Nanofiltration Membrane (NF): Features and Future Applications

Nanofiltration (NF) is a membrane separation technology positioned between Ultrafiltration (UF) and Reverse Osmosis (RO). The concept originated in the 1970s. In its early stage, NF membranes were sometimes referred to as "loose RO membranes" due to their lower salt rejection compared to RO, and "softening membranes" because of their high removal efficiency for divalent ions such as calcium and magnesium. By the 1990s, the term "Nanofiltration Membrane" (NF) became widely accepted, highlighting its nanometer-scale pore size (1–3 nm).

• Pore Size: NF membranes typically have pores of 1–3 nm, while RO membranes are much tighter at 0.2–0.8 nm.
• Separation Capability: NF membranes exhibit a molecular weight cutoff of about 200–2000 Da, bridging the performance gap between UF and RO.
• Material Diversity: RO membranes are mainly made of aromatic polyamide, whereas NF membranes use a wider range of materials, including polyamide, poly-piperazine, and sulfonated polyethersulfone, enabling diverse separation characteristics.
• Operating Pressure: NF systems require significantly lower operating pressure compared to RO, leading to reduced energy consumption.
• Separation Mechanism: RO primarily relies on the "solution-diffusion" mechanism, while NF combines "sieving effect" with the "Donnan effect." As a result, NF membranes remove multivalent salts (e.g., MgSO₄) at efficiencies above 98%, but their rejection rate for monovalent salts such as NaCl usually ranges between 40–60%.

• Industrial Applications: NF membranes are widely used in separation and concentration processes. In water treatment, they are effective in removing sulfates, hardness, color, odor, disinfection by-products, pesticides, synthetic detergents, and dissolved organics.
• Residential Applications: NF membranes are suitable for producing drinking water, applied in household water purifiers, mineral direct-drinking machines, and campus water systems, offering both safety and improved taste.