Nanofiltration (Meter) Membrane Technology

1.1 Principles and Characteristics

Membrane separation is a method that utilizes the differences in selective permeation properties of membranes for components in a mixture, driven by external energy or chemical potential difference, to separate, classify, purify, and enrich two-component or multi-component mixed gases or liquids. Membrane processes with pore sizes in the nanometer range, suitable for separating dissolved components with molecular weights of 200–1000 and molecular sizes of approximately 1 nm, are called nanofiltration (NF). The transmembrane pressure difference required for NF membrane separation is generally 0.5–2.0 MPa, which is 0.5–3 MPa lower than the pressure difference required to achieve the same permeation energy using reverse osmosis membranes. Based on operating pressure and separation limits, NF can be qualitatively classified between reverse osmosis and ultrafiltration; sometimes NF is also called "low-pressure reverse osmosis" or "loose reverse osmosis." The research on NF membranes by J. E. Cadotte in the 1970s, specifically the NS-300 membrane, marked the beginning of research on NF membranes [8]. At the time, the Israeli desalination company used "Hybrid Filtration" to describe the membrane separation process between reverse osmosis and ultrafiltration. Later, the American company Film*.Tech called this membrane technology nanofiltration, a term still used today. Subsequently, nanofiltration (NF) developed rapidly, and membrane modules were commercialized in the mid-1980s. Currently, NF has become one of the hottest research topics in the global membrane separation field.

NF separation is a green water treatment technology that can replace traditional, expensive, and complex wastewater treatment methods in some aspects. Its technical characteristics include: the ability to retain organic matter with a molecular weight greater than 100 and polyvalent ions, while allowing small organic molecules and monovalent ions to permeate; the ability to operate under harsh conditions such as high temperature, acid, and alkali, and resistance to fouling; low operating pressure, high membrane flux, and low operating costs; and the ability to be combined with other wastewater treatment processes to further reduce costs and improve treatment efficiency. In water treatment, NF membranes are mainly used for treating solvent-containing wastewater, effectively removing color, hardness, and odor from the water. NF membranes, with their unique separation properties, have been successfully applied to wastewater treatment in industries such as sugar refining, pulp and paper making, electroplating, machining, and the recovery of chemical reaction catalysts.

1.2 Experimental Research and Application

(1) Daily Chemical Wastewater Treatment. Research on the application of NF membranes in treating daily chemical wastewater [9] shows that NF membranes are acid and alkali resistant, have excellent retention rates, and exhibit good removal rates for heavy metals, without membrane fouling issues. It is estimated that, due to the lower operating costs of NF membranes compared to reverse osmosis technology and their good removal rate for small organic molecules, they 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 matter generated during petroleum extraction and refining. Its composition is very complex, making treatment difficult. Combining membrane methods, especially NF methods, with other methods can effectively treat wastewater and recover useful substances. For example, crude oil wastewater can be separated into an oil-rich aqueous phase and an oil-free saline phase using an NF membrane. The oil-rich phase can then be added to fresh water before entering the oil washing process, thus recovering crude oil and saving water. Previously, a combination of reverse osmosis and phase separation was used to treat petroleum industry wastewater, but this method suffers from severe membrane fouling. Adding an NF membrane before reverse osmosis can solve the membrane fouling problem. Phenolic wastewater from the petroleum industry mainly contains phenol, methylphenol, nitrophenol, and various substituted phenols. These substances are highly toxic and must be removed before discharge. If NF technology is used, not only can the removal rate of phenol reach over 95%, but it can also efficiently remove high-valence ions of heavy metals such as cadmium, nickel, mercury, and titanium from wastewater under lower pressure. The cost is much lower than that of reverse osmosis and other methods [10]. (3) Pesticide wastewater treatment. General water treatment methods cannot remove low-molecular-weight organic pesticides from polluted water. By studying the retention performance of NF membranes for phenol-free pesticides [11], it was found that the retention rate of pesticides other than dichloride was higher than 96.7%, and the adsorption capacity of all pesticides on the NF membrane was affected by its hydrophobicity. NF is also very effective in treating wastewater containing phenolic pesticides.

(4) Wastewater treatment in chemical fiber and dyeing industries. NF can be used for the removal and reuse of dyes and auxiliaries in dyeing process wastewater. When treating dye polymerization slurry, since most dyes have molecular weights in the hundreds to thousands, the NF membrane allows some inorganic salts or small molecules to pass through while trapping larger dye molecules. After the crude dye slurry passes through the NF system, the dye can be enriched, while the concentration of inorganic salts decreases, the desalination rate is greater than 98%, the dye loss rate is less than 0.1%, and it can operate at high temperatures. In addition, NF can also be used for the treatment and recycling of oily wastewater in fiber processing [12].

(5) Domestic sewage treatment. When using a combination of biodegradation and chemical oxidation to treat domestic sewage, the consumption of oxidants is high, and there are many residues. If an NF system is added between them, allowing small molecules (molecular weight less than 100) that can be degraded by microorganisms to pass through, while large organic molecules (molecular weight greater than 100) that cannot be biodegraded are retained and chemically oxidized before being biodegraded, the role of biodegradation can be fully utilized, saving the amount of oxidants or activated carbon used and reducing the content of the final residues [13].

(6) Treatment and recycling of secondary wastewater from thermal power plants. Secondary wastewater from thermal power plants mainly comes from ash flushing, dust removal, and cooling systems. This type of wastewater contains a large amount of suspended solids, ash, high salt content, and some organic matter. NF can be used to treat this type of wastewater into industrial reclaimed water. First, microfiltration removes all suspended particles from the water, reducing the BOD to 99%, COD to 98%, total nitrogen to 73%, and total phosphorus to 17% by mass. Simultaneously, the total bacterial count is reduced to 3-4 CFU/L. Then, acid is added to lower the pH and remove CO2. Finally, NF desalination is performed to achieve the quality required for boiler water. The Eraring power station of the Australian Pacific Thermal Power Plant currently uses NF to treat this type of wastewater, processing 1,000-15,000 m³ of wastewater daily. This reduces the load on the municipal water supply system and saves the power plant US$800,000 annually in operating costs. The power plant plans to expand its power generation capacity, which will correspondingly increase water consumption. It is estimated that by 2010, the volume of this type of wastewater treated will reach 5,000 m³/d, resulting in substantial benefits [14].

(7) Pickling wastewater treatment. The pickling process in steel mills involves immersing steel in a sulfuric acid pickling tank with a mass fraction of approximately 20%. As pickling progresses, the sulfuric acid concentration gradually decreases while the ferrous sulfate concentration continuously increases. When the mass fraction of sulfuric acid in the solution drops to 6%–8% and the concentration of ferrous sulfate exceeds 200–250 g/L, the pickling rate decreases, necessitating replacement of the pickling solution and discharge of pickling wastewater. Pickled steel must be rinsed with clean water to remove surface acidic substances, resulting in the discharge of waste acid water. To protect the environment and conserve resources, the NF process can be used to treat the pickling wastewater. Utilizing the different retention rates of sulfuric acid and ferrous sulfate by the NF membrane, ferrous sulfate is first retained in the concentrate. The concentrate is then sent to a cooling crystallization tank to cool and crystallize FeSO4·7H2O. The permeate then passes through another NF membrane module capable of retaining sulfuric acid, concentrating it to 20% sulfuric acid. The regenerated acid solution is recycled, while the permeate is discharged to a waste acid water treatment plant for further treatment or recycling. This process recovers sulfuric acid and ferrous sulfate, and simultaneously achieves the comprehensive utilization of pickling wastewater and the standard discharge of waste acid water [15].

(8) Papermaking wastewater treatment. Using NF membrane technology to replace traditional chemical treatment methods can more effectively remove dark lignin. Chlorinated lignin produced during the wood pulp bleaching process is negatively charged and easily retained by the negatively charged NF membrane, without causing membrane fouling. In addition, since there are no strict requirements for the removal rate of cations (Na+) during the entire treatment process, the use of reverse osmosis technology is unnecessary. Ultrafiltration/nanofiltration has a good effect on the treatment of kraft paper manufacturing wastewater [16].

1.3 Prospects

NF membranes have separation performance for small organic molecules with molecular weights of several hundred in water, and have a good removal capacity for color, hardness and odor. Moreover, they operate at low pressure and have a large water flux, thus playing a huge role in the field of water treatment. Currently, there are still many technical problems to be solved in the preparation, characterization and separation mechanism of NF membranes. There is also a need to develop inexpensive and high-performance membranes that can provide users with a variety of accurate membrane performance parameters. These are the key factors for nanofiltration technology in wastewater treatment and other applications.