Introduction of Nanofiltration Membrane Technology
1. principle and characteristics
Membrane separation is the use of the difference in the selective osmosis performance of each component of the mixture by the membrane, using external energy or chemical potential as the driving force to separate, classify, purify and enrich the gas or liquid of the two-component or multi-component mixture. Set method. The membrane pore size is at the nanometer level, and the membrane process suitable for separating dissolved components with a molecular weight of 200 to 1000 and a molecular size of about 1 nm is called nanofiltration (NF). The transmembrane pressure difference required for NF membrane separation is generally 0.5 to 2.0 MPa, which is 0.5 to 3 MPa lower than the pressure difference required to achieve the same permeation energy with a reverse osmosis membrane. According to the operating pressure and separation limit, NF can be qualitatively ranked between reverse osmosis and ultrafiltration. Sometimes NF is also called “low pressure reverse osmosis” or “loose reverse osmosis”. In the 1970s, J. E. Cadotte studied NS-300 membrane, which was the beginning of the study of NF membrane. At that time, the Israeli desalination company used “Hybrid Filtration” to represent the membrane separation process between reverse osmosis and ultrafiltration. Later, Film*.Tech in the United States called this membrane technology nanofiltration. Still in use. After that, NF developed rapidly, and membrane modules were commercialized in the mid-1980s. At present, NF has become one of the hotspots in the world of membrane separation research.
NF separation is a green water treatment technology that can replace traditional wastewater treatment methods with high costs and complicated processes in some aspects. Its technical characteristics are: it can intercept organics and multivalent ions with a molecular weight greater than 100, allowing small molecular organics and monovalent ions to pass through; it can operate under harsh conditions such as high temperature, acid, and alkali, and is resistant to pollution; low operating pressure, membrane flux High, low equipment operating costs; can be combined with other sewage treatment processes to further reduce costs and improve treatment effects. In water treatment, NF membrane is mainly used for the treatment of solvent-containing wastewater, which can effectively remove the chroma, hardness and odor in the water. With its special separation performance, NF membranes have been successfully applied to wastewater treatment in industries such as sugar, pulp and paper, electroplating, mechanical processing, and the recovery of chemical reaction catalysts.
2. Experimental research and application
(1) Daily chemical wastewater treatment.
The application research on the treatment of daily chemical wastewater with NF membrane shows that NF membrane is acid and alkali resistant, has excellent rejection rate, has a good removal rate of heavy metals, and does not have membrane pollution problems. It is estimated that because the operating cost of NF membrane is lower than that of reverse osmosis technology, it has a good removal rate of organic small molecules and may cover more than 90% of daily chemical wastewater treatment.
(2) Petroleum industry wastewater treatment.
Petroleum industry wastewater mainly includes wastewater containing various inorganic salts and organic substances produced in the process of petroleum exploration and refining. Its composition is very complex and it is difficult to treat. The membrane method, especially the NF method, is combined with other methods, which can effectively treat wastewater and recover useful substances. For example, the NF membrane is used to separate crude oil wastewater into an oil-rich water phase and an oil-free brine phase, and then the oil-rich phase is added to the fresh water supply before entering the oil washing process, which not only recovers crude oil but saves water. In the past, a combination of reverse osmosis and phase separation was used to treat wastewater from the petroleum industry, but there was a serious problem of membrane pollution. If an NF membrane is added before reverse osmosis, the problem of membrane pollution can be solved. Phenol-containing wastewater in the petroleum industry mainly contains phenol, methyl phenol, nitrophenol and various substituted phenols. These substances are very toxic and must be removed before they can be discharged. If NF technology is used, not only the removal rate of phenol is It can reach more than 95%, and it can efficiently remove high-valent ions such as cadmium, nickel, mercury, titanium and other heavy metals in wastewater under lower pressure, and its cost is much lower than that of reverse osmosis.
(3) Pesticide wastewater treatment.
General water treatment methods cannot remove low-molecular organic pesticides in polluted water. By studying the retention performance of NF membranes for non-phenolic pesticides, it is found that the rejection rate of other pesticides is higher than 96.7% except for dichloride, and the adsorption capacity of all pesticides on the NF membrane is affected by its hydrophobicity. The impact of sex. The use of NF to treat wastewater containing phenolic pesticides is also very effective.
(4) Chemical fiber, printing and dyeing industrial wastewater treatment.
NF can be used for the removal and reuse of dyes and auxiliaries in the drainage of the printing and dyeing process. When processing dye polymerization slurry, since the molecular weight of most dyes is several hundred to several thousand, the NF membrane can let some inorganic salts or small molecules pass through, and intercept the larger dye molecules. After the crude dye slurry passes through the NF system, Dye can be enriched, while the concentration of inorganic salt decreases, the salt rejection rate is more than 98%, the dye loss rate is less than 0.1%, and it can be operated at high temperature. In addition, NF can also be used for the treatment and recycling of oily wastewater in the fiber processing process.
(5) Domestic sewage treatment.
When the common method of combining biodegradation and chemical oxidation is used to treat domestic sewage, the consumption of oxidizing agent is large and the residue is large. If an NF system is added between them to allow small molecules that can be degraded by microorganisms (molecular weight less than 100) to pass, the non-biodegradable organic macromolecules (molecular weight greater than 100) will be intercepted and be chemically oxidized and then biodegraded. It can fully play the role of biodegradation, save the amount of oxidant or activated carbon, and reduce the final residue content.
(6) Treatment and recycling of secondary wastewater from thermal power plants.
The secondary wastewater from thermal power plants mainly comes from ash washing, dust removal and cooling systems. This type of wastewater contains a large amount of suspended solids, ash, high salt and some organic matter. Use NF to treat this type of wastewater into industrial reuse water. First, use microfiltration to remove all suspended particles in the water, with a mass fraction of 99% BOD, 98% COD, 73% total nitrogen and 17% total phosphorus, while reducing the total number of colonies in the water to 3 to 4/L , Then add acid to lower the pH to remove CO2, and finally desalinate by NF to reach the quality of boiler water. The Eraring Power Station of the Pacific Thermal Power Plant in Australia has currently used NF to treat this type of wastewater, treating 1,000 to 15 000 m3 of wastewater per day, which not only reduces the load on the municipal water supply system, but also saves the thermal power plant operating costs by 800,000 US dollars per year. The thermal power plant is preparing to expand the scale of power generation, and the water consumption is also correspondingly increased. It is estimated that by 2010, the amount of waste water treated will reach 5,000 m3/d, which is extremely beneficial.
(7) Treatment of pickling waste liquid.
The pickling process of the steel plant is to immerse the steel in a sulfuric acid pickling tank with a mass fraction of about 20% for pickling. As the pickling progresses, the concentration of sulfuric acid gradually decreases, and the concentration of ferrous sulfate continues to increase. When the mass fraction of sulfuric acid in the solution drops to 6%-8% and the concentration of ferrous sulfate generated exceeds 200-250 g/L, the acid When the washing rate drops, the pickling liquid must be replaced and the waste pickling liquid must be discharged. The pickled steel must be rinsed with clean water to remove acidic substances on the surface, which in turn causes the discharge of waste acid water. In order to protect the environment and save resources, the NF process can be used to treat the pickling waste liquid. Utilizing the difference in the rejection rate of sulfuric acid and ferrous sulfate by NF membrane, ferrous sulfate is first trapped in the concentrated liquid, and then the concentrated liquid is sent to the cooling crystallization tank, and FeSO4·7H2O is cooled and crystallized; the permeate can then intercept sulfuric acid The other NF membrane module of NF, after interception, is concentrated into 20% sulfuric acid, the regenerated acid is recycled and the permeate is discharged to the waste acid water station for further treatment, discharge or recovery. This process recycles sulfuric acid and ferrous sulfate, and at the same time realizes the purpose of recycling and comprehensive utilization of pickling waste liquid and discharge of waste acid water.
(8) Papermaking wastewater treatment.
Using NF membrane technology to replace traditional chemical treatments can more effectively remove dark lignin. The chlorinated lignin produced during the bleaching process of wood pulp is negatively charged and is easily intercepted by the negatively charged NF membrane, and will not pollute the membrane. In addition, because there is no strict requirement on the removal rate of cations (Na+) during the whole treatment process, the use of reverse osmosis technology appears unnecessary. Using ultrafiltration/nanofiltration to treat kraft paper manufacturing wastewater has a good effect.