Summary of 6 kinds of commonly used membrane treatment technologies
At present, membrane technology as an old but emerging technology, technology development is more and more in-depth, the scope of application is more and more extensive, this paper summarizes the existing membrane treatment technology in the world, introduces the reasons and application fields of various membrane technology in detail!
一、 Microfiltration (MF) membrane technology
1. Basic principle of MF
The microfiltration membrane can intercept particles between 0.1-1 μ M. Microfiltration membrane allows macromolecules and soluble solids (inorganic salts) to pass through, but it will intercept suspended solids, bacteria, and high molecular weight colloids. The operating pressure of microfiltration membrane is generally 0.3-7 bar. Microfiltration membrane filtration is the earliest membrane technology developed and applied in the world. Natural or synthetic macromolecular compounds are used as membrane materials. For the microfiltration membrane, the separation mechanism is mainly screening and retention.
2. Application of microfiltration membrane
(1) Water treatment industry: removal of suspended solids, micro particles and bacteria in water;
(2) Electronic industry: semiconductor industry ultra pure water, integrated circuit cleaning water terminal treatment;
(3) Pharmaceutical industry: medical pure water sterilization, pyrogen removal, drug sterilization;
(4) Medical industry: remove bacteria from tissue fluid, antibiotics, serum, plasma protein and other solutions;
(5) Food industry: removal of suspended solids, microorganisms and odor impurities, yeast and mold in beverage, wine, soy sauce, vinegar and other food, clarification and filtration of juice.
(6) Chemical industry: filtration and clarification of various chemicals.
2、 Ultrafiltration (UF) membrane technology
1. Principle of ultrafiltration (UF)
Ultrafiltration (UF) is a kind of membrane separation technology which can purify and separate the solution. Ultrafiltration membrane system is a solution separation device with ultrafiltration membrane silk as filtering medium and pressure difference on both sides of membrane as driving force. Ultrafiltration membrane only allows the solvent (such as water molecules), inorganic salts and small molecules of organic matter in the solution to pass through, and intercepts the macromolecular substances such as suspended solids, colloids, proteins and microorganisms in the solution, so as to achieve the purpose of purification and separation.
The definition of ultrafiltration pore size and molecular weight cut-off (MWCO) has always been vague. It is generally considered that the pore size of ultrafiltration membrane is 0.001-0.1 μ m, and the molecular weight cut-off (MWCO) is 1000-1000000 Dalton. Strictly speaking, the pore size of ultrafiltration membrane is 0.001-0.01 μ m, and the molecular weight intercepted is 1000-300000 Dalton. If the pore size is larger than 0.01 μ m, or the molecular weight intercepted is more than 300000 Dalton, it should be defined as microfiltration membrane or fine filtration membrane.
二. Application of ultrafiltration membrane
The application range of ultrafiltration membrane is extremely wide. Filtration equipment can be used in almost all industries involving filtration. The basic filtration industries are as follows:
Pure water and ultrapure water are used as pretreatment of reverse osmosis and terminal treatment of ultra pure water; industrial water is used to separate bacteria, heat sources, colloids, suspended impurities and macromolecular organics; drinking water and mineral water purification; concentration, purification and clarification of fermentation, enzyme preparation industry and pharmaceutical industry; juice concentration and separation; soybean, dairy, sugar industry, wine and tea The separation, concentration and clarification of juice and vinegar, purification and recovery of industrial wastewater and domestic sewage, and recovery of electrophoretic paint.
Ultrafiltration membrane separation can replace natural sedimentation, plate and frame filtration, vacuum drum, centrifugal separation, solvent extraction, resin purification, activated carbon decolorization and so on. The process is operated at normal temperature without phase change and secondary pollution.
三、 Nanofiltration (NF) membrane technology
1. Principle of nanofiltration
Nanofiltration (NF) is a new type of molecular membrane separation technology, which is one of the research hotspots in the field of membrane separation in the world. The results show that the pore size of NF membrane is more than 1nm, generally 1-2nm; the retention performance of NF membrane for solute is between RO and UF membrane; RO membrane has high removal rate for almost all solutes, but NF membrane only has high removal rate for specific solutes. NF membrane can remove divalent and trivalent ions, organics with Mn ≥ 200, microorganisms, colloids, heat sources and viruses.
One of the most important characteristics of nanofiltration membrane is that the membrane has electric charge, which is the important reason that it still has high desalination performance under very low pressure (only 0.5MPa) and the membrane with molecular weight of hundreds can also remove inorganic salts, which is also the main reason for the low operation cost of NF.
NF is suitable for all kinds of salt water sources. The water utilization rate is generally 75% ~ 85%, and 30% ~ 50% for seawater desalination. There is no acid-base wastewater discharge.
2. Application of nanofiltration membrane in water treatment
(1) Application of nanofiltration membrane in drinking water: the operating pressure of nanofiltration is low, so it is the preferred process for drinking water preparation and deep purification.
At present, the water supply sources in most cities are polluted to varying degrees. However, the removal rate of organic matter in water by conventional treatment process of waterworks is not high. When chlorine is used for disinfection, chlorine and organic matter in water will generate halogenated by-products. Peltier et al's 4-year follow-up study showed that the doc in water decreased to 0.7 MGC / L on average, and the residual chlorine content in effluent decreased from 0.35 mg / L to 0.1 mg / L, and the formation of trihalomethane (THMs) in the final network cable was reduced by 50% compared with that without nanofiltration system. In addition, due to the decrease of biodegradable dissolved organic carbon (bcod), the biological stability of the produced water was improved.
Nanofiltration technology can remove most of Ca and Mg plasma, so desalination is the most widely used field. Membrane water treatment technology is similar to conventional lime softening and ion exchange process in terms of investment, operation, maintenance and price. However, it has the advantages of no sludge, no regeneration, complete removal of suspended solids and organic matter, simple operation and less land occupation. There are many application examples. Nanofiltration can be directly used to soften groundwater, surface water and wastewater, and can also be used as pretreatment of reverse osmosis and solar photovoltaic desalination devices.
(2) Application of nanofiltration membrane in seawater desalination: seawater desalination refers to desalination of seawater with salt content of 35000 mg / L to drinking water below 500 mg / L.
(3) Application of nanofiltration membrane in wastewater treatment
① Domestic sewage: the domestic sewage is generally treated by the combination of biodegradation and chemical oxidation, but the amount of oxidant is too large and there are many residues. The quality of ultrafiltration effluent can meet the requirements of water reuse for hotel toilet flushing, greening and other links. The quality of nanofiltration effluent can meet the sanitary standard of drinking water (gb5749.85), and can be reused to the environment with higher water requirements such as laundry and bathing in hotels Section.
② However, it is difficult to remove the concentration and concentration of inorganic dyes in the textile wastewater by the methods of dissolving, dissolving and retaining inorganic dyes in the textile wastewater.
③ Tannery wastewater: tannery wastewater contains high concentration of organic matter, sulfate and chloride, and the conductivity value of waste liquor in pickling process reaches 75ms / cm.
④ Electroplating wastewater: electroplating plants often produce a large amount of waste liquid. Although the complex treatment steps such as acidification, chemical harmlessness, sedimentation and sludge separation are adopted, the produced water has high salt content and cannot be reused.
⑤ Papermaking wastewater: in the pulp and paper industry, processes such as homogenization, bleaching and papermaking require a large amount of water. It is the best way for pulp and paper mills to save water resources and reduce emissions. The wastewater produced by the traditional activated sludge process also contains some colored compounds, microorganisms, antibodies and a small amount of biological decomposition substances, suspended solids, etc., which can only be used to make packaging paper, but not to produce higher grade paper. In addition, this method can not reduce the content of inorganic salts.
Reverse osmosis (RO) membrane technology
1. Principle of reverse osmosis (RO)
Reverse osmosis is a kind of membrane separation process driven by pressure. In order to generate reverse osmosis pressure, it is necessary to apply pressure to saline solution or wastewater by water pump to overcome the natural osmotic pressure and membrane resistance, so as to make water pass through the reverse osmosis membrane, and prevent the dissolved salt or pollution impurities in water from the other side of the reverse osmosis membrane.
2. Application of reverse osmosis membrane in water treatment
(1) Conventional application in water treatment
Water is an essential material condition for people's survival and production activities. Due to the increasing shortage of fresh water resources, the capacity of reverse osmosis water treatment plants in the world has reached millions of tons per day.
(2) Application in urban sewage
At present, the application of reverse osmosis membrane in advanced treatment of urban sewage, especially the reuse of secondary effluent and reclaimed water in sewage treatment plant, has been highly valued.
(3) Application in heavy metal wastewater treatment
The conventional treatment of wastewater containing heavy metal ions is only a kind of pollution transfer, that is, the dissolved heavy metals in the wastewater are transformed into precipitation or more easily treated form, and the final disposal is often landfill. However, the secondary pollution of groundwater and surface water caused by heavy metals still exists for a long time.
(4) Application in oily wastewater
Oily wastewater is a kind of industrial wastewater with a large amount and wide range. If it is directly discharged into the water body, it will produce oil film on the surface of water body to prevent oxygen from dissolving into the water, resulting in anoxic, biological death and odor in the water, which seriously pollutes the ecological environment. By using membrane treatment technology, oilfield produced water can be used to treat boiler water quality, so the treated water can be reused for boiler water supply of power station.
Dialysis membrane technology
1. Technical principles of various dialysis membranes
(1) Dialysis: dialysis (d) is a process of solute transport from upstream to downstream of membrane under the action of its own concentration gradient.
Dialysis is the first membrane separation technology discovered and studied. However, due to the limitation of its own system, the dialysis process is slow and inefficient, and the selectivity of dialysis process is not high. Therefore, dialysis process is mainly used to remove low molecular weight components in solutions containing various solutes, such as blood dialysis, which uses dialysis membrane instead of kidney to remove urea, creatinine, phosphate and uric acid Toxic low molecular weight components to alleviate the condition of patients with renal failure and uremia.
(2) Electrodialysis: electrodialysis (ED) is under the action of direct current electric field, with potential difference as driving force, using ion-exchange membrane to select anion and cation in solution to separate electrolyte from solution, so as to realize concentration, desalination, refinement and purification of solution.
(3) Reverse electrodialysis (EDR): according to the principle of ED, the polarity of positive and negative electrodes is reversed each other every certain time (generally 15-20min). It can automatically clean the dirt formed on the surface of ion exchange membrane and electrode, so as to ensure the long-term stability of the efficiency of ion exchange membrane and the quality and quantity of fresh water. In the late 1980s, the use of inverted electrodialyzer greatly increased the electrodialysis operating current and water recovery rate, and extended the operation cycle. EDR is especially unique in wastewater treatment, and its concentrated water circulation and water recovery rate can reach 95%.
(4) Liquid membrane electrodialysis (edlm): liquid membrane electrodialysis is to replace solid ion exchange membrane with liquid membrane with the same function. Its experimental model is to pack the liquid membrane solution into a thin layer partition plate with semi permeable glass paper, and then put it into the electrodialysis device for operation. It is possible to find an efficient separation method for the concentration and extraction of precious metals, heavy metals and rare metals by using extractant as liquid membrane electrodialysis (LMED), because the search for a membrane with special selectivity for some ions is related to the improvement of the extraction efficiency of electrodialysis. It is very promising to improve the separation efficiency of electrodialysis and combine it with liquid membrane directly. For example, when solid ion exchange membrane electrodialysis of platinum group metals (osmium, ruthenium, etc.) salt solution, metal dioxide precipitation will be formed on the membrane, which will cause premature loss of the membrane and destroy the whole process. The application of liquid membrane has no such disadvantage.
(5) Packed bed electrodialysis (EDI): packed bed electrodialysis (EDI) is a new type of water treatment method combining electrodialysis with ion exchange method. Its biggest feature is that the mixed bed ion exchange resin filled in the fresh water chamber of electrodialysis is automatically regenerated by using H + and oh - produced by hydrolysis, so as to realize continuous deep desalination. It integrates the advantages of electrodialysis and ion exchange, and improves the limiting current density and current efficiency. Packed bed electrodialysis technology is highly advanced and practical, and has broad application prospects in the fields of electronics, medicine and energy. It is expected to become the mainstream technology of pure water manufacturing.
(6) Bipolar membrane electrodialysis (edmb): bipolar membrane is a new type of ion exchange composite membrane, which is generally composed of laminated cation exchange membrane. The water molecules of the membrane are immediately decomposed into H + and oh -, which can be used as the supply source of H + and oh -. The outstanding advantages of bipolar membrane electrodialysis are simple process, high energy efficiency and less waste discharge. At present, the main application field of bipolar membrane electrodialysis process is acid-base preparation. For example, the two compartment membrane composed of bipolar membrane and cation membrane can realize the conversion of organic acid salt (sodium gluconate, sodium gulonate, etc.) and obtain alkali (NaOH), but the concentration (maximum concentration of acid is 2mol · L-1, the maximum concentration of alkali is 6mol · L-1) and purity are limited. At present, the developed application fields include flue gas desulfurization, regeneration of ion exchange resin, inorganic process of potassium and sodium, etc.
(7) Electrodeless electrodialysis: electrodialysis is an improved form of traditional electrodialysis. Its main feature is to remove the polar chamber and polar water of traditional electrodialysis. For example, the electrodes of the device are close to one or more layers of ion-exchange membranes, which are electrically interconnected, so as to prevent metal ions from entering the ion-exchange membrane, and at the same time prevent the plate from scaling, thus prolonging the service life of the electrode. Due to the cancellation of the pole chamber and the discharge of endless water, the utilization rate of raw water is greatly improved. Electrodeless hydrodialysis came into being in 1991, and its technology has been improved continuously in the process of application. At present, the operation mode of the device is mostly in the form of frequent pole reversal. At present, electrodialyzer with automatic control has been used in 20 provinces and cities in China, and recently exported to Southeast Asia.
2. Application of dialysis membrane
(1) Industrial wastewater treatment
Electrodialysis can be used to treat electroplating wastewater and heavy metal wastewater, and extract metal ions from wastewater. It can not only recycle water and useful resources, but also reduce pollution discharge. The feasibility of passivation solution treatment in copper production process was studied by Wan Shigui and other self-made ion-exchange membrane electrolyzers. The results showed that not only copper and zinc could be recovered, but also Cr3 + could be oxidized to Cr6 + to regenerate passivation solution. The technology of recovering heavy metals and acids from pickling wastewater by electrodialysis combined with ion exchange has been applied in industry.
(2) Drinking water and process water treatment
In Southwest China, electrodialysis is used to produce salt from Salt Spring brine. The content of NaCl is increased to 120g / L. compared with the original salt boiling method, the output is increased and the cost is reduced.
(3) Food industry
Blending is the key link in liquor production, and the quality of blending water is very important. It not only affects the internal quality of liquor, but also affects the appearance quality of liquor. Using electrodialysis to measure the blending water can significantly improve the water quality and reach the national standard.
(4) Biochemical industry
N-acetyl-L-cysteine was separated and purified by electrodialysis desalination with high performance ion exchange membrane and satisfactory results were obtained. According to the characteristics of bipolar membrane electrodialysis system, that is, H + is precipitated from the positive membrane and oh - from the negative membrane of the bipolar membrane. Bipolar membrane electrodialysis technology can be applied to the separation of soybean protein. It has many advantages: the whole production process does not need to add acid and alkali, the resources can be recycled, the water consumption is less, and the salt content in the separated protein is significantly reduced.
六、Forward osmosis (FO) technology
1. Principle of forward osmosis (FO)
A semi permeable membrane that can only penetrate the solvent but not the solute molecules is used to separate the solvent from the solution. Under the action of osmotic pressure, the solvent molecules will spontaneously pass through the membrane from the solvent side into the solution side under the action of osmotic pressure, which is called "forward osmosis".
2. Application of forward osmosis membrane in water treatment
(1) Wastewater treatment
The application of fo in the field of wastewater has been reported in many literatures, including the concentration of early high concentration industrial wastewater, the treatment of landfill leachate, the treatment of domestic sewage, the concentration of anaerobic digestion liquid of municipal sewage treatment plant and the life support system of directly treating wastewater into drinking water on space station. Although fo process is not a terminal process in these studies, it has high desalination performance in the pretreatment stage.
In recent years, with the continuous development of fo process, it has attracted many scholars' attention. The combination of fo technology and traditional membrane separation technology is a research hotspot in recent years.
(2) Deep purification of water quality
With the development of reclaimed water reuse technology, the most successful application of fo in drinking water purification is to treat domestic sewage directly into drinking water in space station.
(3) Seawater desalination
In fo system, similar to RO, the water molecules in the feed solution penetrate into the permeable side of the membrane through the semi permeable membrane, and the salt solution is intercepted on the other side of the membrane. Therefore, using fo as a desalination process and method has always been the focus of researchers, and there are many patents.