Nanofiltration (NF): Letting monovalent salts pass through while retaining divalent salts and dyes — the intermediate force in industrial wastewater fractionation and resource recovery.

Nanofiltration (NF): Letting Monovalent Salts "Pass the Gate" While Intercepting Divalent Salts and Dyes — The Intermediate Force in Industrial Wastewater Fractionation and Resource Recovery

The pore size of Nanofiltration / NF membranes is approximately 1 nm, with a Molecular Weight Cut-Off (MWCO) between 200–2000 Da, positioning them exactly between Ultrafiltration / UF and Reverse Osmosis / RO. Its "quirky temperament" is this: it retains SO₄²⁻, hardness, and medium-molecular-weight organics, yet lets NaCl pass through unimpeded. It is precisely this "semi-permeable gate" that makes it an irreplaceable link in dye desalination and concentration, Landfill Leachate fractionation, concentrate volume reduction, and Zero Liquid Discharge / ZLD.

Gaowutong · Industrial Water Treatment Technology Series · For industry technical personnel · All data are annotated with public literature and engineering sources

I. Mechanism: Size Sieving + Charge Repulsion, a "Charged Sieve"

Nanofiltration / NF membranes are mostly thin-film composite (TFC) structures, with a negatively charged polyamide active layer as the skin. Three mechanisms operate simultaneously during permeation, which is also the source of its selectivity:

1. Size sieving: Pore size approximately 1 nm, MWCO 200–2000 Da. Organics with molecular weights above the cut-off limit (dyes, humic substances, polymers) are essentially retained, while small molecules below the limit readily pass through.
2. Donnan charge repulsion: The membrane surface is negatively charged, strongly repelling multivalent anions (SO₄²⁻, PO₄³⁻) and weakly repelling monovalent anions (Cl⁻). This is the core of NF's ability to "separate salts."
3. Dielectric exclusion and steric hindrance: The difference in dielectric constant inside and outside the membrane pores creates an additional energy barrier for high-valence ions, further enhancing divalent/monovalent selectivity.

The superposition of these three mechanisms gives rise to NF's signature salt rejection order: MgSO₄ > Na₂SO₄ > MgCl₂ > NaCl. In actual measurements of a hollow fiber TFC Nanofiltration / NF membrane, MgSO₄ rejection can reach 96.20%, while rejection of the anionic dyes Reactive Brilliant Blue X-BR and Acid Red B is 99.99% and 99.90%, respectively.

II. Parameter Window: MWCO, Pressure, Flux and Recovery

The operating parameters of Nanofiltration are not like Reverse Osmosis, which "forces through with high pressure," nor like Ultrafiltration, which features "low pressure and high flow." It sits in the middle ground, emphasizing the balance between flux and rejection. The commonly monitored windows in engineering practice are as follows:

One point must be emphasized: increasing pressure does improve rejection and flux, but the returns diminish. In a pilot test of Nanofiltration for RO concentrate from a coal chemical plant, when pressure was raised from 1.8 MPa to 2.2 MPa, the average COD removal rate only increased from 54.0% to 59.9%—the extra electricity cost must be carefully calculated to determine whether it is worth it.

III. Real-World Engineering Ledger

The 8 sets of data below are all sourced from publicly available pilot tests and engineering reports/journal literature, categorized by industry for easy comparison against your own water quality to find references.

IV. Membrane Fouling and Control: NF's "Lifespan Killer"

NF separation performance depends on the active layer, and the active layer is most vulnerable to three things: organic fouling, inorganic scaling, and biological slime. Engineering experience is roughly as follows:

• Organic fouling is the most common category—dyes, humic substances, and surfactants adsorb onto the membrane surface to form a film, manifesting as flux decline and pressure differential rise. In the comparison of standalone Nanofiltration for coking wastewater, Membrane Fouling was more severe and the operating cycle shorter than that of the dual-membrane process, but the performance recovery after chemical cleaning differed very little from the dual-membrane process, indicating that fouling is reversible, and the key lies in the cleaning strategy.
• Inorganic scaling is dominated by CaSO₄ and CaCO₃—because NF precisely retains SO₄²⁻ and Ca²⁺. Under high-salinity conditions, antiscalants must be dosed, and the recovery rate must be controlled within the scaling critical threshold.
• Cleaning strategy is generally "alkali first, then acid": alkaline cleaning targets organic/biological fouling, acid cleaning targets inorganic scaling; chemical cleaning (e.g., once every 10 d) + daily flushing (every day) is a common rhythm.
• Water temperature and shear also have an impact: rising temperature and crossflow rotation speed reduce transmembrane pressure differential and mitigate fouling, but have little effect on COD rejection—this means that operating condition optimization can be used to extend service life without compromising water quality.

V. Technical Comparison and Positioning: Where Exactly Should It Stand

Six, 6 Engineering Truths

1. Don't expect NF to remove salts completely. Monovalent salts (NaCl) have a high passage rate through NF, and the TDS removal rate of single-stage NF typically falls within the 60%–85% range (TDS removal of 57%–68% in the coking case, 79.87% in the high-salinity case); to meet reuse standards, RO is often still required downstream.
2. Divalent salt rejection is the selling point, and also the source of scaling risk. SO₄²⁻ is retained on the concentrate side, and upon encountering Ca²⁺, CaSO₄ may precipitate. Setting the recovery rate is essentially a trade-off between "desired concentration factor" and "scaling threshold."
3. Pretreatment determines membrane lifespan. In the high-salinity case, Ultrafiltration first removed approximately 79.5% of COD before NF could run stably at 30 d. Skipping pretreatment means paying the debt with membrane flux and cleaning frequency.
4. The removal rate improvement gained from pressure is limited. In a certain pilot test, increasing from 1.8 MPa to 2.2 MPa yielded only 5.9 percentage points more removal. Before raising pressure, first ask: is this slight water quality improvement worth the extra power consumption and membrane pressure?
5. Concentrate must find an outlet, otherwise the problem isn't solved. There are three feasible directions: recirculation to the biological stage for continued degradation (coking case), resource recovery and reuse (Na₂SO₄ 21.2 g/L in printing and dyeing concentrate directly returned to dyeing, ΔE<1), and evaporation crystallization for zero liquid discharge. Merely shifting pollution into the concentrate does not count as compliance.
6. The true value of NF is "fractionation." In the printing and dyeing case, NF achieved a dye recovery rate of 91.4% and NaCl removal of 95.3%; in another case, Na₂SO₄ was rejected at 93%, 3.8 kg/m³ of the feed was recovered, and approximately 0.27 US$/m³ of revenue was generated—when resources can be sold, NF has its strongest economic persuasiveness.

References

1. Pilot-scale study on desalination and fractional treatment of high-salinity wastewater by integrated Nanofiltration-Reverse Osmosis membrane process. Wastewater Treatment Engineering Network, 2024-01-17. https://www.dowater.com/jishu/2024-01-17/4993830.html
2. Coagulation-Nanofiltration combined process for printing and dyeing finishing wastewater treatment (Institute of Process Engineering, Chinese Academy of Sciences, State Key Laboratory of Biochemical Engineering). Wastewater Treatment Engineering Network, 2023-03-16. https://www.dowater.com/jishu/2023-03-16/3536312.html
3. Membrane combined process for advanced treatment of coking wastewater (Ultrafiltration-Nanofiltration combination, continuous operation 20 d). Wastewater Treatment Engineering Network, 2019-11-05. https://m.dowater.com/jishu/2019-11-05/1094880.html
4. Pilot-scale study on advanced treatment of coking wastewater by Nanofiltration process. Wastewater Treatment Engineering Network, 2018-03-30. https://m.dowater.com/jishu/2018-03-30/683653.html
5. Nanofiltration treatment of high-salinity wastewater from coal chemical industry (Reverse Osmosis concentrate from a coal chemical plant in Yulin, Shaanxi, pilot test 1 m³/h, continuous operation 90 d). Botaida Water Treatment, 2021-11-15. http://www.botaida.com/news/202111154500.html
6. Wang R, Du M, Wang H, Li J. Highly efficient recovery of Na₂SO₄ and desalted water from industrial dyeing wastewater with zero liquid discharge by an integrated process of electrocatalytic membrane reactor coupled with ozonation and nanofiltration. Separation and Purification Technology, 2025, 379: 134927. doi:10.1016/j.seppur.2025.134927
7. Characterization and application of a thin-film composite nanofiltration hollow fiber membrane for dye desalination and concentration. Chemical Engineering Journal, 2013, 223: 172–178.
8. Xing C, Han J, Pei X, et al. Tunable Graphene Oxide Nanofiltration Membrane for Effective Dye/Salt Separation and Desalination. ACS Applied Materials & Interfaces, 2021. doi:10.1021/acsami.1c16141
9. GB/T 19923-2005 Urban Sewage Reclamation and Utilization — Water Quality for Industrial Uses.
10. GB/T 50335-2002 Code for Design of Wastewater Reclamation and Reuse Engineering (Water Quality Standard for Make-up Water of Circulating Cooling Systems).