Principle of Nanofiltration (NF)
Nanofiltration (NF) is a new molecular-scale membrane separation technology, which is one of the hotspots in the field of membrane separation in the world. The NF membrane pore size is above 1nm, generally 1-2nm; the interception performance of solute is between RO and UF membrane; RO membrane has the high removal rate for almost all solutes, but NF membrane has the high removal rate only for specific solutes. The NF membrane is capable of removing divalent and trivalent ions, organic substances with Mn≥200, and microorganisms, colloids, heat sources, viruses, and the like. A very special feature of the nanofiltration membrane is that the membrane body carries a charge, which is an important reason that it can removal inorganic salts with the higher salt rejection performance and molecular weight cut-off of hundreds of molecules at very low pressure (Only 0.5MPa). which is also the reason for the low cost of NF operation. NF is suitable for various brine sources. The water utilization rate is generally 75%~85%, and the seawater desalination is 30%~50%. There is no acid-base wastewater discharge.
Application of NF Membrane in Water Treatment
● NF Membrane Applied in Drinking Water
Because of the low operation pressure, NF is a priority process for drinking water preparation and deep purification.
At present, the water supply sources of most cities are polluted to varying degrees, and the conventional treatment process of the tap water plant does not have a high removal rate of organic matters in the water. When chlorine is used for disinfection, chlorine will form halogenated by-products with organic matters in the water . A four-year follow-up study by Peltier showed that: the DOC in the water was reduced to an average of 0.7 mg C/L after using the nanofiltration system, and the residual chlorine content of the effluent was reduced from 0.35 mg/L to 0.1 mg/L. The formation of trihalomethanes (THMs) in the final networks was reduced by 50% compared to the absence of a nanofiltration system. In addition, the biostability of the product water is improved due to the reduction of biodegradable dissolved organic carbon (BCOD).
Nanofiltration technology can remove most of the Ca and Mg ions, so desalination is the most widely used field of nanofiltration technology. Membrane water treatment technology is similar to conventional lime softening and ion exchange processes in terms of investment, operation, maintenance, and price, while no sludge, no regeneration, complete removal of suspended solids and organic matters, easy operation and less land occupation, as well as many application cases. Nanofiltration can be directly used for the softening of groundwater, surface water and wastewater, and can also be used as a pretreatment for reverse osmosis (RO), solar photovoltaic desalination system (Photovoltaic powered desalination system).
● NF Membrane Applied in Seawater Desalination
Seawater desalination refers to desalting seawater with the salinity of 35000mg/L to drinking water less than 500mg/L.
● NF Membrane Applied in Wastewater Treatment
(1) Domestic Sewage
Domestic sewage is generally treated by biodegradation/chemical oxidation, but the amount of oxidant is too large and there are many residues. Xue Gang and others used micro-flocculation fiber ball to filter. Ultrafiltration and nanofiltration combined process are used for a small test on the hotel's bathing wastewater. The water quality of ultrafiltration effluent can be reused to the hotel toilet flushing and greening etc. The water quality of the nanofiltration effluent can reach the sanitary standards of drinking water, which can be reused to the hotel for washing, bathing, and other higher water-use requirements.
(2) Textile and Dyeing Wastewater
The dyes contained in textile wastewater are difficult to remove biologically. Hassani studied the effects of acidity, activity, concentration of direct and disperse dye solutions, pressure, total dissolved solids and inorganic salts on the interception performance of nanofiltration membranes.
(3) Tannery Wastewater
The tannery wastewater contains a high concentration of organic matters, sulfates and chlorides, and the waste liquid conductivity of the pickling process reaches 75 mS/cm. Bes-Pia used NF technology to recover the tannery wastewater, and the high concentration of sulfate concentrate water obtained is returned to the pickling section, and the product water of the chlorides is returned to the soaking drum.
(4) Electroplating Wastewater
Electroplating plants often produce large amounts of waste liquid. Despite the complicated processing steps such as acidification, chemical decontamination, sedimentation and sludge separation, the salt content of the product water is still high and it cannot be reused.
(5) Papermaking Wastewater
In the pulp and paper industry, processes such as homogenization, bleaching, and papermaking require large amounts of water. Achieving a (semi-) closed cycle of the water system is the best way for pulp and paper mills to save water and reduce emissions. The product water of the traditional activated sludge process contains some colored compounds, microorganisms, antibodies and a small amount of biodegradable matters, suspended solids, etc., which can only be used to make packaging paper and cannot be used for the production of higher-grade paper. In addition, this method does not reduce the content of inorganic salts. Koyuncu compared the practicality of two processing technologies that water→nanofiltration and papermaking wastewater→activated sludge→nanofiltration. The experiment showed that the effluent quality of the two methods is similar, but the second method has better water yield, and the effluent can be used for high grade paper production. However, the nanofiltration product water still contains a certain amount of monovalent salts, and it is necessary to increase the low-pressure reverse osmosis device to remove the salts to ensure the quality of the circulating water.
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