Nanofiltration

Nanofiltration is a pressure-driven membrane separation process that uses semipermeable membranes with nanometer-scale pores to remove certain ions and organic molecules from a feed stream. The membranes provide moderate selectivity, rejecting multivalent ions and larger solutes while allowing many monovalent ions to pass, placing nanofiltration between ultrafiltration and reverse osmosis in the spectrum of membrane technologies. It is widely used for water treatment, pretreatment for desalination, and selective concentration or clarification in the food, pharmaceutical, and biotech industries.

Most commercial NF membranes are thin-film composites, often based on polyamide, and carry a negative surface charge under ordinary water-treatment conditions. Filtration results from a combination of size exclusion and charge-based rejection (Donnan effect). Consequently, divalent and higher-valent ions are rejected more strongly than monovalent ions, and natural organic matter, color compounds, and some dyes are removed efficiently.

NF systems operate at moderate pressures, typically 5 to 40 bar, with fluxes that depend on feed composition, temperature, and membrane design. Recovery rates commonly reach 70–90% for well-behaved feeds, though fouling and concentration polarization can reduce performance.

Applications include drinking water softening by removing hardness-causing ions, removal of color and organic contaminants, reduction of sulfates and other multivalent ions, and pretreatment for reverse osmosis. In the food and beverage industry, nanofiltration is used for whey or milk standardization, sugar concentration, and juice clarification; in pharmaceuticals and biotech, for purification and concentration of small molecules.

Limitations include partial rejection of monovalent ions, sensitivity to fouling and chlorine, and the need for pretreatment and periodic cleaning. Materials commonly used are polyamide thin-film composites and, less often, cellulose-based membranes.