4, nanofiltration membrane technology
4.1 nanofiltration (NF) principle
Nanofiltration (NF) is a new kind of molecular membrane separation technology, which is one of the hotspots in the field of membrane separation. NF membrane pore size of 1nm or more, usually 1-2nm; solute retention between RO and UF membrane; RO membrane almost all of the solute has a high removal rate, but the NF membrane only for specific Solute has a high removal rate. NF membrane can remove divalent, trivalent ions, Mn ≥ 200 of organic matter, as well as microorganisms, colloids, heat, viruses and so on. One of the great features of the nanofiltration membrane is that the membrane bulk carries a charge, which is important at very low pressure (only 0.5 MPa) still has a high desalination performance and a molecular weight of hundreds of membranes that can also remove inorganic salts The reason is that NF is the main reason for lower operating costs. NF for a variety of salt water, water utilization is generally 75% to 85%, desalination in 30% to 50%, no acid and alkali wastewater discharge.
4.2 Application of nanofiltration membrane in water treatment
4.2.1 Application of nanofiltration membrane in drinking water
Nanofiltration operation pressure is small, is the preparation of drinking water and deep purification of the preferred process.
At present, most of the city's water supply are subject to varying degrees of pollution, and the water treatment plant conventional treatment process of organic matter removal rate is not high, when the use of chlorine disinfection disinfection, chlorine and organic compounds in water will produce halogenated by-products The Peltier and other four years of follow-up study showed that: the use of nanofiltration system after the DOC in the water decreased to an average of 0.7mgC / L, the residual chlorine content from 0.35mg / L down to 0.1mg / L, the final network of trihalomethanes (THMs ) Was reduced by 50% compared to the absence of a nanofiltration system. In addition, the biodegradability of water production is improved due to the reduction of biodegradable dissolved organic carbon (BCOD).
Nanofiltration technology can remove most of the Ca, Mg plasma, so desalination (desalination) is the application of nanofiltration technology most areas. Membrane water treatment technology in the investment, operation and maintenance and price and other aspects of conventional lime softening and ion exchange process is similar, but with no sludge, no regeneration, complete removal of suspended solids and organic matter, easy to operate and the province Advantages, application examples more. Nanofiltration can be directly used for softening of groundwater, surface water and wastewater, and can also be used as pretreatment for reverse osmosis (RO), solar photovoltaic desalination (Photovoltaicpowereddesalinationsystem).
4.2.2 Application of nanofiltration membrane in seawater desalination
Seawater desalination refers to the salt content of 35000mg / L desalination desalination to 500mg / L below the drinking water.
4.2.3 Application of nanofiltration membrane in wastewater treatment
(1) Domestic sewage
Domestic sewage is generally treated with biodegradation / chemical oxidation, but the amount of oxidant used too much, more residue. Xue Gang and other micro-flocculation fiber ball filtration. Ultrafiltration. Nano filter combination of the hotel bathing wastewater for a small test. Ultrafiltration effluent quality can be used to reuse to the hotel toilet flushing, green and other aspects of water requirements, satisfied that the effluent water quality can reach the drinking water health standards (GB5749.85), can be used to the hotel laundry, bath and other water requirements higher Link.
(2) textile, printing and dyeing wastewater
Haskani studied the effects of acidity, activity, direct and disperse dye aqueous solution concentration, pressure, total dissolved solids and inorganic salts on the retention performance of nanofiltration membranes.
(3) tannery wastewater
Tannery wastewater contains high concentrations of organic matter, sulfate and chloride, pickling process waste liquid conductivity up to 75mS / cm. Bes-Pia using NF technology to recover the tannery wastewater, the resulting high concentration of sulphate concentrated water back to the pickling section, and chloride water to crack the cracking reaction drum.
(4) electroplating wastewater
Electroplating plants tend to produce large amounts of waste, despite the acidification, chemical harmless, sedimentation and separation of sludge and other complex processing steps, high salinity, can not be reused.
(5) papermaking wastewater
In the pulp and paper industry, homogenization, bleaching and paper making processes require a lot of water. Achieving the (semi) closed cycle of the water system is the best way for pulp mills and paper mills to conserve water to reduce emissions. The traditional activated sludge process also contains some of the colored compounds, microorganisms, antibodies and a small amount of biodegradables, suspended solids, etc., can only be used to make wrapping paper, can not be used for higher levels of paper production. In addition, the method can not reduce the inorganic salt content. Koyuncu compares the practicability of the two treatment processes of water → nanofiltration and papermaking wastewater → activated sludge → nanofiltration. The experiment shows that the effluent quality of the two methods is similar, the second method produces better water flux, For high-grade paper. But satisfied that the filter water still contains a certain amount of monovalent salt, need to increase the low-pressure reverse osmosis device to remove salt in order to ensure the quality of recycled water.