Ultrafiltration Membrane Buying Tips

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Ultra filtration process separates macromolecules and suspended materials are separated from wastewater by using membrane and pressure difference. This process is suitable for both dilute solutions and large polymerized macromolecules.

The performance of the system depends on ultrafiltration system hence their selection should be done keeping following considerations:

The cut-off rating of the membrane should be 0.2 to 0.3 times the value of the molecular weight targeted for retention. This type of selection will ensure robust process performance. Some other factors that should be kept before buying ultra filtration membranes are:

• Shape and nature of the targeted species
• pH and Ionic strength
• Membrane characteristic
• Tangential Flow velocity - gel layer and TMP
• Concentration vs. diafiltration
• Membrane use/life

Shape and nature of the targeted species
The retention process will depend on the types, shapes and nature of the targeted species. For example some of the proteins and molecules are spherical in shape and their retention will be affected by their relative alignment with respect to the pores in the membrane. Hence membrane should be selected on the basis of the impurities to be treated.

pH and Ionic strength 
The shape of the protein may change according to the pH of the solution. Hence the retention of the particular protein/membrane system may change accordingly. The protein and the membranes have different charges. This difference in charges can have a dramatic effect on retention.

Ultrafiltration Membrane characteristic
Ultrafiltration membrane pores are categorized by their nominal retention capability. membranes capability to retain approximately 80 - 90% of species at the rated molecular weight is the basis on which the users and the manufacturers provide a rating to the ultra filtration membrane. Amembrane with 10 kD (kilo Dalton) rating simply means that the membrane can retain 80 - 90% of a 10.000 molecular weight protein from a "typical" solution.

A membrane having lower kD value will retain a higher percentage of the target species whereas membrane with higher kD value will retain a lesser amount. The membrane that is more open will provide higher filtrate rate (flux). However due to larger pores, the retention rates will be lowered. For food and beverage applications, ultrafiltration membrane should be more open whereas for BioPharmaceutical users normally use ultra filtration membrane with small pores.

Applicability Considerations
For various applications different types of ultra filtration membrane should be chosen. The available modules are plate-and-frame, spiral-wound, and tubular configurations. In order to obtain high quality pure water, spiral-wound and capillary configurations are generally used. Open configuration like plate-and-frame and tubular should be preferred for more concentrated solutions.

Membrane Materials
The materials of which membrane is made should be checked properly. The membrane module for high quality water solution should be compatible with chemicals like hydrogen peroxide used in sanitizing the membranes on a periodic basis. The most common materials used for making ultra filtration membrane are polysulfone and cellulose acetate.

Other Considerations
The membrane that is too wide open should not be selected. As it will result in operation at the steep angle (vertical part) of the retention curve. This finally will yield product that is of low quality and may also result in the variation of production yield. The membrane should also be chosen on the basis of pressure drop, flow velocity, power consumption, membrane fouling and module cost.