Ultrafiltration Membrane Filtration Principle

Ultrafiltration is a screening process using membrane separation technology. The pressure difference on both sides of the membrane is used as the driving force, and the ultrafiltration membrane is used as the filter medium. Many tiny micropores densely covered on the surface only allow water and small molecular substances to pass through and become permeate, while substances in the original solution whose volume is larger than the micropore size of the membrane surface are trapped on the liquid inlet side of the membrane and become concentrated solution, thus realizing The purpose of purification, separation and concentration of stock solution.
There are about 6 billion micropores of 0.01 microns on the wall of each meter long ultrafiltration membrane, and the pore size only allows water molecules, beneficial minerals and trace elements in the water to pass through, and the smallest bacteria are more than 0.02 microns in volume. Therefore, bacteria and colloids, rust, suspended matter, sediment, macromolecular organic matter, etc., which are much larger than bacteria, can be intercepted by the ultrafiltration membrane, thereby realizing the purification process.

1. Water production process of ultrafiltration membrane

Tap water first enters the ultrafiltration membrane tube. Under the action of the water pressure difference, many 0.01-micron micropores densely distributed on the membrane surface only allow water molecules, beneficial minerals and trace elements to pass through, becoming purified water. The harmful substances such as bacteria, rust, colloid, sediment, suspended solids, macromolecular organic matter are trapped in the ultrafiltration membrane tube and discharged when the ultrafiltration membrane is rinsed.

2. Ultrafiltration membrane washing process

After the ultrafiltration membrane is used for a period of time, the trapped bacteria, rust, colloid, suspended matter, macromolecular organic matter and other harmful substances will adhere to the inner surface of the ultrafiltration membrane, which will gradually reduce the water production of the ultrafiltration membrane, especially tap water. When the quality pollution is serious, it is more likely to cause the blockage of the ultrafiltration membrane. Regular flushing of the ultrafiltration membrane can effectively restore the water production of the membrane.

3. Ultrafiltration membrane filter element

The bundled ultrafiltration membrane wire is made into the ultrafiltration filter element as shown in the following figure after the casting process. The epoxy head fills the gap between the membrane filament and the membrane filament to form the isolation between the original solution and the permeate. The original solution first enters the pores of the ultrafiltration membrane, and becomes the permeate after being filtered by the ultrafiltration membrane, preventing the original solution. directly into the permeate without filtration.

4. Calculation of the total area of ​​the ultrafiltration membrane filter element membrane wire:

Under the condition that the water production per unit membrane filament area is constant, the larger the membrane area of ​​the filter element is, the more the total water production of the filter element will be.
Its calculation formula is:
Inside S=πdL×n
Outside S=πDL×n
Among them: S is the total inner surface area of ​​the membrane filament, d is the inner diameter of the ultrafiltration membrane filament;
S is the total outer surface area of ​​the membrane filament, D is the outer diameter of the ultrafiltration membrane filament;
L is the length of the ultrafiltration membrane;
n is the number of ultrafiltration membrane filaments.

5. Factors affecting the water yield of ultrafiltration membranes

  1. The influence of temperature on water production: the increase of temperature increases the activity of water molecules, and the viscosity decreases, so the water production increases. On the contrary, the water production is reduced, so the difference in water production between winter and summer is very large even for the same ultrafiltration system.
  2. The influence of operating pressure on water production: in the low pressure section, the water production of the ultrafiltration membrane is proportional to the pressure, that is, the water production increases with the increase of the pressure, but when the pressure value exceeds 0.3MPa, even if the pressure increases The increase of water production is also very small, mainly because the ultrafiltration membrane is compressed under high pressure and the water permeability resistance is increased.
  3. Influence of influent turbidity on water production: the greater the influent turbidity, the less water produced by the ultrafiltration membrane, and the greater the influent turbidity is more likely to cause blockage of the ultrafiltration membrane.
  4. The influence of flow rate on water production: the influence of flow rate on water production is not as obvious as that of temperature and pressure. Too slow flow rate will easily lead to blockage of ultrafiltration membrane, and too fast will affect water production.

6. Performance characterization of ultrafiltration membranes

The performance of ultrafiltration membrane usually refers to the physical and chemical properties and separation and permeation performance of the membrane. The physical and chemical properties mainly include the mechanical strength of the membrane, chemical resistance, heat-resistant temperature range and applicable PH value range, etc. The separation and permeation performance mainly refers to the membrane. Water flux and cut molecular weight and rejection.

7. Ultrafiltration membrane materials and characteristics

Main materials: polyacrylonitrile (PAN), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyethersulfone (PES), etc.

PAN film:

  • With excellent chemical stability, acid resistance, alkali resistance and hydrolysis resistance, it can be widely used in various fields;
  • Membrane filament has good strength and flexibility;
  • After hydrophilic modification, the water yield is large, and it has strong anti-pollution properties.
  • There are few formula materials for the film filament, and the process is easy to control, and there will be no odor problem of the film itself due to the large amount of PVC raw material formula materials.

PVDF membrane:

  • UV resistance, excellent resistance to pollution and chemical erosion;
  • The heat-resistant temperature can reach 140℃, and ultra-high temperature steam and ethylene oxide can be used for sterilization;
  • Can be used in a wide range of PH (1-13), can be used in strong acid and alkali and various organic solvents.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

AQU200 UF filter membranes

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