Ultrafiltration
Ultrafiltration is one of the important means of water treatment. Ultrafiltration is one of the membrane separation technologies driven by pressure. For the purpose of separating large and small molecules, the membrane pore size is between 20-1000A °. Hollow fiber ultrafiltration (membrane) has the advantages of high filling density per unit container and small footprint.
Basic principles of ultrafiltration
The separation mechanism of ultrafiltration is mainly the “mechanical screening” mechanism, which means that raw water containing “impurities” flows through the surface of the ultrafiltration membrane under the driving force of pressure. Substances larger than the pore size of the membrane in the raw water are intercepted, while substances smaller than the pore size of the membrane pass through the membrane, achieving the purpose of separation.
The microporous pore size range on the surface of the ultrafiltration membrane is approximately 0.001~0.1 μ Between m, the cutoff molecular weight (MWCO) is approximately 1000 to 500000 Daltons. Ultrafiltration membranes have a high removal rate for suspended solids, colloids, bacteria, and macromolecular substances in water. They have a partial removal rate for BOD, COD, and small molecule substances in water, while they can hardly intercept small molecule substances such as water and soluble solids.
Characteristics of ultrafiltration
- The separation process does not undergo phase changes and consumes less energy.
- The separation process can be carried out at room temperature, suitable for the concentration or purification of some heat sensitive substances such as juices, biological preparations, and certain drugs.
- The separation process has low operating pressure, simple equipment and process flow, and is easy to operate, manage, and maintain.
- Wide application range, with solute forces ranging from 1000 to 500000 Daltons or solute sizes ranging from 0.001 to 0.1 μ Around m, ultrafiltration separation technology can be used. In addition, the use of serialized ultrafiltration membranes with different molecular weights can classify the molecular weight of each component in a mixture of solutes with different molecular weights.
Types of ultrafiltration
Internal pressure filtration and external pressure filtration
- Internal pressure filtration
The raw water enters from the inside of the hollow fiber membrane filament and, driven by pressure, penetrates through the membrane filament radially from the inside out to become the permeate. This filtration method is called internal pressure filtration. - External pressure filtration
The raw water enters from the outside of the hollow fiber membrane filament and, driven by pressure, penetrates through the membrane filament radially from the outside to the inside to become the permeate. This filtration method is called external pressure filtration.
Full filtration and cross flow filtration
- Full filtration
Full filtration, also known as dead end filtration, refers to the process where the raw water enters the membrane module and flows out of the membrane module in an equal amount through the liquid, leaving the retained material inside the membrane module. When the quality of raw water is good (usually referring to its turbidity ≤ 10NTU), full filtration is usually used. - Cross flow filtration
Cross flow filtration refers to the process in which a portion of the raw water enters the membrane component and vertically penetrates through the membrane to form a filtrate that flows out of the membrane component. All intercepted substances are accompanied by the remaining portion of the raw water to form a concentrated solution that flows out of the membrane component. When the raw water quality is poor, cross flow filtration is often used.