Positive infiltration technology has developed rapidly in recent years, to fill the "zero discharge" of industrial wastewater treatment blank. As a low-energy, green solution, the positive infiltration technology has broad application prospects in industrial water treatment, desalination and zero discharge of wastewater.
The four major advantages of infiltration technology highlight the bright future of the application market
Positive infiltration technology is emerging in recent years, water treatment technology, the initial R & D that is directed to a variety of difficult wastewater treatment and reuse and material separation, is a vast space for development of water treatment technology. At present, the research of this technology focuses on the selection of membrane materials and draw solutions. Based on the principle of positive infiltration, the scientific and technological achievements of forward osmosis membrane materials and liquid extracts are introduced, and the selection of the materials for the forward osmosis membrane and the selection of the liquid extracts are analyzed. Finally, the advantages of positive infiltration technology are summarized and the future application of infiltration is prospected.
With the increasing awareness of environmental protection, people pay more and more attention to the conservation and utilization of resources. As a limited resource necessary for human production and life, water has drawn more and more attention.
From simple water conservation to the development of water multiple reuse, the concept of water reuse is no longer strangers in the country, the water reuse and reuse water quality also has strict standards and requirements. The simple reuse of water from the initial use, such as for flushing, flushing roads, has now developed as a process water; from the initial part of the reuse, the development of the final zero emissions. With the improvement of reuse water quality standards and reuse rates, higher requirements are set for the treatment processes of reuse water.
For the reuse of water with high water requirements, almost all membrane processing technology will be used. As the most cost-effective desalination technology, reverse osmosis technology has been widely recognized. However, with the improvement of the standards of water quality and recovery of reverse osmosis, the disadvantages of reverse osmosis technology are also emerging. Problems such as poor anti-pollution ability and scaling and scaling have become a bottleneck for the development of the technology. It is precisely because of the shortcomings of reverse osmosis technology, and promote the development of positive infiltration technology.
First, the principle of penetration
For the principle of positive infiltration technology is no stranger to the study of reverse osmosis technology is the beginning of the principle of infiltration, this natural infiltration phenomenon is actually based on the principle of infiltration technology.
Positive infiltration is natural infiltration, refers to the water from the higher hydration level (or lower osmotic pressure) side of the region through the selection of permeable membrane flow to the lower hydration level (or higher osmotic pressure) side of the process . Positive permeation is the application of the osmotic pressure difference across the membrane as a driving force that allows water to spontaneously pass through the selective permeable membrane from the feedstock (with lower osmolarity) to the drive fluid (with higher Osmotic pressure) side. When applying a pressure less than the osmotic pressure to the solution with high osmotic pressure, the water will still flow from the feed solution side to the drive fluid side. This process is called pressure-damping permeation. The driving force of pressure damping penetration is still osmotic pressure, so it is also a positive infiltration process.
Second, is penetrating the core technology
Positive infiltration If as a commercial water purification technology, to solve two major problems: First, to make high-throughput water through the semipermeable membrane, and to ensure that the membrane life and prolonged anti-pollution ability; the second is to be able to learn The solute of the driving fluid is separated from the solution. This is also the two core technical issues that are being infiltrated: one is the choice of the material and structure of the membrane being infiltrated, and the other is the choice of driving solution.
1, is permeable membrane material and structure
In the infiltration technology, semi-permeable membrane material is the core material. Early researchers using asymmetric reverse osmosis composite membrane to study the positive infiltration process and found that this type of membrane is not suitable for positive infiltration, mainly due to the composite membrane porous support layer generated within the concentration polarization phenomenon, greatly reducing the infiltration process s efficiency. Therefore, the research on the material of the positive osmosis membrane focuses on finding the membrane material with high permeation efficiency, in order to reduce the polarization of the internal concentration and solve the problem of membrane flux and pollutant rejection rate. In addition, but also to ensure the physical strength of the film and chemical resistance.
Currently the best commercially available forward osmosis membrane material is the HTI company's supportive high-strength film, the film is a three-layer structure: a compact skin, porous support layer and mesh support layer. The dense skin and porous support layer are hydrophilic, electrically neutral and have a thickness of about 50 Î¼m. It is reported that the material is made of cellulose acetate polymer materials, the structure of circular fibers added to enhance the mechanical strength of the material.
In addition, a research team based in Statkraft, Norway, has developed composite positive osmosis membrane materials similar to those of reverse osmosis membranes for use in PRO processes that utilize the energy of a mixture of freshwater and seawater to gain energy. Another team from the research team used a polyetherimide hollow fiber membrane of higher strength as a support layer to form a hollow fiber composite positive osmosis membrane by interfacial polymerization. Compared with the reverse osmosis membrane material, the composite positive osmosis membrane support layer has a high opening rate, can effectively reduce the internal concentration polarization.
National University of Singapore has developed polybenzimidazole (PBI) hollow fiber nanofiltration membrane material with a positive charge on the surface of the membrane and a high rejection rate for divalent cations. It has been demonstrated in laboratories that it has good Positive permeability. The membrane material of the outer skin structure is more dense, the inner surface of the hole, the water permeability is the best reported data.
In addition to the problem of internal concentration polarization, another important issue that needs to be addressed is the chemical resistance of the positive osmosis membrane, primarily its resistance to acids and bases and its tolerance to oxidants. As the target of positive infiltration is mostly applied to the more polluted wastewater, the water itself has a wide pH range and may contain a large amount of oxidizing substances, thus requiring the membrane to have good chemical resistance. At the same time, the use of higher concentrations of acid-base or oxidants is inevitable due to the high levels of contamination and higher frequency of cleaning, in order to remove the large number of complex membrane surface contaminants. Therefore, in order to prolong the service life of the film, it is necessary to improve the chemical resistance of the film.
Chemical resistance of the membrane is mainly due to two factors: First, the membrane material itself performance; the other is a film of adhesive, glue performance. The current organic film material generally poor oxidation resistance, slightly better acetate fiber material, but the basic can not meet the use of oxidants for cleaning, only to improve the operational stability. With the development of materials science and technology, the application of new membrane materials may completely solve such problems.
Taken together, as a positive infiltration membrane should have the following characteristics: (1) dense, low porosity cortex, with high rejection; (2) the membrane cortex has good hydrophilicity, high water flux ; (3) membrane support layer as thin as possible, high porosity; (4) high mechanical strength; (5) with acid and alkali resistance to chemical corrosion in a wide pH range and a variety of different composition Solution conditions normal operation.
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