Removal of three causing substances: Studies have shown that nanofiltration membranes can remove most of the toxic and harmful organic matter and Ames mutagen in the water, so that the mutagenic ratio MR value of TA98 and TA100 strains at each test dose is less than 2, Ames test The result was negative. Further research will investigate the retention characteristics of endocrine disrupting substances in drinking water by nanofiltration technology and provide a basis for safe and high-quality drinking water.
The development and application of nanofiltration (NF) membranes is about 20 years later than reverse osmosis membranes. 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 mean a membrane separation process between reverse osmosis and ultrafiltration, which was called loose RO membrane.
Later, Filmtec in the United States referred to this membrane technology as nanofiltration, and it has been used today. After that, nanofiltration technology developed rapidly, and membrane modules were commercialized in the mid-1980s. At present, nanofiltration technology has become one of the research hotspots in the field of membrane separation in the world.
(1) Definition of nanofiltration membranes So far, the understanding of the precise definition, mechanism and characteristics of nanofiltration membranes is far from sufficient. The more uniform interpretation of the definition of nanofiltration membrane in the academic circles includes the following seven aspects:
① Nanofiltration membrane is between reverse osmosis and ultrafiltration membrane, and the separation skin on the membrane surface may have a nano-scale microporous structure.
② Compared with the reverse osmosis membrane, the NaCI removal rate is above 95%. Generally, the membrane with the NaCI removal rate below 90% can be called a nanofiltration membrane.
③ The reverse osmosis membrane has a high removal rate for almost all solutes, while the nanofiltration membrane only has a removal rate for specific solutes.
④ The pore size of the nanofiltration membrane is above 1nm, generally 1~2nm.
⑤ It mainly removes lysosomes about one nanometer, and the molecular weight cut-off is 200-1000 Daltons.
⑥ The reverse osmosis membrane is almost all made of polyamide, and the nanofiltration membrane material can be made of various materials, such as cellulose acetate, cellulose acetate-triacetate, sulfonated polysulfone, sulfonated polyethersulfone, and aromatic polyamide composite materials And inorganic materials.
⑦ Generally, a polymer electrolyte is formed on the surface of the nanofiltration membrane, which often has a strong negative charge.
(2) Nanofiltration principle Like membrane separation processes such as ultrafiltration and reverse osmosis, nanofiltration is also a membrane separation process driven by pressure difference, which is an irreversible process.
The separation mechanism can be described using charge models (space charge model and fixed charge model), pore model, and electrostatic repulsion and three-dimensional obstruction models that have only been proposed in recent years.
Compared with other membrane separation processes, one advantage of nanofiltration is that it can retain small molecular weight organics that pass through the ultrafiltration membrane, and it can also dialyze some of the inorganic salts retained by the reverse osmosis membrane—that is, it can synchronize "concentration" and desalination. get on.
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 that must be applied to achieve the same permeation energy with a reverse osmosis membrane. Under the same applied pressure, the flux of nanofiltration is much larger than that of reverse osmosis, and when the flux is constant, the pressure required for nanofiltration is much lower than that of reverse osmosis.
Therefore, when nanofiltration is used instead of reverse osmosis, the "concentration" process can be carried out more efficiently and quickly, and a larger "concentration" multiple can be achieved. Generally speaking, in the membrane separation process using nanofiltration membranes, the rejection rate of various solutes in the solution has the following rules:
① Increase with the increase of molar mass;
②In the case of a given feed concentration, it increases with the increase of the transmembrane pressure difference;
③In the case of a given pressure, it decreases as the concentration increases;
④ For anions, it rises in the order of NO3-, CI-, OH-, SO42-, CO42-.
⑤ For cations, it rises in the order of H+, Na+, K+, Ca2+, Mg2+, Cu2+.
(3) Application of nanofiltration membrane These properties of nanofiltration membrane determine its unique and broad application in drinking water treatment, which is briefly described as follows.
① Softening: Membrane softening water mainly uses the selective permeability characteristics of the nanofiltration membrane to different valence ions to realize the softening of water. The film softening can remove the turbidity, chroma and organic matter while removing the hardness, and the effluent quality is significantly better than other softening processes. Moreover, membrane softening has the advantages of no need for regeneration, no pollution, simple operation, and less floor space, and has obvious social and economic benefits. Membrane softening is very common in the United States. In the past 10 years, new water softening plants in Florida have adopted membrane softening instead of conventional lime softening and ion exchange processes. In recent years, with the continuous improvement of nanofiltration performance, the price of nanofiltration membrane modules has been declining, and the membrane softening method has been superior or close to conventional methods in terms of investment, operation, and maintenance.
② Used to remove organics in water: In addition to softening in drinking water treatment, nanofiltration membranes are mostly used for decolorization, removal of natural and synthetic organics (such as pesticides, etc.), tertiary substances, and disinfection by-products (trihalomethanes and haloacetic acids) And its precursors and volatile organic compounds to ensure the biological stability of drinking water.
Removal of three causing substances: Studies have shown that nanofiltration membranes can remove most of the toxic and harmful organic matter and Ames mutagen in the water, so that the mutagenic ratio MR value of TA98 and TA100 strains at each test dose is less than 2, Ames test The result was negative. Further research will investigate the retention characteristics of endocrine disrupting substances in drinking water by nanofiltration technology and provide a basis for safe and high-quality drinking water.
Removal of disinfection by-products and their precursors: disinfection by-products mainly include trihalomethanes (THMs), haloacetic acids (HAAs) and possibly trichloroacetaldehyde hydroxide (CH). Foreign scientific and technological workers have carried out extensive research in this area, and the average rejection rates of the three kinds of disinfection by-product precursors by nanofiltration membranes are 97%, 94% and 86% respectively. Through the selection of suitable nanofiltration membranes, drinking water quality can meet higher standards for safe and high-quality drinking water.
In addition, nanofiltration water is low corrosive, which has a positive impact on the service life of the drinking water pipe network and the dissolution of metal ions in the pipe, which is beneficial to protect all materials of the water distribution system. Tests show that the use of the necessary post-treatment nanofiltration membrane system can reduce the dissolution of lead in the pipe network by 50%, and at the same time make the concentration of other dissolved metal ions meet the requirements of drinking water quality standards.
Removal of volatile organic compounds (VOC): It has a high removal rate for trace volatile organic compounds in drinking water.
Application in direct drinking water in pipelines; Nanofiltration can intercept ions with more than two valences and other particles, and only water molecules and some monovalent ions (such as sodium, potassium, and chloride ions) are passed through. Nanofiltration can be used to produce direct drinking water, and certain ions are still retained in the effluent, and treatment costs can be reduced.
The development and application of nanofiltration (NF) membranes is about 20 years later than reverse osmosis membranes. 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 mean a membrane separation process between reverse osmosis and ultrafiltration, which was called loose RO membrane.
Later, Filmtec in the United States referred to this membrane technology as nanofiltration, and it has been used today. After that, nanofiltration technology developed rapidly, and membrane modules were commercialized in the mid-1980s. At present, nanofiltration technology has become one of the research hotspots in the field of membrane separation in the world.
(1) Definition of nanofiltration membranes So far, the understanding of the precise definition, mechanism and characteristics of nanofiltration membranes is far from sufficient. The more uniform interpretation of the definition of nanofiltration membrane in the academic circles includes the following seven aspects:
① Nanofiltration membrane is between reverse osmosis and ultrafiltration membrane, and the separation skin on the membrane surface may have a nano-scale microporous structure.
② Compared with the reverse osmosis membrane, the NaCI removal rate is above 95%. Generally, the membrane with the NaCI removal rate below 90% can be called a nanofiltration membrane.
③ The reverse osmosis membrane has a high removal rate for almost all solutes, while the nanofiltration membrane only has a removal rate for specific solutes.
④ The pore size of the nanofiltration membrane is above 1nm, generally 1~2nm.
⑤ It mainly removes lysosomes about one nanometer, and the molecular weight cut-off is 200-1000 Daltons.
⑥ The reverse osmosis membrane is almost all made of polyamide, and the nanofiltration membrane material can be made of various materials, such as cellulose acetate, cellulose acetate-triacetate, sulfonated polysulfone, sulfonated polyethersulfone, and aromatic polyamide composite materials And inorganic materials.
⑦ Generally, a polymer electrolyte is formed on the surface of the nanofiltration membrane, which often has a strong negative charge.
(2) Nanofiltration principle Like membrane separation processes such as ultrafiltration and reverse osmosis, nanofiltration is also a membrane separation process driven by pressure difference, which is an irreversible process.
The separation mechanism can be described using charge models (space charge model and fixed charge model), pore model, and electrostatic repulsion and three-dimensional obstruction models that have only been proposed in recent years.
Compared with other membrane separation processes, one advantage of nanofiltration is that it can retain small molecular weight organics that pass through the ultrafiltration membrane, and it can also dialyze some of the inorganic salts retained by the reverse osmosis membrane—that is, it can synchronize "concentration" and desalination. get on.
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 that must be applied to achieve the same permeation energy with a reverse osmosis membrane. Under the same applied pressure, the flux of nanofiltration is much larger than that of reverse osmosis, and when the flux is constant, the pressure required for nanofiltration is much lower than that of reverse osmosis.
Therefore, when nanofiltration is used instead of reverse osmosis, the "concentration" process can be carried out more efficiently and quickly, and a larger "concentration" multiple can be achieved. Generally speaking, in the membrane separation process using nanofiltration membranes, the rejection rate of various solutes in the solution has the following rules:
① Increase with the increase of molar mass;
②In the case of a given feed concentration, it increases with the increase of the transmembrane pressure difference;
③In the case of a given pressure, it decreases as the concentration increases;
④ For anions, it rises in the order of NO3-, CI-, OH-, SO42-, CO42-.
⑤ For cations, it rises in the order of H+, Na+, K+, Ca2+, Mg2+, Cu2+.
(3) Application of nanofiltration membrane These properties of nanofiltration membrane determine its unique and broad application in drinking water treatment, which is briefly described as follows.
① Softening: Membrane softening water mainly uses the selective permeability characteristics of the nanofiltration membrane to different valence ions to realize the softening of water. The film softening can remove the turbidity, chroma and organic matter while removing the hardness, and the effluent quality is significantly better than other softening processes. Moreover, membrane softening has the advantages of no need for regeneration, no pollution, simple operation, and less floor space, and has obvious social and economic benefits. Membrane softening is very common in the United States. In the past 10 years, new water softening plants in Florida have adopted membrane softening instead of conventional lime softening and ion exchange processes. In recent years, with the continuous improvement of nanofiltration performance, the price of nanofiltration membrane modules has been declining, and the membrane softening method has been superior or close to conventional methods in terms of investment, operation, and maintenance.
② Used to remove organics in water: In addition to softening in drinking water treatment, nanofiltration membranes are mostly used for decolorization, removal of natural and synthetic organics (such as pesticides, etc.), tertiary substances, and disinfection by-products (trihalomethanes and haloacetic acids) And its precursors and volatile organic compounds to ensure the biological stability of drinking water.
Removal of three causing substances: Studies have shown that nanofiltration membranes can remove most of the toxic and harmful organic matter and Ames mutagen in the water, so that the mutagenic ratio MR value of TA98 and TA100 strains at each test dose is less than 2, Ames test The result was negative. Further research will investigate the retention characteristics of endocrine disrupting substances in drinking water by nanofiltration technology and provide a basis for safe and high-quality drinking water.
Removal of disinfection by-products and their precursors: disinfection by-products mainly include trihalomethanes (THMs), haloacetic acids (HAAs) and possibly trichloroacetaldehyde hydroxide (CH). Foreign scientific and technological workers have carried out extensive research in this area, and the average rejection rates of the three kinds of disinfection by-product precursors by nanofiltration membranes are 97%, 94% and 86% respectively. Through the selection of suitable nanofiltration membranes, drinking water quality can meet higher standards for safe and high-quality drinking water.
In addition, nanofiltration water is low corrosive, which has a positive impact on the service life of the drinking water pipe network and the dissolution of metal ions in the pipe, which is beneficial to protect all materials of the water distribution system. Tests show that the use of the necessary post-treatment nanofiltration membrane system can reduce the dissolution of lead in the pipe network by 50%, and at the same time make the concentration of other dissolved metal ions meet the requirements of drinking water quality standards.
Removal of volatile organic compounds (VOC): It has a high removal rate for trace volatile organic compounds in drinking water.
Application in direct drinking water in pipelines; Nanofiltration can intercept ions with more than two valences and other particles, and only water molecules and some monovalent ions (such as sodium, potassium, and chloride ions) are passed through. Nanofiltration can be used to produce direct drinking water, and certain ions are still retained in the effluent, and treatment costs can be reduced.
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