The application of nanofiltration membranes in the water treatment industry

The development and application of nanofiltration (NF) membranes were approximately 20 years later than those of reverse osmosis membranes. In the 1970s, J.E. Cadote began studying the NS-300 membrane, marking the beginning of the study of NF membranes.

At that time, the Israeli desalination company used "hybrid filtration" to refer to the membrane separation process between reverse osmosis and ultrafiltration, known as loose reverse osmosis (LOOSE RO) membrane.

Later, Filmtec in the United States referred to this membrane technology as nanofiltration, which has been used to this day. Afterwards, nanofiltration technology developed rapidly, and membrane components were commercialized in the mid-1980s. At present, nanofiltration technology has become one of the hotspots in the field of membrane separation research worldwide.

(1) So far, the accurate definition, mechanism, and characteristics of nanofiltration membranes are still far from sufficient. The definition of nanofiltration membrane in the academic community generally includes the following seven aspects:

① The nanofiltration membrane is between reverse osmosis and ultrafiltration membranes, and its surface separation layer may have a nanoscale microporous structure.

② Compared to reverse osmosis membranes, the removal rate of NaCI is generally above 95%. Membranes with a removal rate of NaCI below 90% can generally be called nanofiltration membranes.

③ Reverse osmosis membranes have a high removal rate for almost all solutes, while nanofiltration membranes only have a removal rate for specific solutes.

④ The pore size of the nanofiltration membrane is above 1nm, usually 1-2nm.

⑤ Mainly remove solute particles of about one nanometer, and intercept molecular weights between 200-1000 Daltons.

⑥ Almost all reverse osmosis membranes are made of polyamide materials, while nanofiltration membrane materials can be made of various materials, such as cellulose acetate, cellulose acetate triacetate, sulfonated polysulfone, sulfonated polyethersulfone, aromatic polyamide composite materials, and inorganic materials.

⑦ Generally, the surface of nanofiltration membranes forms high polymer electrolytes, which often have strong negative charge.

(2) The principle of nanofiltration is similar to membrane separation processes such as ultrafiltration and reverse osmosis. Nanofiltration is also an irreversible membrane separation process driven by pressure difference.

The separation mechanism can be described using charge models (space charge model and fixed charge model), pore models, as well as electrostatic repulsion and steric hindrance models proposed in recent years.

Compared with other membrane separation processes, one advantage of nanofiltration is that it can intercept small molecular weight organic matter that passes through the ultrafiltration membrane, and can also dialyze some inorganic salts intercepted by the reverse osmosis membrane - that is, it can synchronize "concentration" and desalination.

The transmembrane pressure difference required for NF membrane separation is generally 0.5-2.0MPa, which is 0.5-3MPa lower than the pressure difference required to achieve the same permeation energy using reverse osmosis membranes. Under the same external pressure, the flux of nanofiltration is much higher than that of reverse osmosis, and when the flux is constant, the pressure required for nanofiltration is much lower than that of reverse osmosis.

So when using nanofiltration instead of reverse osmosis, the "concentration" process can be carried out more effectively and quickly, and achieve a larger "concentration" multiple. Generally speaking, in the membrane separation process using nanofiltration membranes, the retention rates of various solutes in the solution follow the following pattern:

① Increasing with the increase of molar mass;

② At a given feed concentration, it increases with the increase of transmembrane pressure difference;

③ Under a given pressure, it decreases with increasing concentration;

④ For anions, they increase in the order of NO3-, CI -, OH -, SO42-, and CO42-.

⑤ For cations, they increase in the order of H+, Na+, K+, Ca2+, Mg2+, and Cu2+.

(3) The performance of nanofiltration membranes determines their unique and broad applications in drinking water treatment, as summarized below.

① Softening: Membrane softening of water is mainly achieved by utilizing the selective permeation characteristics of nanofiltration membranes for different valence ions. Membrane softening can remove turbidity, chromaticity, and organic matter while removing hardness, and its effluent quality is significantly better than other softening processes. Moreover, membrane softening has advantages such as no regeneration, no pollution generation, simple operation, and low footprint, which have significant social and economic benefits.

Membrane softening has become common in the United States, and new softening water plants in Florida have been using membrane softening for over a decade, replacing conventional lime softening and ion exchange processes. In recent years, with the continuous improvement of nanofiltration performance, the price of nanofiltration membrane components has been continuously decreasing. The membrane softening method has been superior or close to conventional methods in terms of investment, operation, and maintenance.

② Used for removing organic compounds from water: In addition to softening, nanofiltration membranes are often used for decolorization, removal of natural and synthetic organic compounds (such as pesticides), carcinogens, disinfection by-products (trihalomethane and haloacetic acid), their precursors, and volatile organic compounds in drinking water treatment, ensuring the biological stability of drinking water.

Removal of three pathogenic substances: Research has shown that nanofiltration membranes can remove most of the toxic and harmful organic compounds and Ames mutagens in water, resulting in a mutagenicity ratio MR value of less than 2 for TA98 and TA100 strains at each experimental dose, and a negative Ames test result. Further research will examine the retention characteristics of endocrine disruptors in drinking water by nanofiltration technology, providing 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 possible trichloroacetaldehyde hydroxides (CH). Foreign technology workers have conducted extensive research in this area, and the average retention rates of nanofiltration membranes for the precursors of these three disinfection by-products are 97%, 94%, and 86%, respectively. By selecting appropriate nanofiltration membranes, drinking water quality can meet higher standards for safe and high-quality drinking water quality.

In addition, the effluent from nanofiltration is low corrosive and has a positive impact on the service life of the drinking water pipeline network and the dissolution of metal ions in the pipeline, which is beneficial for protecting all materials of the water distribution system. Experiments have shown that using a necessary post-treatment nanofiltration membrane system can reduce the dissolution of lead in the pipeline network by 50%, while also ensuring that the concentration of other dissolved metal ions meets the requirements of drinking water quality standards.

Removal of volatile organic compounds (VOCs): It has a high removal rate for trace amounts of volatile organic compounds in drinking water.

Application in pipeline direct drinking water: Nanofiltration can intercept ions and other particles with divalent or higher ions, passing through only water molecules and some monovalent ions (such as sodium, potassium, chloride ions). Nanofiltration can be used to produce direct drinking water, retaining certain ions in the effluent and reducing treatment costs.

More information can be found on the official website of Shenzhen Hongjie Water Technology Co., Ltd. www.execro.com. If you need it, you can call the company's hotline for free at 180 3800 0078, and we will be happy to serve you.