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The difference between ultrafiltration, nanofiltration, reverse osmosis, microfiltration
- Dec 25, 2018 -

1. Ultrafiltration (UF): The filtration precision is 0.001-0.1 micron, which is one of the high-tech in the 21st century. It is a membrane separation technology that utilizes differential pressure to filter out harmful substances such as rust, sediment, suspended matter, colloids, bacteria, macromolecular organic substances in water, and retain some mineral elements beneficial to the human body. It is the core component in the production process of mineral water and mountain spring water. The water recovery rate in the ultrafiltration process is as high as 95% or more, and the flushing and backwashing can be easily realized, which is not easy to block and has a relatively long service life. Ultrafiltration does not require electric pressure and pressure. It can be filtered only by tap water pressure. The flow rate is large and the cost of use is low. It is more suitable for comprehensive purification of household drinking water. Therefore, the purification of drinking water in the future will be based on ultrafiltration technology, combined with other filter materials, in order to achieve a wider range of treatment, more comprehensive elimination of pollutants in the water.


2. Nanofiltration (NF): The filtration precision is between ultrafiltration and reverse osmosis. The desalination rate is lower than that of reverse osmosis. It is also a membrane separation technique that requires electricity and pressure, and the water recovery rate is low. That is to say, in the process of making water with nanofiltration membrane, nearly 30% of tap water will be wasted. This is unacceptable to the average family. Generally used in industrial pure water manufacturing.


3. Reverse osmosis (RO): The filtration precision is about 0.0001 micron. It is an ultra-high precision membrane separation technology developed by the United States in the early 1960s. It can filter out almost all impurities (including harmful and beneficial) in water, and only allows water molecules to pass. That is to say, in the process of making water with a reverse osmosis membrane, nearly 50% of tap water will be wasted. This is unacceptable to the average family. Generally used in the manufacture of pure water, industrial ultrapure water, and pharmaceutical ultrapure water. Reverse osmosis technology requires pressurization, power up, low flow rate, low water utilization rate, and is not suitable for purification of a large amount of drinking water.


4, microfiltration (MF): filtration accuracy is generally 0.1-50 microns, common PP filter, activated carbon filter, ceramic filter, etc. are in the microfiltration category, for simple coarse filtration, filtration of sediment in water, Large particles such as rust, but can not remove harmful substances such as bacteria in the water. The filter element is usually not cleanable and is a disposable filter material that needs to be replaced frequently. 1 PP cotton core: generally only used for coarse filtration that is not required to remove large particles such as sediment and rust in water. 2 Activated carbon: It can eliminate the color and odor in the water, but it can't remove the bacteria in the water, and the removal effect on the sediment and rust is also very poor. 3 Ceramic filter: The minimum filtration accuracy is only 0.1 micron. Usually, the flow rate is small and it is difficult to clean.


First, reverse osmosis membrane (RO membrane):


RO is the abbreviation of English Reverse Osmosis membrane. Chinese means (reverse osmosis). Generally, the flow of water is from low concentration to high concentration. Once the water is pressurized, it will flow from high concentration to low concentration, which is called reverse osmosis principle: Since the pore size of the RO membrane is one-fifth of a millimeter (0.0001 micrometers) of the hair filament, it is generally invisible to the naked eye, and the bacteria and virus are 5,000 times larger. Therefore, only water molecules and some mineral ions beneficial to the human body can pass. Other impurities and heavy metals are discharged from the waste water pipe. All seawater desalination processes and astronaut wastewater recycling treatments use this method. Therefore, the RO membrane is also called the high-tech artificial kidney in vitro.


1. What is reverse osmosis?


Reverse osmosis is a new membrane separation technology developed in the 1960s. It relies on reverse osmosis membrane to separate the solvent and solute in solution. The full name of reverse osmosis is REVERSE OSMOSIS, abbreviated as “ RO".


2. The principle of reverse osmosis:


First of all, we must understand the concept of "infiltration". Infiltration is a physical phenomenon. When two kinds of water containing different salts, such as separated by a semi-permeable membrane, it is found that the water with one side with less salt will pass through. The membrane penetrates into the water with high salt content, and the salt contained is not penetrated. Thus, the salt concentration on both sides is gradually merged to equal. However, it takes a long time to complete the process. It is called osmotic pressure. However, if a pressure is applied to the water side having a high salt content, the above-mentioned permeation can be stopped as a result, and the pressure at this time is called the osmotic pressure. If the pressure is increased, it can penetrate in the opposite direction and the salt remains. Therefore, the principle of reverse osmosis desalination is to apply a pressure greater than the natural osmotic pressure in the salty water (such as raw water), so that the penetration proceeds in the opposite direction, and the water molecules in the raw water are pressed to the other side of the membrane. It becomes clean water, so as to remove impurities and salt from water.


3. The origin of RO reverse osmosis:


In 1950, the American scientist DR.S. Sourirajan had an unintentional discovery that the seagull picked up a large mouthful of sea water from the sea when it was flying at sea. After a few seconds, it spit out a small amount of seawater, which was a problem because of the animals that were breathed by the lungs on land. It is absolutely impossible to drink high-salt seawater. After dissection, it was found that there is a film in the seagull. The film is very precise. The seawater is inhaled through the seagull and pressurized, and then the water molecules are transformed into fresh water through the permeated membrane through pressure, while the seawater containing impurities and highly concentrated salt is spit out. Outside the mouth, this is the basic theoretical framework of the reverse osmosis method; and in 1953, the University of Florida applied desalination to remove salt equipment. In 1960, it was supported by the US federal government project to support Dr. Dr., UCLA University School of Medicine. S.Sidney Lode teamed up with Dr. Dr.S. Soirirajan to study reverse osmosis membranes, investing about $400 million a year in research to be used by astronauts, so that spacecraft do not have to carry large amounts of drinking water to take off until 1960. More and more scholars and experts have been working on the research, which has made the quality and quantity more precise, thus solving the problems in human drinking water.


 


Second, ultrafiltration membrane (UF):


A microporous membrane with a uniform pore size and a nominal pore size range of 0.001-0.02 microns. The membrane filtration method using an ultrafiltration membrane with a pressure difference as a driving force is ultrafiltration membrane filtration. Ultrafiltration membranes are mostly made of acetate fibers or polymer materials with similar properties. It is most suitable for the separation and enrichment of solutes in solution, and is also commonly used for the separation of colloidal suspensions that are difficult to accomplish by other separation techniques, and its application fields are expanding.


Membrane filtration based on pressure difference can be divided into three types: ultrafiltration membrane filtration, microporous membrane filtration and reverse osmosis membrane filtration. Their distinction is based on the minimum particle size or molecular weight that the membrane layer can trap. When the nominal pore size range of the membrane is used as a classification standard, the pore size range of the microporous membrane (MF) is 0.02 to 10 μm; the ultrafiltration membrane (UF) is 0.001 to 0.02 μm; and the reverse osmosis membrane (RO) is 0.0001 to 0.001 μm. It can be seen that the ultrafiltration membrane is most suitable for the separation and enrichment of solute in the treatment solution, or the separation of the colloidal suspension which is difficult to accomplish by other separation techniques. The membrane forming technique of ultrafiltration membranes, that is, the technique of obtaining the desired size and narrow distribution of micropores is extremely important. There are many control factors for the pores. For example, ultrafiltration membranes with different pore sizes and pore size distributions can be obtained according to the type and concentration of the solution during the film formation, evaporation and coagulation conditions. The ultrafiltration membrane is generally a polymer separation membrane, and the polymer materials used as the ultrafiltration membrane mainly include cellulose derivatives, polysulfone, polyacrylonitrile, polyamide, and polycarbonate. The ultrafiltration membrane can be used in the form of a flat membrane, a roll membrane, a tubular membrane or a hollow fiber membrane, and is widely used in, for example, the pharmaceutical industry, the food industry, environmental engineering, and the like.


We all know that the sieve is used to sieve things, it can release small objects, and the larger ones are left behind. However, have you heard of a sieve that can sieve molecules? Ultrafiltration Membrane - This super sieve separates molecular sieves of varying sizes! So, what exactly is an ultrafiltration membrane?


The ultrafiltration membrane is a porous membrane with super "sieving" separation function. Its aperture is only a few nanometers to tens of nanometers, which means that there is only one hair strand! By applying a suitable pressure to one side of the membrane, solute molecules larger than the pore size can be sieved to separate particles having a molecular weight of more than 500 Daltons and a particle diameter of more than 2 to 20 nm. The structure of the ultrafiltration membrane is symmetrical and asymmetrical. The former is isotropic, has no cortex, and the pores in all directions are the same, belonging to deep filtration; the latter has a dense surface layer and a bottom layer mainly composed of finger structures, and the surface layer thickness is 0.1 micron or less. And having ordered micropores, the bottom layer having a thickness of 200 to 250 microns, belonging to surface filtration. Ultrafiltration membranes used in industry are generally asymmetric membranes. The membrane materials of ultrafiltration membrane mainly include cellulose and its derivatives, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, polysulfone amide, sulfonated polysulfone, cross-linked Polyvinyl alcohol, modified acrylic polymer, and the like.


Ultrafiltration membrane was one of the earliest developed polymer separation membranes. In the 1960s, ultrafiltration equipment was industrialized. Ultrafiltration membranes are widely used in industrial applications and have become one of the new chemical unit operations. It is used in separation, concentration, purification of biological products, pharmaceutical products and food industry; it is also used in terminal treatment equipment in blood treatment, wastewater treatment and ultrapure water preparation. In China, the ultrafiltration membrane has been successfully used for the concentration and purification of Chinese herbal medicine. With the advancement of technology, ultrafiltration membranes will be improved and strengthened, and their contribution to human society will increase.