Water treatment commonly used filtration membranes: microfiltration, ultrafiltration, nanofiltration, reverse osmosis What are the differences?

The following types of filtration membranes are commonly used in water treatment. They are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), then, do you know how much their filtration precision? And what substances can be intercepted?



01 Microfiltration (MF)
Filtration precision is generally 0.1-50 microns, a variety of common PP filter, activated carbon filter, ceramic filter belongs to the category of microfiltration, used for simple coarse filtration, filtration of sediment in the water, rust and other large particles of impurities, but can not remove bacteria and other harmful substances in the water.

The cartridge usually can not be cleaned, for the disposable filter material, need to be replaced often.1 .PP cotton cartridge: generally only used for coarse filtration of low requirements, to remove large particles in the water such as sediment, rust, etc. 2. activated carbon: can eliminate the foreign color and odor in the water, but can not remove the bacteria in the water, and the removal of sediment, rust, the removal of the effect is very poor.3. ceramic cartridge: the smallest filtration precision of 0.1 microns only, usually small flow rate, not easy to clean.

02 Ultra Filtration (UF)

A pore size specifications consistent, rated pore size range of 0.001-0.02 micron microporous filtration membrane. The membrane filtration method using ultrafiltration membrane with pressure difference as the driving force is ultrafiltration membrane filtration. Most ultrafiltration membranes are made of acetate fibers or polymer materials with similar properties. Suitable for handling the separation and thickening of solutes in solution, but also commonly used in other separation techniques are difficult to complete the separation of colloidal suspensions, and its field of application is constantly expanding.

Membrane ultrafiltration with pressure difference as the driving force of filtration at any time can be distinguished as ultrafiltration membrane filtration, microporous membrane filtration and reverse osmosis membrane filtration three categories. They are distinguished on the basis of the size of small particles or molecular weight size that can be retained by the membrane layer. When the rated pore size range of the membrane is used as a criterion for differentiation, the rated pore size range of the microporous membrane (MF) is 0.02-10 μm; the ultrafiltration membrane (UF) is 0.001-0.02 μm; and the reverse osmosis membrane (RO) is 0.0001-0.001 μm. There are many pore controlling factors, such as different pore size and pore size distribution of UF membranes, depending on the type and concentration of the solution during membrane production, evaporation and coalescence conditions, and so on. Pore size distribution of ultrafiltration membrane.
Ultrafiltration membrane is generally a polymer separation membrane, used as ultrafiltration membrane polymer materials are mainly cellulose derivatives, polysulfone, polyacrylonitrile, polyamide and polycarbonate. Ultrafiltration membrane can be made into flat membrane, roll membrane, tubular membrane or hollow fiber membrane and other forms, widely used in such as the pharmaceutical industry, food industry, environmental engineering and so on.
Ultrafiltration membrane is one of the early development of polymer separation membrane, in the 60's ultrafiltration device has been industrialized. The industrial application of ultrafiltration membrane is very wide, has become one of the new chemical unit operation. Used for separation, concentration, purification of biological products, pharmaceutical products and the food industry; also used for blood treatment, wastewater treatment and ultrapure water preparation in the terminal treatment device. In China has successfully utilized ultrafiltration membrane for the concentration and purification of Chinese herbal medicine.
Ultrafiltration process is the core component in the production process of mineral water and spring water. Its water recovery rate is as high as 95% or more, and it can conveniently realize flushing and backwashing, is not easy to clog, and has a relatively long service life. Ultrafiltration does not need to add electricity to pressurize, relying only on tap water pressure can be filtered, the flow is large, the use of low-cost, more suitable for family drinking water purification. Therefore, the future purification of drinking water will be based on ultrafiltration technology, and combined with other filtration materials, in order to achieve a wider range of treatment, more comprehensive elimination of pollutants in the water.

03Nano-filtration (NF)
The filtration precision is between ultrafiltration and reverse osmosis, the desalination rate is lower than that of reverse osmosis, and it is also a kind of membrane separation technology that needs to be energized and pressurized, and the water recovery rate is lower.
That is to say the process of making water with nanofiltration membrane will definitely waste nearly 30% of tap water. This is unacceptable to the average family and is generally used for industrial pure water manufacturing.


04 Reverse Osmosis Membrane (RO Membrane)
RO is the abbreviation of English Reverse Osmosis membrane, the Chinese meaning is (reverse osmosis), the general flow of water from the low concentration to the high concentration of water, once the water is pressurized, it will be from the high concentration to the low concentration of the flow, that is, the so-called reverse osmosis principle:

Due to the RO membrane pore size is five millionths of a hair (0.0001 micron), the general eye can not be seen, bacteria, viruses are 5000 times it. Therefore, only water molecules and some beneficial mineral ions can pass through, other impurities and heavy metals are discharged by the waste water pipe, all the desalination process, as well as the astronauts wastewater recycling treatment are used in this way, so the RO membrane is also known as the high-tech artificial kidney outside the body.


Ultrafiltration is a membrane separation process that utilizes a pressure-activated membrane to retain colloids, particles and relatively high molecular weight substances in water under the action of an external driving force (pressure), while water and small solute particles pass through the membrane. Water and small solute particles pass through the membrane. Substances with high molecular weight can be retained by screening through the micropores on the membrane surface. Dissolution, diffusion principle: permeate dissolved in the membrane, and along its driving force gradient diffusion transfer, the formation of chemical equilibrium between the phases on the surface of the nanofiltration membrane. b. Electrical effect: nanofiltration membrane and electrolyte ions to form an electrostatic interaction between the electrolyte salt ions with different charge strengths, resulting in differences in the membrane on the retention rate of the ions in the multivariate system containing ions with different valence due to the DONAN (DONNAN) In the multivariate system containing ions of different valence states, due to the DONNAN effect, the selectivity of the membrane for different ions is not the same, and the proportion of different ions passing through the membrane is not the same.　

The ion selectivity of the nanofiltration process is due to negatively charged groups on or in the nanofiltration membrane, which interact with each other through electrostatic forces to impede the permeation of multivalent ions. The possible charge density of a nanofiltration membrane is 0.5 to 2 meq/g. Nanofiltration membranes are between RO and UF membranes. The principle of reverse osmosis is that in water with salts (such as raw water), applying a pressure greater than the natural osmosis pressure, so that osmosis is carried out in the opposite direction, and the water molecules in the raw water are pressurized to the other side of the membrane, turning into clean water, so as to achieve the purpose of removing impurities and salts from the water. The essence is dehydration technology. Polymers for Filtration and Separation Membranes

With more than 2.3 billion people living in water-stressed countries today, how can this precious resource be managed as efficiently as possible? This question is particularly important. Today's water filtration processes rely on high-performance yet cost-effective membrane materials that can withstand high-pressure, high-temperature environments and constant chemical exposure.