The material world is made up of microscopic units such as atoms, molecules, and cells. However, these very small material units always live together in symbiosis. The second law of thermodynamics reveals that microscopic particles tend to be mixed in disorder. The foundation of membrane separation technology is separation membrane. Separation Mo is a membrane with selective permeability, some molecules (or particles) can pass through the membrane, while others are blocked. This separation always relies on a certain difference between different molecules (or particles). The simplest difference is the size. In the three-dimensional space, everything is large and small, and the membrane has pores.
Full-volume filtration is also called direct-flow filtration, dead-end filtration, and is similar to conventional filter cloth filtration. The processed material enters the mold assembly, and the same amount of permeate flows out of the mold assembly, and the interception remains in the mold assembly. In order to ensure the recoverability of membrane performance, the retentate must be unloaded from the mold assembly in time. Therefore, regular backwashing (reverse process of filtration) and other measures are required to remove deposits on the membrane surface and restore membrane flux. After the mold components are contaminated, they cannot be disassembled and cleaned. The in-line cleaning method (CIP) is usually used for ultrafiltration/microfiltration. The water treatment process generally adopts the full filtration mode.
The treated material liquid flows through the membrane surface at an agreed speed, the permeate passes through the membrane in a vertical direction, and most of the retentate is carried out of the mold assembly by the concentrated liquid. The cross-flow filtration mode reduces the thickness of the concentration polarization layer on the membrane surface, which can effectively reduce membrane pollution. Both reverse osmosis and nanofiltration adopt cross-flow filtration methods.
Membrane system refers to the membrane separation device unit. The pressure-driven membrane system is mainly composed of pretreatment system, booster pump, mold assembly (pressure vessel and membrane element), pipeline valve and control system.
All kinds of raw water contain a certain concentration of suspended solids and dissolved substances. Suspended matter is mainly composed of inorganic particles, colloids, microorganisms, algae and other biological particles. The soluble substances are mainly soluble salts (such as chloride) and insoluble salts (such as carbonate, sulfate and silicate). In the reverse osmosis process, the volume of influent water is decreasing, and the concentration of suspended matter and dissolved substances is increasing. Suspended particles will be deposited on the membrane, blocking the water inlet channel and increasing frictional resistance (pressure drop). Insoluble salts will precipitate out of the concentrated water and form scale on the grinding surface, reducing the flux of the RO membrane. This phenomenon of forming a deposited layer on the membrane surface is called membrane fouling, and membrane fouling is the degradation of the performance of the membrane system.