Applications of Nanofiltration Membranes in the Water Treatment Industry

The development and application of nanofiltration (NF) membranes lagged behind reverse osmosis membranes by approximately 20 years. Research on NF membranes began in the 1970s with J.E. Cadotte's study of the NS-300 membrane. At that time, the Israel Desalination Company used the term "hybrid filtration" to describe the membrane separation process between reverse osmosis and ultrafiltration, calling it loose reverse osmosis (looseRO) membranes. Later, the American company Filmtec named this membrane technology nanofiltration, a name that remains in use today. Subsequently, nanofiltration technology developed rapidly, and membrane modules were commercialized in the mid-1980s. Currently, nanofiltration technology has become one of the hot topics in the world's membrane separation field.

(1) Definition of Nanofiltration Membranes To date, the accurate definition, mechanism, and characteristics of nanofiltration membranes are far from complete. The academic consensus on the definition of nanofiltration membranes includes the following seven aspects:

① Nanofiltration membranes lie between reverse osmosis and ultrafiltration membranes, and their membrane surface separation layer may have a nanoscale microporous structure.

② Compared to reverse osmosis membranes, which generally achieve NaCl removal rates above 95%, nanofiltration membranes are typically defined as those with NaCl removal rates below 90%.

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

④ Nanofiltration membranes have pore sizes above 1 nm, typically 1–2 nm.

⑤ They primarily remove solute particles around one nanometer in size, with a molecular weight cutoff of 200–1000 Daltons.

⑥ Reverse osmosis membranes are almost always made of polyamide, while nanofiltration membranes can be made from a variety of materials, such as cellulose acetate, cellulose acetate-triacetate, sulfonated polysulfone, sulfonated polyethersulfone, aromatic polyamide composites, and inorganic materials.

⑦ Nanofiltration membranes generally have a strong negative charge due to the formation of polymer electrolytes on their surface.

(2) The principle of nanofiltration is similar to that of ultrafiltration and reverse osmosis membrane separation processes. Nanofiltration is also a membrane separation process driven by pressure difference and is an irreversible process. The separation mechanism can be described using charge models (space charge model and fixed charge model), micropore models, and the electrostatic repulsion and steric hindrance models proposed in recent years. Compared with other membrane separation processes, one advantage of nanofiltration is that it can retain small molecular weight organic matter that permeates through ultrafiltration membranes, while also dialyzing some of the inorganic salts retained by reverse osmosis membranes—that is, it can simultaneously carry out "concentration" and desalination. 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 osmotic energy using reverse osmosis membranes. Under the same applied pressure, the flux of nanofiltration is much greater than that of reverse osmosis, while at a certain flux, the pressure required for nanofiltration is much lower than that of reverse osmosis. Therefore, when nanofiltration replaces reverse osmosis, the "concentration" process can be carried out more effectively and rapidly, achieving a greater "concentration" factor. Generally speaking, in membrane separation processes using nanofiltration membranes, the retention rates of various solutes in the solution follow these patterns:

① Increase with increasing molar mass;

② Increase with increasing transmembrane pressure at a given feed concentration;

③ Decrease with increasing concentration at a given pressure;

④ For anions, the retention rates increase in the order of NO3-, Cl-, OH-, SO42-, CO42-;

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

(3) Applications of Nanofiltration Membranes These properties of nanofiltration membranes determine their unique and extensive applications in drinking water treatment, briefly described below:

① Softening: Membrane softening mainly utilizes the selective permeation characteristics of nanofiltration membranes for ions of different valence states to soften water. While reducing hardness, membrane softening can also remove turbidity, color, and organic matter, resulting in effluent water quality significantly superior to other softening processes. Moreover, membrane softening offers advantages such as no regeneration required, no pollution generation, simple operation, and small footprint, resulting in significant social and economic benefits. Membrane softening is already widespread in the United States; in Florida, new water softening plants over the past decade have all adopted membrane softening, replacing conventional lime softening and ion exchange processes. In recent years, with the continuous improvement of nanofiltration performance and the continuous decline in the price of nanofiltration membrane modules, membrane softening has become superior to or approached conventional methods in terms of investment, operation, and maintenance.

② Used for removing organic matter from water: In drinking water treatment, besides softening, nanofiltration membranes are widely used for decolorization, removal of natural and synthetic organic matter (such as pesticides), carcinogenic, mutagenic, and teratogenic substances, disinfection byproducts (trihalomethanes and haloacetic acids) and their precursors, and volatile organic compounds, ensuring the biological stability of drinking water.

Removal of carcinogenic, mutagenic, and teratogenic substances: Studies have shown that nanofiltration membranes can remove most toxic and harmful organic matter and Ames mutagens from water, resulting in mutagenicity ratios (MR values) of less than 2 for TA98 and TA100 strains at various experimental doses, and negative Ames test results. Further research will examine the retention characteristics of nanofiltration technology for endocrine disruptors in drinking water, providing a basis for safe and high-quality drinking water.

Removal of disinfection byproducts and their precursors: Disinfection byproducts mainly include trihalomethanes (THMs), haloacetic acids (HAAs), and possibly trichloroacetaldehyde hydroxide (CH). Extensive research has been conducted abroad, with nanofiltration membranes achieving average retention rates of 97%, 94%, and 86% for these three disinfection byproduct precursors, respectively. By selecting appropriate nanofiltration membranes, drinking water quality can meet higher standards for safe and high-quality drinking water.

Furthermore, nanofiltration effluent is low in corrosiveness, positively impacting the service life of drinking water pipe networks and reducing the leaching of metal ions from pipes, thus protecting all materials in the water distribution system. Experiments show that nanofiltration membrane systems employing necessary post-treatment can reduce lead dissolution in pipe networks by 50%, while ensuring that the concentrations of other leached metal ions meet drinking water quality standards. Removal of volatile organic compounds (VOCs): High removal rates are achieved for trace amounts of volatile organic compounds in drinking water.