What is the difference between microfiltration, ultrafiltration, nanofiltration and reverse osmosis?

Ultrafiltration (UF):

The filtration accuracy is 0.001-0.1 micron, which belongs to one of the high and new technologies in the 21st century. It is a membrane separation technology using differential pressure, which can filter out rust, sediment, suspended solids, colloids, bacteria, macromolecular organics and other harmful substances in water, and retain some mineral elements beneficial to human body. It is the core component in the production process of mineral water and mountain spring water. The recovery rate of water in ultrafiltration process is more than 95%, and it can easily realize flushing and backwashing, which is not easy to block and has a relatively long service life. Ultrafiltration does not need to be powered and pressurized. It can be filtered only by tap water pressure. It has large flow and low cost. It is more suitable for the comprehensive purification of household drinking water. Therefore, in the future, the purification of drinking water will focus on ultrafiltration technology, combined with other filter materials, so as to achieve a wide treatment range and more comprehensively eliminate pollutants in water.

Nanofiltration (NF):

The filtration accuracy is between ultrafiltration and reverse osmosis, and the desalination rate is lower than reverse osmosis. It is also a membrane separation technology requiring power on and pressure, and the recovery rate of water is low. In other words, nearly 30% of tap water will be wasted in the process of making water with nanofiltration membrane. This is unacceptable to ordinary families. It is generally used in the manufacture of industrial pure water.

Reverse osmosis (RO):

The filtration accuracy is about 0.0001 micron. It is an ultra-high precision membrane separation technology using differential pressure developed in the United States in the early 1960s. It can filter out almost all impurities in water (including harmful and beneficial) and only allow water molecules to pass through. In other words, more than 50% of tap water will be wasted in the process of making water with reverse osmosis membrane. This is unacceptable to ordinary families. It is generally used in the manufacture of purified water, industrial ultrapure water and pharmaceutical ultrapure water. Reverse osmosis technology needs pressurization and power on, with small flow and low utilization rate of water, which is not suitable for the purification of a large number of drinking water.

Microfiltration (MF):

The filtration accuracy is generally 0.1-50 microns. Various common PP filter elements, activated carbon filter elements and ceramic filter elements belong to the category of microfiltration. They are used for simple coarse filtration to filter large particle impurities such as sediment and rust in water, but they can not remove harmful substances such as bacteria in water. The filter element usually cannot be cleaned. It is a disposable filter material and needs to be replaced frequently. ① PP cotton core: generally, it is only used for coarse filtration with low requirements to remove sediment, rust and other large particles in water. ② Activated carbon: it can eliminate the color and odor in the water, but it can not remove the bacteria in the water, and the removal effect of sediment and rust is also very poor. ③ Ceramic filter element: the minimum filtration accuracy is only 0.1 micron. Usually, the flow is small and it is not easy to clean.

Reverse osmosis membrane (RO membrane):

Ro is the abbreviation of reverse osmosis membrane in English, which means (reverse osmosis) in Chinese. Generally, the flow mode of water is from low concentration to high concentration. Once pressurized, water will flow from high concentration to low concentration, that is, the so-called reverse osmosis principle: because the pore diameter of RO membrane is five millionths (0.0001 microns) of hair, it is generally invisible to the naked eye, and bacteria and viruses are 5000 times that of it, Only water molecules and some mineral ions beneficial to human body can pass through, and other impurities and heavy metals are discharged from the waste water pipe. This method is used in all seawater desalination processes and astronaut wastewater recovery and treatment. Therefore, RO membrane is also known as high-tech artificial kidney in vitro.

What is reverse osmosis?

Reverse osmosis is a new membrane separation technology developed in the 1960s. It is a process of separating solvent and solute in solution under pressure by reverse osmosis membrane The full English name of reverse osmosis is "reverse osmosis", abbreviated as "ro".

Principle of reverse osmosis:

First of all, we should understand the concept of "penetration" Infiltration is a physical phenomenon When two kinds of water containing different salts are separated by a semi permeable membrane, it will be found that the water on the side with less salt content will penetrate through the membrane into the water with high salt content, but the salt content will not penetrate. In this way, the salt concentration on both sides will be gradually fused to be equal However, it takes a long time to complete this process, which is also known as osmotic pressure. However, if you try to add a pressure on the water side with high salt content, the result can also stop the above infiltration. The pressure at this time is called osmotic pressure. If the pressure is increased again, it can penetrate in the opposite direction and the salt remains. Therefore, the principle of reverse osmosis desalination is to apply a greater pressure than the natural osmotic pressure to the water with salt (such as raw water), so as to make the infiltration proceed in the opposite direction, and press the water molecules in the raw water to the other side of the membrane to become clean water, so as to achieve the purpose of removing impurities and salt in the water.

Origin of RO reverse osmosis:

In 1950, Dr. s. sourirajan, an American scientist, once inadvertently found that when seagulls were flying at sea, they took a large sip of sea water from the sea surface and spit out a small sip of sea water after a few seconds. He was in doubt, because animals breathing by lungs on land can never drink high salt sea water. After dissection, it is found that there is a layer of film in the seagull, which is very precise. The sea water is inhaled into the body through the seagull and pressurized, and then the water molecules penetrate through the film through the pressure to convert into fresh water, while the sea water containing impurities and highly concentrated salt is spitted out of the mouth, which is the basic theoretical framework of backward reverse osmosis; In 1953, it was applied by the University of Florida to the desalination and desalination equipment. In 1960, with the special support of the federal government, Dr. S. Sidney lode, Professor of the medical school of U.C.L.A University, and Dr. s. soiirajan began to study the reverse osmosis membrane. About US $400 million was invested in research a year to be used by astronauts, so that the spacecraft did not need to carry a large amount of drinking water until the scholars who put into research in 1960 There are more and more experts to improve the quality and quantity, so as to solve the problem of human drinking water.

Ultrafiltration membrane (UF):

A microporous filter membrane with uniform pore size and rated pore size range of 0.001-0.02 microns. Ultrafiltration membrane is adopted, and the membrane filtration method driven by pressure difference is ultrafiltration membrane filtration. Ultrafiltration membranes are mostly made of acetate fiber or polymer materials with similar properties. It is most suitable for the separation and concentration of solute in the treatment solution. It is also commonly used for the separation of colloidal suspension which is difficult to be completed by other separation technologies. Its application field is expanding.

Membrane filtration driven by pressure difference can be divided into ultrafiltration membrane filtration, microporous membrane filtration and reverse osmosis membrane filtration. They are distinguished according to the minimum particle size or molecular weight that can be retained by the film. When the rated pore size range of membrane is taken as the distinguishing standard, the rated pore size range of microporous membrane (MF) is 0.02 ~ 10 μ m; Ultrafiltration membrane (UF) is 0.001 ~ 0.02 μ m; Reverse osmosis membrane (RO) is 0.0001 ~ 0.001 μ m。 Therefore, ultrafiltration membrane is most suitable for the separation and concentration of solute in solution, or the separation of colloidal suspension which is difficult to be completed by other separation technologies. The membrane technology of ultrafiltration membrane, that is, the technology to obtain the expected size and narrow distribution of micropores, is extremely important. There are many factors controlling pore size. For example, ultrafiltration membranes with different pore size and pore size distribution can be obtained according to the type and concentration of solution during membrane preparation, evaporation and coagulation conditions. Ultrafiltration membranes are generally polymer separation membranes. The polymer materials used as ultrafiltration membranes mainly include cellulose derivatives, polysulfone, polyacrylonitrile, polyamide and polycarbonate. Ultrafiltration membrane can be made into flat membrane, coiled membrane, tubular membrane or hollow fiber membrane, which is widely used in pharmaceutical industry, food industry, environmental engineering, etc.

We all know that the sieve is used to sift things. It can release small objects and intercept the larger ones. But have you ever heard of a sieve that can sieve molecules? Ultrafiltration membrane - this super sieve can separate molecules of different sizes! So, what is ultrafiltration membrane?

Ultrafiltration membrane is a kind of porous membrane with super "screening" separation function. Its aperture is only a few nanometers to tens of nanometers, that is, only 1 ‰ of a hair! When proper pressure is applied to one side of the membrane, solute molecules larger than the pore size can be screened out to separate particles with molecular weight greater than 500 daltons and particle size greater than 2 ~ 20 nm. The structure of ultrafiltration membrane can be divided into symmetrical and asymmetric. The former is isotropic, without 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 dominated by finger structure. The thickness of the surface layer is 0.1 μ m or less, and has ordered micropores. The thickness of the bottom layer is 200 ~ 250 μ m, which belongs to surface filtration. The ultrafiltration membrane used in industry is generally asymmetric membrane. The membrane materials of ultrafiltration membrane mainly include cellulose and its derivatives, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, polysulfone amide, sulfonated polysulfone, crosslinked polyvinyl alcohol, modified acrylic polymer, etc.

Ultrafiltration membrane is one of the earliest developed polymer separation membranes. The ultrafiltration device was industrialized in the 1960s. Ultrafiltration membrane is widely used in industry and has become one of the new chemical unit operations. It is used to separate, concentrate and purify biological products, pharmaceutical products and food industry; It is also used as a terminal treatment device in blood treatment, wastewater treatment and ultrapure water preparation. In China, ultrafiltration membrane has been successfully used to concentrate and purify Chinese herbal medicine. With the progress of technology, the screening function of ultrafiltration membrane will be improved and strengthened, and its contribution to human society will be greater and greater.