peroxide. However, people who have used the above methods know that although the effect of activated carbon is better, but the failure is very fast, the need to constantly replace, which determines the use of this method of sewage treatment cost is relatively high. At the same time, the adsorption efficiency of pesticides in water is low because of competitive adsorption. From this point of view, this method should not be used for a long time, or a large number of use. Oxidation, which turns large molecules of pesticides into smaller ones, encourages bacteria to repopulate the water. It is worth paying special attention to that, due to the screening effect of nanofiltration membrane technology on low molecular weight neutral solute molecules, so to some extent determines that the technology has a very good interception effect on pesticide harmful substances in water, not only that, compared with activated carbon adsorption, due to the low cost of the technology, so the treatment of pesticide wastewater is particularly economic.
Post time: Dec-26-2022