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Types of ultrafiltration membranes and their application characteristics. Based on their structure, ultrafiltration membranes can be mainly divided into four types: plate membranes, spiral-wound membranes, tubular membranes, and hollow fiber membranes. Plate membrane: It was the first type of membrane to be developed, but due to the difficulty in maintaining an appropriate flow rate on its surface and the complex sealing issues associated with it, its application is very limited. Pre-treatment requirements are not strict ； Roll-type membranes: Developed from plate-type membranes; due to the grid structure of roll-type membranes, dead zones are created and backwashing is not possible, so they are generally not suitable for industrial raw water treatment. They are suitable for the separation of materials under high temperature and high pressure, and the pre-treatment requirements are not strict ； Tubular membrane: Due to its high energy consumption, it is not economically suitable for ordinary water treatment; it is generally used for fluids with high solid content or high oil concentration. Among the four types of membranes, it requires the least stringent pretreatment. Hollow fiber membranes: Due to their low pressure, absence of dead zones in the channels, high flux, and the possibility of backwashing, they are an excellent choice for most water types, except those with high oil content or high solid levels. Among the four types of membranes, they are the most widely used in water treatment. Note: Since hollow fiber membranes are the most widely used, in the subsequent information, except for the common features, everything else will be explained using hollow fiber membranes as an example. Application scope: Ultrafiltration is widely used in the field of water treatment. Classified by application scenarios, it can be mainly used for: 1.1. Pre-treatment of raw water (surface water, groundwater, tap water). It is employed in pre-treatment processes as a substitute for clarifiers or sand filters, and for pre-treatment before RO treatment and ion exchange. In these applications, ultrafiltration removes solids and colloids from the raw water, thereby improving the performance of equipment in subsequent stages; for example, it helps reduce the frequency of backwashing for ion exchangers and the need to replace reverse osmosis membrane elements. However, it requires frequent cleaning/rinsing. The membrane type generally has a molecular cut-off of 100,000. 1.2. Purification processes: removal of particles (such as 18WΩ water), elimination of microorganisms and pyrogens, followed by RO or ion exchange treatment. Ultrafiltration is used to remove colloids and solids from water; it allows for a high water permeability and requires less frequent cleaning. It does not need to be cleaned or rinsed often, except when the system pressure drops to an level that makes operation difficult or when bacteria start to form. In the pharmaceutical and electronics industries, ultrafiltration is used at the point of use to remove microorganisms and pyrogens. The membrane type generally has a molecular cutting capacity of 10,000 to 100,000 molecules. 1.3. Water circulation and reuse after biological treatment and clarification (secondary and tertiary)