Ultrafiltration (Ultra-filtration, UF) is a membrane separation technology that purifies and separates solutions. The ultrafiltration membrane system is a kind of solution separation device taking ultrafiltration membrane filaments as filter medium, and using the pressure difference on both sides of the membrane as the driving force. The ultrafiltration membrane only allows the solvent (such as water molecules), inorganic salts and small molecular organic substances in the solution to pass, while the macromolecular substances such as suspended matters, colloids, proteins and microorganisms in the solution will be intercepted, therefore the purpose of purification and separation can be achieved.
The filtration diameter and the intercepted molecular weight of ultrafiltration has not been clearly defined. In general, it is considered that the filtration diameter of the ultrafiltration membrane is 0.001 to 0.1μm and the molecular weight intercepted (Molecular weigh cut-off, MWCO, MWCO) is 1,000 to 1,000,000 Daltons. Strictly speaking, the filtration diameter of ultrafiltration membrane is 0.001 to 0.01μm and the molecular weight intercepted 1,000 to 300,000 Dalton. If the filtration diameter exceeds 0.01μm or the molecular weight intercepted over 300,000 Daltons, which should be defined as a microfiltration membrane or a precision filtration membrane.
Ultrafiltration membranes are widely used in water treatment projects. Ultrafiltration technology plays an increasingly important role in the fields of reverse osmosis pretreatment, drinking water treatment and reclaimed water reuse etc. The ultrafiltration plays a key role in the disinfection and turbidity removal in terms of the concentration process of alcohol and beverages, the depyrogenation of medicines and the pharmaceutical materials.
The types of ultrafiltration membrane can be divided into three types: plate, tube and spiral-wound. The second type is classified into hollow fiber, capillary and tube according to the different tube diameters. At present, the ultrafiltration membranes used in the water treatment market is almost capillary tube, and few works use the hollow fiber ultrafiltration membranes.
The assembly of ultrafiltration membrane filaments into a module that can be connected with the ultrafiltration system is referred to as an ultrafiltration membrane module. The hollow fiber ultrafiltration membrane modules are divided into three types: internal pressure, external pressure and submersion. And the driving force of the submersion type is the pressure difference between the vacuum and the atmospheric pressure inside the membrane tube. However, for ultrafiltration membranes with high filtration accuracy, such pressure difference generally cannot meet the filtration drive force required, therefore the submersion type membrane module is suitable for ultrafiltration membranes or microfiltration membranes with low filtration accuracy. When the ultrafiltration membrane of the external pressure type is forward-flushing and backflushing, the flow rate of the liquid on the surface of the membrane is extremely uneven, which affects the flushing effect on the surface of the membrane. Accordingly, in terms of the three membrane types, the ultrafiltration membrane used for water treatment adopts internal pressure type, which has better advantages.