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Application of various membrane technologies in water treatment(2)

Author: Site Editor     Publish Time: 2022-10-14      Origin: Site


3. Microfiltration and ultrafiltration membrane technology

Basic Principles

Both ultrafiltration and microfiltration are liquid phase separation processes driven by differential hydrostatic pressure, and there is no fundamental difference in principle. Under a certain pressure, when a mixed solution containing high molecular solutes and low molecular solutes flows across the membrane surface, the solvent and low molecular solutes (e.g. inorganic salts) smaller than the membrane pore pass through the membrane and become permeate to be collected; high molecular solutes (e.g. organic colloids) larger than the membrane pore are retained by the membrane and recovered as concentrated solution. The membrane separation process that can retain molecules above 500 molecular weight and below 106 molecules is called ultrafiltration; the membrane separation process that can only retain larger molecules (often referred to as dispersed particles) is called microfiltration.

Applications of ultrafiltration and microfiltration membranes

Ultrafiltration and microfiltration technologies can effectively remove particulate matter, including microorganisms such as cryptococci, giardia, bacteria and viruses. Disinfection by-products can also be reduced by somewhat lowering the concentration of disinfection by-product precursors and limiting the amount of oxidant required during disinfection. However, the removal of organic matter from water is very low, only below 20%. Ultrafiltration and microfiltration are used in a wide range of applications and can be applied to treat different water qualities.

water treatment

4. Nanofiltration membrane technology

Nanofiltration (NF) principle

Nanofiltration (NF) is a new molecular membrane separation technology, is currently one of the world's membrane separation field research hotspots. NF membrane pore size in more than 1nm, generally in 1-2nm; solute retention performance between RO and UF membrane; RO membrane for almost all solutes have a high rate of removal, but NF membrane only for specific solutes have a high rate of removal. NF membrane can remove divalent, trivalent ions , organic matter with Mn ≥ 200, as well as microorganisms, colloids, pyrogens, viruses, etc. A great feature of NF membranes is that the membrane body is electrically charged, which is an important reason for its high desalination performance at very low pressures (only 0.5 MPa) and the ability to remove inorganic salts even from membranes with a MW of several hundred, as well as the low operating costs of NF.

Nanofiltration membranes in drinking water applications

Nanofiltration operates under low pressure and is the preferred process for the preparation and deep purification of drinking water.

Currently, most urban water sources are contaminated to varying degrees, and conventional treatment processes in waterworks do not remove much organic matter from the water, and when chlorine is used for disinfection, chlorine and organic matter in the water can generate halogenated by-products.

Nanofiltration technology is able to remove most of the Ca and Mg plasma, so desalination is the area where nanofiltration technology is most used. Membrane water treatment technology is similar to conventional lime softening and ion exchange processes in terms of investment, operation and maintenance and price, but has the advantages of no sludge, no regeneration, complete removal of suspended matter and organic matter, ease of operation and space saving, so there are many examples of application. Nanofiltration can be used directly for the softening of groundwater, surface water and wastewater, and as a pre-treatment for Reverse Osmosis (RO), Photovoltaic powereddesalinationsystem, etc.

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