Nanofiltration Sits Between RO and UF: The Separation Logic Behind the Membrane

Where nanofiltration sits

Nanofiltration is a membrane-separation technology driven by pressure difference that sits between reverse osmosis and ultrafiltration and retains nano-scale particles in water. Most NF membranes derive from RO membranes, including CA, CTA, aromatic polyamide composite and sulfonated polyethersulfone membranes. Because its operating pressure is lower than RO, NF is also called low-pressure RO or loose RO.

The NF membrane is a charged membrane capable of electrostatic adsorption, so under the same water quality and conditions it needs less pressure than an RO membrane. Its pore size and surface features produce different Donnan potentials for ions of different charge and valence, and its mechanism combines sieving and solution-diffusion with a charge-repulsion effect. The practical result: it effectively removes divalent and multivalent ions and substances above MW 200, and only partially removes monovalent ions and substances below MW 200. Separation performance is clearly better than UF and MF, while against RO it offers partial monovalent-ion removal, low process osmotic pressure, low operating pressure and energy savings.

Put numerically, NF pore size lies between RO and UF. It has higher removal of divalent and multivalent ions and of organics with MW 200-1000, and lower removal of monovalent ions and small molecules. Operating pressure is about 1.0 MPa, and because low monovalent and small-molecule removal means lower osmotic pressure, NF saves about 15% energy versus RO under the same conditions.

Why the charge matters more than the pore

The principle approximates mechanical sieving, but the membrane itself carries charge, which is why it keeps high desalting performance and can remove inorganic salts even at very low pressure with a membrane of several-hundred MWCO. That combination produces a distinctive feature set: it retains organics above MW 100 and multivalent ions while passing small organics and monovalent ions; it operates under harsh conditions of high temperature, acid and alkali with fouling resistance and a wide tolerance; it has a high concentration factor; and it runs at low operating pressure with high flux and low running cost, since pressure is the only driving force and energy input is minimal.

Those features make NF a green water-treatment technology that can replace traditional costly, cumbersome methods in specific places. It is used for ultrapure-water preparation, juice concentration, peptide and amino-acid separation, antibiotic concentration and purification, whey-protein concentration and NF-membrane/bioreactor coupling, and it is increasingly found in electronics, food and pharmaceuticals.

Industrial streams where it pays

Daily-chemical wastewater is a good fit. Studies show the membrane resists acid and alkali, has excellent retention and good heavy-metal removal with no fouling problem, and because NF running cost is lower than RO while it still removes small organics well, it may cover over 90% of daily-chemical wastewater treatment.

Petroleum-industry wastewater, from oil extraction and refining, carries various inorganic salts and organics and is very complex and hard to treat. Membrane methods, especially NF combined with others, can treat it and recover useful substances. NF first separates crude-oil wastewater into an oil-rich aqueous phase and an oil-free brine phase; the oil-rich phase is added to fresh supply water and enters the oil-washing step, recovering crude and saving water. Where RO plus phase separation was used before, severe fouling resulted, and adding an NF stage ahead of RO solves it. Phenolic petroleum wastewater contains phenol, cresol, nitrophenol and various substituted phenols, all highly toxic and requiring removal before discharge; NF removes phenol above 95% and at lower pressure efficiently removes high-valence heavy-metal ions such as cadmium, nickel, mercury and titanium at much lower cost than RO.

Pesticide wastewater defeats conventional methods when the pesticides are low-molecular organics. Studies on NF retention of phenol-free pesticides found that except for dichlorides, all pesticides had retention above 96.7%, and adsorption on NF was affected by hydrophobicity. NF is also very effective on phenolic-pesticide wastewater. In chemical-fiber and dyeing wastewater, NF removes and reuses dyes and auxiliaries. Treating dye polymer slurry, since most dyes have MW in the hundreds to thousands, NF passes some inorganic salts or small molecules while intercepting larger dye molecules: after the NF system the crude dye slurry is enriched in dye while salt concentration drops, with desalination above 98% and dye loss below 0.1%, and it can run at high temperature. It also treats and recycles oily wastewater from fiber processing.

Domestic sewage treatment is another application. The common biodegradation plus chemical oxidation route consumes much oxidant and leaves many residues. Placing an NF system between them lets biodegradable small molecules, MW below 100, pass while intercepting non-biodegradable macromolecules, MW above 100, for chemical oxidation and then biodegradation, exploiting biodegradation fully, saving oxidant or activated carbon and lowering final residues. In papermaking, NF replaces traditional chemical methods to remove dark lignin more effectively: chlorinated lignin from wood-pulp bleaching is negatively charged and is easily retained by the negatively charged NF membrane without fouling, and since cation removal is not strictly required, RO is unnecessary. UF and NF treatment of kraft-pulp wastewater works well.

Pickling waste liquor is a clean recovery case. Steel pickling immerses steel in about 20% sulfuric acid; as pickling proceeds, acid concentration falls and ferrous-sulfate concentration rises, and when acid drops to 6%-8% and ferrous sulfate exceeds 200-250 g/L the rate falls and the liquor must be replaced. Using NF's different retention of sulfuric acid and ferrous sulfate, ferrous sulfate is first retained in concentrate sent to a cooling crystallizer yielding FeSO4.7H2O, while the permeate passes another NF module that retains sulfuric acid and concentrates it to 20% for reuse, with its permeate going to the waste-acid station. Both acid and ferrous sulfate are recovered. Thermal-power secondary wastewater, from ash sluicing, dust removal and cooling, can also be turned into industrial reuse water: microfiltration first removes all suspended particles and 99% BOD, 98% COD, 73% total nitrogen and 17% total phosphorus, and drops total bacteria to 3-4 per litre; acid then lowers pH to remove CO2; and NF desalts to boiler-water quality. Australia's Eraring power station already does this, treating 1,000-15,000 m3/day and saving about US$800,000 a year in operating cost, with expansion to an estimated 5,000 m3/day planned by 2010.

Drinking water: the harder sell

In water supply NF mainly produces softened and potable pure water, removing hardness from Ca and Mg, trihalomethane (THM) precursors, odour, colour, pesticides, synthetic detergents, soluble organics and evaporation residues. Sibille and colleagues studied groundwater at Auvers-sur-Oise in France, comparing NF with biological treatment, ozone and biological activated carbon filtration, for drinking water. NF significantly improved water quality, reduced bacteria counts and organic concentration, made subsequent disinfection more effective and reduced chloroform formation. The study also noted that small amounts of readily bacteria-absorbed biodegradable organic matter (BOM) and assimilable organic carbon (AOC) can pass through NF membranes.

Other work piloted NF for advanced treatment of a city's tap water drawn from the heavily polluted Huai River, using a cyclic NF process. Against single-stage NF, the cyclic process at the same low pressure gave higher yield, lower energy and less concentrate discharge. Even at 80% recovery, membrane permeate TOC was still 50% lower than tap water, and mutagen removal was significant enough to turn Ames-positive water negative.

Adoption is uneven. Although NF engineering applications are widely promoted in the water-supply industry of the USA, Japan and others, in China the conditions for wide engineering use are not yet mature and it is still at the trial stage, mainly because domestic NF membrane performance indicators are not yet up to standard. There are reported cases, though: China's first industrial large-scale membrane-softening system, the Shandong Changdao Nanhuangcheng NF demonstration project, designed by the Hangzhou Water Treatment Center of the State Oceanic Administration, began producing fresh water in April 1997 and ran continuously for 27 months with desalinated water meeting the national drinking-water standard.

Three fundamental problems of membrane separation remain the reason NF water treatment is hard to apply widely, and water-treatment workers worldwide are researching solutions. Recovery of valuable substances is meanwhile a growing motive: soybean whey contains about 1% oligosaccharides and a little salt; sulfite-pulp and paper-pulp processes produce calcium-sulfite waste liquor containing 2%-2.5% hexose and pentose; and sugar-industry molasses contains a little salt. Alongside this, nano-science, which studies the special properties of matter at 0.1-100 nm, is feeding in new options such as nano-TiO2 photocatalytic oxidation, suggesting that a distinctly nano water-treatment technology is not far off.