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Key Steps in Liquid Filtration

Oct. 14, 2020


Filtration refers to an operation of separating solid particles suspended in liquid. A porous material is used to pass the liquid in the suspension, and the intercepted solid particles are stored on the filter medium to form a filter cake. From the distinction of filtration accuracy, it can be divided into Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis. Let us take a look at the differences between them.

1. Microfiltration

Microfiltration belongs to precision filtration, and its basic principle is the process of sieve separation. Microfiltration is suitable for the separation of cells, bacteria and particles. In biological separation, it is widely used for the separation and concentration of bacteria, generally used for pre-treatment. The membrane pore size is greater than 0.1 micron, and it can also be used for clarification and security filtration of general liquids. The application scope of the microporous membrane is mainly to trap particles, bacteria and other pollutants from the liquid to achieve the purpose of purification, separation and concentration. The material of HONGTEK's microfiltration membranes include: Polypropylene (PP), Polyester, Glass Fiber, Nylon, Polyethersulfone (PES), Polytetrafluoroethylene (PTFE), Polyvinylidene Fluoride (PVDF), etc.


2. Ultrafiltration

Ultrafiltration is a membrane process between microfiltration and nanofiltration. The pore size of the membrane is between 0.001 and 0.1 microns. Ultrafiltration is a membrane separation technology that can purify, separate, and concentrate solutions. The ultrafiltration process can usually be understand as a screening process related to the size of the membrane pore size. The driving force is the pressure difference between the two sides of the membrane, and the ultrafiltration membrane is used as the filter medium. Under a certain pressure, when water flows through the membrane surface, only water and small molecules smaller than the membrane pore size are allowed to pass through, so as to achieve the purpose of purification, separation and concentration of the solution. Ultrafiltration membranes can separate macromolecular organics (such as proteins, bacteria), colloids, suspended solids, etc., and are widely used in the clarification of liquid materials and the separation and purification of macromolecular organics. HONGTEK's ultrafiltration membrane materials include Polyvinyl Chloride (PVC) and Polyvinylidene Fluoride (PVDF).


3. Nanofiltration

Nanofiltration is a membrane separation technology between ultrafiltration and reverse osmosis. The membrane pore size is 0.0001~0.001 microns, its molecular weight cut-off is in the range of 80~1000, and the pore size is a few nanometers, so it is called nanofiltration. Based on the superior characteristics of nanofiltration separation technology, it has shown broad application prospects in pharmaceutical, biochemical, food industry and many other fields. For nanofiltration, the retention characteristics of the membrane are characterized by the retention rate of standard NaCl, MgSO4, CaCI2 solutions. Usually the rejection rate is in the range of 60-90%, and the corresponding molecular weight cut-off is in the range of 100~1000. Therefore, the nanofiltration membrane can separate small molecular organic matter from water and inorganic salts, and achieve simultaneous desalination and concentration.


4. Reverse Osmosis

Reverse osmosis is a membrane that uses the selective permeability of reverse osmosis membranes to only permeate solvents (usually water) while retaining ionic substances or small molecules. The static pressure on both sides of the membrane is used as the driving force to separate liquid mixtures. The interception object of reverse osmosis is all ions, only let water pass through the membrane, the interception rate of NaCI is above 98%, and the effluent is ion-free water. The reverse osmosis method can remove soluble metal salts, organic matter, bacteria, colloidal particles, and heat-generating substances, that is, it can intercept all ions. It is used in the production of purified water, softened water, non-ionized water, product concentration, and wastewater treatment. At present, reverse osmosis membranes have been widely used in many industries such as medicine, electronics, chemicals, food, and desalination. Reverse osmosis technology has become the preferred water treatment technology in modern industry.


Water plays an important role in our lives. It is the source of mankind and one of the most important material resources indispensable for human survival and development. Filtering is widely used in our daily life, and it is also an essential technology in our lives. The filtering technology will continue to be innovated and improved in the future.

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