In the march toward zero liquid discharge (ZLD) for industrial wastewater, the nanofiltration (NF) membrane plays an irreplaceable role as a central hub. It does not attempt to reject all ions indiscriminately like a reverse osmosis (RO) membrane, nor is it limited to simple physical sieving like an ultrafiltration membrane. Instead, with its unique ion selectivity, the NF membrane builds a bridge for salt separation between reverse osmosis and evaporation crystallization. This transforms ZLD from a mere exercise in pollutant elimination into a closed-loop process capable of recovering valuable salt resources.

Why “Salt Separation” Matters: The Dilemma of Mixed Waste Salts

To appreciate the value of nanofiltration, one must first understand a persistent pain point in conventional ZLD processes. Many industrial wastewaters—from coal chemical, flue gas desulfurization, and textile dyeing operations—contain large quantities of sodium chloride and sodium sulfate. If this mixed stream is simply concentrated by reverse osmosis and sent directly to an evaporator-crystallizer, the result is a “mixed salt” containing an unrefined blend of NaCl and Na₂SO₄.

Such mixed salts are compositionally complex and of low purity, classifying them as hazardous waste. With disposal costs often reaching thousands of yuan per ton, they offer zero economic return and impose immense landfill pressure. The philosophy of modern ZLD, therefore, has shifted from “unidirectional salt disposal” to “fractional salt recovery”—turning sodium sulfate into anhydrous sodium sulfate (thenardite) and sodium chloride into industrial-grade salt, giving the inorganic salts in wastewater a new lease of life. The key to achieving this lies precisely in the “salt separation” capability of nanofiltration.

Nanofiltration: A “Smart Gatekeeper” Navigating Between Ions

In terms of pore size, an NF membrane sits between ultrafiltration and reverse osmosis, at roughly 1 nanometer. However, its true sophistication comes from the “charge effect” (the Donnan effect). The membrane surface typically carries a negative charge, and based on the principle of like-charge repulsion and opposite-charge attraction:

• Divalent and multivalent ions (such as sulfate SO₄²⁻, calcium Ca²⁺, and magnesium Mg²⁺) are strongly repelled, with rejection rates typically exceeding 98%.
• Monovalent ions (such as chloride Cl⁻ and sodium Na⁺) pass through much more easily, with rejection rates usually between 20% and 60%. By adjusting operating pressure and membrane chemistry, one can even achieve “negative rejection,” actively promoting their transport.

It is like stationing a smart gatekeeper at a passage: those with a larger footprint and carrying two charges (sulfate) must stay back, while those with a small frame and a single charge (chloride) can pass through smoothly. This unique mechanism of selective transport makes nanofiltration the perfect choice for precisely separating mixtures of monovalent and divalent salts.

NF as the “Central Hub”: How It Integrates with RO and Evaporation Crystallization

In a typical ZLD salt separation process, nanofiltration occupies a pivotal middle position, working in close coordination with upstream RO and downstream evaporation crystallization. The overall logic runs as follows:

Stage 1: Bulk Concentration by RO (Front-End Rough Concentration)
After softening, hardness removal, and ultrafiltration pretreatment, the wastewater is first heavily concentrated by conventional RO or seawater reverse osmosis (SWRO) membranes. The RO permeate is directly reused as high-quality water, while the RO concentrate, typically enriched to 3–5% total dissolved solids (TDS), still contains an intimate mixture of monovalent and divalent salts. Sending this stream directly to evaporation would be extremely energy-intensive and would produce only mixed waste salt.

Stage 2: Precise Splitting by NF (Core Salt Separation)
This is where the RO concentrate enters the nanofiltration system, and the classic “salt redistribution” occurs:

• NF Permeate Side: The vast majority of sodium chloride (NaCl) passes through, along with only a small residual amount of sulfate, forming a relatively clean, NaCl-dominant brine.
• NF Concentrate Side: The vast majority of sodium sulfate (Na2SO4Na2​SO4​) is retained, effectively parting ways with the sodium chloride. This concentrate stream is rich in divalent salts, with organics and hardness ions also concentrated here.

The elegance of this step lies in the fact that NF splits a chaotic, high-salinity wastewater into two compositionally distinct streams, clearing the biggest hurdle for their respective downstream resource-recovery pathways.

Stage 3: Respective Concentration and Crystallization (Back-End Refining)
After separation, the two streams are still not concentrated enough for economical entry into a crystallizer (which generally requires well over 100,000 mg/L TDS). Here, specialized RO is reintroduced for targeted concentration:

• Sodium Chloride Product Line: The NF permeate is sent to dedicated high-pressure RO systems (such as DTRO or HPRO) or electrodialysis (ED) to concentrate the NaCl solution nearly to saturation (e.g., TDS of 8–12%). It then enters an MVR (mechanical vapor recompression) evaporative crystallizer, yielding high-purity sodium chloride crystals (purity can exceed 99%, meeting industrial salt standards).
• Sodium Sulfate Product Line: The NF concentrate first undergoes polishing softening to prevent scaling, followed by further dewatering via another stage of NF or high-pressure RO to bring sodium sulfate close to saturation. Depending on the process design, it then enters a chilled crystallizer (exploiting the strong temperature dependence of sodium sulfate solubility to precipitate mirabilite, Na₂SO₄·10H₂O) or an evaporative crystallizer to produce anhydrous sodium sulfate (purity can exceed 98%).

Without NF at the front end to split sulfate and chloride apart, the downstream RO concentration and crystallization equipment would be forced to deal with a complex, mixed-salt system that is highly prone to scaling and incapable of yielding pure single salts—producing only worthless waste mixed salt. NF is the critical decoupler that turns a dead end into two independent production lines.

Technical Advantages and Real-World Challenges

The introduction of NF salt separation brings fundamental advantages: the mass of hazardous mixed salt can be slashed from 100% to below 10%, and the recovered industrial salts help offset treatment costs, enabling true “near-zero” or even liquid zero discharge. At the same time, targeted salt-specific concentration reduces the scale and energy consumption of the crystallizers.

Of course, challenges are plain to see. NF has demanding feedwater quality requirements. The influent must be thoroughly rid of calcium, magnesium, silica, and strong oxidizing agents; otherwise, scaling on the membrane surface (particularly calcium sulfate and barium sulfate scale) will rapidly cripple membrane performance. In addition, organic fouling is another formidable enemy that NF systems must guard against, typically requiring protection through advanced oxidation or activated carbon adsorption.

Conclusion

In the technological landscape of zero liquid discharge, the nanofiltration membrane is far more than a new type of filtration material. Drawing on its unique wisdom of ion screening, it links the immense concentrating power of reverse osmosis with the precise separation ability of evaporation crystallization in flawless harmony. It acts like a master chemical splitter, routing sodium chloride and sodium sulfate to their proper places in the inorganic brine world, turning yesterday’s pollutant waste into today’s valuable resources. One could say that the presence of an efficient and stable NF salt separation stage is the golden thread that distinguishes traditional “mixed-salt ZLD” from modern “resource-recovery ZLD.”

This article is for technical popularization purposes and does not include specific project parameters. Daltonen provides nanofiltration membrane products and zero liquid discharge process design services.