Filtration is an indispensable treatment in the purification process of fluids, mainly used to remove particulate matter or other suspended matter in the fluid. The principle of the filter is to use a porous medium to remove contaminants from the fluid (liquid or gas) to achieve the desired level of cleanliness. Filters are often considered to be a simple mesh or screen, and filtration or separation is performed on a surface. This is the way of the past. Most filter walls now have a certain thickness, which means that the filter equipment has a depth, which helps to remove contaminants in the form of "curved channels". The filter is a device for removing a small amount of solid particles in the liquid. When the fluid enters the filter cartridge with a certain size filter, the impurities are blocked, and the clean filtrate is discharged from the filter outlet. When the cleaning is required, the screw is used to clean the tube. The bottom screw plug, drain the fluid, remove the flange cover, take out the filter cartridge, and reload it after processing. It is well known that the core element of the filter is a filter membrane, which is a film which is formed on a microporous support layer (support) and which is filled with finer pores. There are many materials for making filters, which are classified into organic films (such as polysulfone hollow fiber membranes) and inorganic membranes (such as ceramic membranes). The membrane filter has high filtration precision, stable particle size control, and easy backwashing to restore performance. Therefore, it is extremely convenient to use and maintain.

1. Filtration mechanism and influencing factors 1.1 Filtration mechanism There are two main types of fluid filtration mechanisms. One is based on the size of the particles to separate, such as interception, sieving and surface capture, etc. The other is adsorption, ie the particles adhere to the filter under chemical/charge. This requires each pharmaceutical company to choose different filter membranes according to their actual needs.
1.2 Factors affecting filtration 1.2.1 The characteristics of the fluid are related to the characteristics of the fluid. For example, the viscosity of the fluid and the chemical/ionic composition, the greater the viscosity of the fluid, the slower the flow rate under the same pressure conditions, the more contact between the fluid and the membrane, and the better the filtration effect; for example, the mixing of the fluid and the membrane/ The contact time also has a large effect on the filtration effect, and the longer the mixing/contact time, the better the filtration effect. In addition, it should be noted that the characteristics of the fluid only affect the adsorption retention effect of the membrane on the fluid without affecting the exclusion of the particle size.
1.2.2 Operating conditions Relevant to actual operating conditions, such as particle flow rate and filtration pressure. In order to achieve good filtering results, generally choose a lower flow rate, the lower the flow rate, the better the interception effect. Practice has shown that the structural movement of the membrane is detrimental to filtration. Once the structure of the membrane changes during the filtration process, the particles and fibers can be precipitated from the depth filter, affecting the filtration effect. However, the velocity/pressure differential has only a significant effect on the adsorption cut-off and has a relatively small effect on size exclusion.
1.2.3 Types of Particles The type of particles is also closely related to the filtration effect. The particles are classified into two types: deformable particles and non-deformable particles. Under a certain pressure, the deformable particles will enter the filter membrane and cause more filter mesh plugging, which will affect the filtration effect, such as gel filtration. However, when the immutable particles are filtered, a layer-like object is formed on the filter.
1.2.4 Types of filtration membranes Depending on the type of membranes, the pore sizes and structures of different membranes are different. Some membranes are rigid and some membranes are mobile. The nominal pore size of the pre-filtration membrane does not have a uniform national standard. Different manufacturers have their own definitions and methods, so it is necessary to pay attention to the selection and replacement of the business. It is also a 0.22μm pre-filtration membrane, which is filtered by different manufacturers. The effect will vary greatly. The common pore size of the sterilization filter is defined by regulations, and each merchant performs the same standard, which is relatively simple when selecting and replacing.
1.2.5 Filter material According to the material of the filter, the filter material is divided into two types: hydrophilic (water infiltable) and hydrophobic (water infiltable). Hydrophilic filters are mainly used for filtration and sterilization filtration of water or water/organic solution mixtures, such as cellulosic materials (regenerated cellulose, mixed cellulose ester), PVPP polycarbonate, PVDF modified polyvinylidene fluoride The hydrophobic filter is intercepted or "guided" into the filter through water, mainly used in solvent, acid, alkali and chemical filtration, tank / equipment respirator, process gas, fermentation intake / exhaust filtration, such as PTFE polytetrafluoroethylene, PVDF polyvinylidene fluoride, polypropylene, polysulfone, polycarbonate, and the like.

2, filter characteristics and structure Filters are generally divided into deep filter (primary filter), surface filter (intermediate filter) and membrane filter (depth filter) three kinds. The fibers of the deep filter are detached and cannot give an exact pore diameter. The thickness is generally 3 to 20 mm. It usually has adsorption and has a large soil-containing capacity. The fibers of the surface filter are generally thermally bonded or coated. Coated, can give the rated pore size, relatively thin (<1mm), less adsorption capacity; membrane filter is characterized by hard texture, not easy to break, with tortuous channels and very high internal surface area, there is a certain Open cell rate, for integrity testing, often used for deep filtration, such as sterile filters.