Nanofiltration (NF) membranes separate substances at a molecular level. They operate as a pressure-driven process, pushing liquid through the membrane material to remove specific dissolved components from water or other solutions. NF membranes filter out particles ranging from approximately 1 to 10 nanometers in size.
The Science Behind Nanofiltration
Nanofiltration membranes achieve separation through size exclusion and charge-based interactions. Their pores, ranging from 1 to 10 nanometers, physically block larger molecules and particles. This mechanism is similar to a sieve, where only particles smaller than the holes can pass through.
Beyond size, the surface of NF membranes carries an electrical charge, which plays a role in their selectivity. This charge can repel ions with similar charges, a phenomenon known as Donnan exclusion. For example, negatively charged membranes reject multivalent ions like calcium (Ca2+) and magnesium (Mg2+), while allowing smaller, monovalent ions such as sodium (Na+) and chloride (Cl-) to pass through.
Applied pressure forces the liquid, known as the permeate, through the semi-permeable membrane. Molecules larger than the pore size or those repelled by the membrane’s charge are retained on the feed side, forming a concentrated stream called the retentate. This pressure differential ensures continuous flow and separation.
Where Nanofiltration is Used
Nanofiltration membranes are used across various industries to selectively remove dissolved substances. In water treatment, they soften water by removing hardness-causing divalent ions like calcium and magnesium, while allowing beneficial monovalent ions to pass. NF membranes also remove natural organic matter and contaminants like pesticides and heavy metals from drinking water, improving its quality.
In industrial wastewater treatment, NF membranes are used for dye removal in the textile industry, allowing for dye concentration, recovery, and pollution reduction. They are also used in chemical recovery processes, such as removing sulfates from seawater in oilfield operations to prevent scaling. Additionally, NF treats landfill leachate by removing pollutants, decreasing total organic carbon (TOC) and chemical oxygen demand (COD) levels.
The food and beverage industry uses NF technology for concentrating and demineralizing lactose in dairy processing, purifying cheese by-products. NF membranes also concentrate fruit juices, such as maple syrup, offering an energy-efficient alternative to traditional boiling methods that preserves flavor. This technology helps retain beneficial minerals while removing undesirable impurities.
Nanofiltration in the Membrane Spectrum
Nanofiltration bridges the gap between ultrafiltration (UF) and reverse osmosis (RO) within membrane filtration technologies. UF membranes have larger pore sizes, ranging from 0.01 to 0.1 micrometers, primarily removing suspended solids, bacteria, protozoa, and some viruses. In contrast, RO membranes have the smallest pore sizes, around 0.0001 micrometers, removing nearly all dissolved solids, including monovalent ions and very small molecules.
NF membranes feature pore sizes between 0.001 and 0.01 micrometers, making them tighter than UF but more permeable than RO. This allows NF to selectively remove multivalent ions and larger organic molecules, while permitting water and monovalent ions to pass. This selective rejection is a differentiator, as RO membranes remove almost all dissolved substances, including some beneficial minerals.
Nanofiltration is a choice when partial demineralization or selective removal of specific dissolved solids is desired, without the extensive removal achieved by RO. For instance, in water softening, NF removes hardness-causing ions without significantly altering the overall dissolved solids content as much as RO would. This balance of selectivity and permeability makes NF a suitable solution for many separation challenges.