Nanofiltration

Nanofiltration is a pressure-driven membrane separation process that retains many dissolved ions and small molecules while water and some smaller solutes pass through.

Donnan exclusion: Electrostatic partitioning that excludes ions from a charged membrane or phase. Membrane charge repels co-ions and helps nanofiltration retain dissolved salts.
Semipermeable membrane: A barrier that permits some substances to pass while restricting others. Nanofiltration depends on selective passage through a semipermeable barrier.
Water softening: Treatment that reduces water hardness by removing calcium and magnesium ions. Nanofiltration can reject hardness ions while allowing much of the water to pass.
Reverse osmosis: A pressure-driven membrane process that rejects most dissolved salts while water passes through. Reverse osmosis generally retains smaller dissolved species more strongly than nanofiltration.
Membrane scaling: Formation of mineral deposits on a membrane when dissolved salts precipitate. Concentrated rejected ions can precipitate and constrain recovery or operating conditions.
Steric exclusion: Separation based on whether molecules can fit through pores or available free volume. Molecular size helps determine which solutes pass through nanofiltration membranes.
Membrane technology: Separation methods that use selective barriers to divide components of a mixture. Nanofiltration is one pressure-driven method within this broader family.
Desalination: The removal of dissolved salts from water. Nanofiltration can partially desalinate brackish water and pretreat feed for other processes.
Ultrafiltration: A membrane process that retains colloids and macromolecules while passing most dissolved ions. Ultrafiltration typically removes larger material but lets dissolved salts pass.
Membrane cleaning: Physical or chemical treatment used to restore membrane performance after fouling or scaling. Cleaning frequency and effectiveness affect the long-term cost of nanofiltration.

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Membrane filtrationBroader topic: It bridges size-based filtration and the stronger ion rejection of reverse osmosis.