At a Glance
Costs vary significantly with scale, membrane chemistry, and application. Use untangle.bio for project-specific estimates.
How Nanofiltration Works
Nanofiltration uses semi-permeable membranes with molecular weight cut-offs between 200 and 1,000 Da, bridging the gap between ultrafiltration and reverse osmosis. NF membranes separate molecules by a combination of size exclusion and charge effects (Donnan exclusion). Most NF membranes carry a slight negative surface charge, giving them preferential rejection of divalent and multivalent ions over monovalent species.
Two Outputs
Retentate (heavy): Sugars, organic acids, divalent ions (Ca²+, Mg²+, SO₄²−), peptides, and other molecules above the MWCO that are retained by the membrane.
Permeate (light): Water, monovalent salts (Na+, Cl−, K+), and very small molecules below the MWCO that pass through.
Operating Modes
| Mode | Purpose | Description |
|---|---|---|
| Sugar/Salt Fractionation | Selective separation | Retain sugars (MW 180–342 Da) while passing monovalent salts (MW 58–75 Da) through the membrane |
| Organic Acid Concentration | Product concentration | Concentrate lactic, citric, or succinic acid in the retentate by removing water and salts |
| Divalent Ion Removal | Water softening | Remove Ca²+, Mg²+, and SO₄²− while passing monovalent ions — >90% divalent rejection typical |
| Color / Flavor Removal | Polishing | Remove pigments, polyphenols, and off-flavor compounds from sugar streams or fermentation products |
MWCO Selection Guide
Rule of thumb: Choose MWCO at least 2–3× smaller than the target molecule to ensure >90% rejection. NF charge effects enhance separation of charged vs. neutral species.
| MWCO | Retains | Passes | Common Use |
|---|---|---|---|
| 200–300 Da | Glucose, sucrose, all larger molecules | NaCl, KCl, small monovalent ions | Sugar demineralization, tight NF |
| 300–500 Da | Disaccharides, organic acids, divalent ions | Monovalent salts, water | Sugar/salt fractionation, softening |
| 500–700 Da | Small peptides, antibiotics, dyes | Glucose, salts, amino acids | Antibiotic concentration, dye removal |
| 700–1,000 Da | Larger peptides, oligosaccharides | Monosaccharides, salts, small organics | Loose NF, oligosaccharide fractionation |
Best Molecules for NF Separation
| Molecule | MW | NF Behavior | Use Case |
|---|---|---|---|
| Glucose | 180 Da | >95% retained by tight NF | Sugar recovery, demineralization |
| Sucrose | 342 Da | >99% retained by most NF | Sugar concentration, purification |
| Lactic Acid | 90 Da | Partially retained (charge-dependent) | Organic acid concentration at high pH |
| Citric Acid | 192 Da | Well retained (trivalent anion at neutral pH) | Citric acid purification, concentration |
| NaCl | 58 Da | <30% rejected (passes freely) | Removed in permeate during desalting |
| Lysine | 146 Da | Partially retained (charge-dependent) | Amino acid recovery, fractionation |
Cost Considerations
Capital Cost (CAPEX)
NF systems are more expensive than UF systems of equivalent area due to higher-pressure housings and specialized membranes. Spiral-wound modules are the most cost-effective format for large-scale applications. Flat-sheet cassettes offer easier cleaning for fouling-prone feeds but at higher cost per unit area.
Key CAPEX Drivers
| Factor | Impact |
|---|---|
| Operating pressure | Higher TMP requires stainless-steel high-pressure housings and larger pumps |
| Membrane material | Thin-film composite (TFC) standard; ceramic NF membranes 5–10× more but last years longer |
| Membrane area | Primary cost driver — determined by flux (10–50 LMH typical) and throughput |
| Feed pretreatment | Fouling-prone feeds need MF/UF pretreatment, adding to total system cost |
Operating Cost (OPEX)
NF operates at higher pressures than UF (5–40 bar vs. 1–5 bar), resulting in significantly higher energy costs. Membrane replacement is typically every 1–3 years depending on feed quality and cleaning regime. Chemical cleaning (CIP) with acids and bases is essential for maintaining flux. Scaling from divalent ions (especially CaSO₄) is a common OPEX concern requiring anti-scalant dosing.
Frequently Asked Questions
What is the difference between NF and UF?
Ultrafiltration (1–100 kDa MWCO) separates proteins from small molecules using size exclusion alone. Nanofiltration (200–1,000 Da MWCO) operates in the small-molecule range and separates by both size and charge — for example, retaining sugars while passing monovalent salts. NF requires higher pressures (5–40 bar vs. 1–5 bar for UF).
Can NF separate glucose from NaCl?
Yes, this is a classic NF application. Tight NF membranes (200–300 Da MWCO) achieve >95% glucose rejection while passing >70% of NaCl. The 3× molecular weight difference (180 Da vs. 58 Da) combined with charge exclusion of neutral glucose vs. ionic NaCl enables effective fractionation.
Why does NF reject divalent ions more than monovalent ions?
Most NF membranes have a negatively charged surface layer. Divalent anions (SO₄²−) experience stronger electrostatic repulsion (Donnan exclusion) than monovalent anions (Cl−). Similarly, divalent cations (Ca²+, Mg²+) are co-rejected to maintain electroneutrality. This charge-based selectivity is independent of size and is unique to NF.
What pretreatment is needed before NF?
NF membranes are sensitive to fouling from particles, colloids, and biological material. Most bioprocess feeds require upstream microfiltration or ultrafiltration to remove cells, debris, and macromolecules. Anti-scalant dosing may be needed if the feed contains high concentrations of divalent ions to prevent CaSO₄ or CaCO₃ scaling.
Related Separation Techniques
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