Basic Wastewater Treatment Terminology: What Is Nanofiltration?
Definition 1: A membrane-separation technology driven by pressure difference that, between reverse osmosis and ultrafiltration, retains nano-scale particles in water.
It has the following two characteristics:
It is worth noting that the technologies and standards in this field are also continuously developing and improving.
Nanofiltration is mainly used for purification of drinking and industrial water, wastewater treatment, and concentration of valuable components in process fluids. Most NF membranes derive from RO membranes — e.g., CA, CTA, aromatic polyamide composite and sulfonated polyethersulfone membranes. But versus RO its operating pressure is lower, so NF is also called 'low-pressure RO' or 'loose RO'.
An NF membrane is a charged membrane capable of electrostatic adsorption. Under the same water quality and conditions it needs less pressure than an RO membrane. So in separation principle NF is both similar to and different from RO. Its pore size and surface features determine its unique performance and give different Donnan potentials for ions of different charge and valence; its mechanism combines sieving and solution–diffusion with a charge-repulsion effect, effectively removing divalent and multivalent ions and substances above MW 200, partially removing monovalent ions and substances below MW 200. NF's separation performance is clearly better than UF and MF, and versus RO it offers partial monovalent-ion removal, low process osmotic pressure, low operating pressure and energy savings [2].
As a novel membrane-separation technology, nanofiltration's principle approximates mechanical sieving. But the NF membrane itself carries charge, which is why it retains high desalting performance and can remove inorganic salts even at very low pressure with a membrane of several-hundred MWCO.
Nanofiltration is increasingly used in electronics, food and pharmaceuticals — e.g., ultrapure-water preparation, juice concentration, peptide and amino-acid separation, antibiotic concentration/purification, whey-protein concentration, and NF-membrane/bioreactor coupling. Versus UF or RO, NF poorly retains monovalent ions and organics below MW 200 but has higher removal of divalent/multivalent ions and organics of MW 200–500; based on this it is mainly applied to water softening, purification, and separation/grading/concentration of substances of roughly hundred-level MW (e.g., dyes, antibiotics, peptides, polysaccharides), plus decolorization and deodorization. It is mainly used in drinking water to remove hardness (Ca, Mg), trihalomethane precursors, odor, color, pesticides, synthetic detergents, soluble organics and evaporation residues.
As awareness of environmental protection and resource comprehensive utilization rises, people hope to recover valuable substances while treating wastewater — e.g., 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, etc.
NF separation is a green water-treatment technology that in some aspects can replace traditional costly, cumbersome methods. Its features: retains organics above MW 100 and multivalent ions while passing small organics and monovalent ions; operates under harsh conditions of high temperature, acid and alkali with fouling resistance; low operating pressure, high flux, low running cost; and can combine with other processes to further cut cost and improve performance. In water treatment NF membranes are mainly used for solvent-containing wastewater, effectively removing color, hardness and odor, and have been successfully applied to wastewater in sugar, pulp and paper, electroplating, machining and chemical-catalyst recovery.
Nanofiltration is a green water-treatment technology and the latest international development in membrane separation, replacing in some aspects traditional costly, cumbersome methods. Its nano-scale charged pores specially filter by retaining organics above MW 200 and multivalent ions while passing small organics and monovalent ions; it runs under harsh high-temperature, acid and alkali conditions with wide tolerance, high concentration factor and fouling resistance; low operating pressure, high flux, low running cost and minimal energy (pressure is the only driving force).
NF membranes have excellent thermal stability, acid, alkali and solvent resistance, playing an immeasurable role in valuable-substance recovery from wastewater, widely applied to various organic-wastewater recoveries — e.g., pesticide-waste liquor treatment, whey and antibiotic desalination, metal recovery from electroplating liquor, and various petrochemical wastewaters. In water supply, NF mainly produces softened and potable pure water, effectively removing color, hardness and odor [2].
(1) Daily-chemical wastewater treatment. Studies on NF for daily-chemical wastewater show the membrane resists acid/alkali, has excellent retention and good heavy-metal removal with no fouling problem. It is estimated that because NF running cost is lower than RO and it well removes small organics, it may cover over 90% of daily-chemical wastewater treatment.
(2) Petroleum-industry wastewater treatment.
Petroleum-industry wastewater mainly includes streams from oil extraction and refining containing various inorganic salts and organics, very complex and hard to treat. Membrane methods, especially NF combined with others, can both treat wastewater and recover useful substances. For example, 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. Previously RO plus phase separation was used but suffered severe fouling; adding an NF stage before RO solves it. Phenolic petroleum wastewater mainly contains phenol, cresol, nitrophenol and various substituted phenols, highly toxic and must be removed before discharge; NF not only removes phenol above 95% but at lower pressure efficiently removes high-valence heavy-metal ions like cadmium, nickel, mercury and titanium at much lower cost than RO.
(3) Pesticide wastewater treatment. Conventional methods cannot remove low-molecular organic pesticides. Studies on NF retention of phenol-free pesticides found that except dichlorides, all pesticides had retention above 96.7%, and all pesticides' adsorption on NF was affected by hydrophobicity. NF is also very effective for phenolic-pesticide wastewater.
(4) Chemical-fiber and dyeing wastewater treatment. NF can remove and reuse dyes and auxiliaries from dyeing discharge. Treating dye polymer slurry, since most dyes have MW 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 >98% and dye loss <0.1%, and it can run at high temperature. In addition NF can treat and recycle oily wastewater from fiber processing.
(5) Domestic sewage treatment. The common biodegradation + chemical oxidation method consumes much oxidant and leaves many residues. Adding an NF system between them lets biodegradable small molecules (MW <100) pass while intercepting non-biodegradable macromolecules (MW >100) for chemical oxidation then biodegradation, fully exploiting biodegradation, saving oxidant or activated carbon and lowering final residues.
(8) Papermaking wastewater treatment. NF replaces traditional chemical methods to more effectively remove dark lignin. Chlorinated lignin from wood-pulp bleaching is negatively charged and easily retained by the negatively charged NF membrane without fouling. Also, since cation (Na+) removal is not strictly required, RO is unnecessary. UF/NF treatment of kraft-pulp wastewater works well.
(7) Pickling-waste-liquor treatment. Steel pickling immerses steel in about 20% sulfuric acid; as pickling proceeds, acid concentration falls and ferrous-sulfate concentration rises; when acid drops to 6%–8% and ferrous sulfate exceeds 200–250 g/L, the rate falls and the liquor must be replaced and discharged. Pickled steel is rinsed with fresh water, also producing waste acid. To protect the environment and save resources, NF can treat the liquor. 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; the permeate passes another NF module retaining sulfuric acid, concentrated to 20% for reuse, while its permeate goes to the waste-acid station for further treatment/discharge. This process recovers both sulfuric acid and ferrous sulfate and achieves comprehensive reuse and compliant discharge.
(6) Treatment and reuse of thermal-power secondary wastewater. This wastewater mainly comes from ash sluicing, dust removal and cooling systems, containing much suspended solids, ash, high salts and some organics. NF can turn it into industrial reuse water. First microfiltration removes all suspended particles and 99% BOD, 98% COD, 73% total nitrogen and 17% total phosphorus, and drops total bacteria to 3–4/L; then acid lowers pH to remove CO2; finally NF desalts to boiler-water quality. Australia's Eraring power station already uses NF for this, treating 1,000–15,000 m³/day, easing municipal supply and saving the plant US$800,000/year in operating cost. The plant plans to expand, raising treated volume to an estimated 5,000 m³/day by 2010, with very considerable benefits.
NF pore size lies between RO and UF; it has higher removal of divalent/multivalent ions and organics of MW 200–1000, but lower removal of monovalent ions and small molecules. Versus RO, NF operating pressure is lower (about 1.0 MPa); and because its low monovalent/small-molecule removal gives lower osmotic pressure, under the same conditions NF saves about 15% energy versus RO [3]. Thus in water treatment NF is widely used for drinking-water concentration/purification, water softening, desalting and concentrating organics and bioactive substances, removing THM precursors, grading and concentrating organics of different MW, and wastewater decolorization.
Sibille et al. studied groundwater in Auvers-sur-Oise, France, comparing NF with biological treatment (ozone–biological activated carbon filtration) for drinking water. Results show NF significantly improves water quality, reduces bacteria counts and organic concentration, making subsequent disinfection more effective and reducing chloroform formation. But the study also notes that small amounts of readily bacteria-absorbed biodegradable organic matter (BOM: biological organic matter) and assimilable organic carbon (AOC) can pass through NF membranes.
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; the main problem is that domestic NF membranes' performance indicators are not yet up to standard. There are reported engineering cases, e.g., China's first industrial large-scale membrane-softening system — the Shandong Changdao Nanhuangcheng NF demonstration project — a real application of NF to high-hardness island brackish-water purification. Designed by the Hangzhou Water Treatment Center of the State Oceanic Administration, it began producing fresh water in April 1997 and ran continuously and normally for 27 months, with desalinated water meeting the national drinking-water standard.
Scholars used NF to pilot advanced treatment of a city's tap water (with heavily polluted Huai River water as raw water), studying a cyclic NF process. Results show that versus single-stage NF, the cyclic process at the same low pressure has higher yield and lower energy and less concentrate discharge. Even at 80% recovery, membrane permeate TOC is still 50% lower than tap water; mutagen removal is significant, turning Ames-positive water negative.
NF membranes have high flux, can retain organic and inorganic pollutants, yet have high passage for some essential ions, so applying NF to drinking-water advanced purification has greater advantages than other membrane technologies. The main problems of applying NF membranes to water supply are:
These three problems are fundamental to membrane separation and the main reason NF water-treatment is hard to widely apply. Water-treatment workers worldwide are researching broadly for solutions. Wider adoption of NF in water supply depends on further solving these problems.
NF membranes separate organic small molecules of several-hundred MW and well remove color, hardness and odor, with low operating pressure and high water flux, so they will play a huge role in water treatment. Much remains in NF preparation, characterization and mechanism; cheap, high-performance membranes and accurate performance parameters for users are still needed — all key to NF in wastewater treatment and other applications.
Nano-TiO2 photocatalytic oxidation technology — introducing nano-science, especially nano-TiO2 photocatalytic oxidation and NF membrane technology, their principles, roles and application methods in water treatment — suggests the novel nano water-treatment technology is not far off. Nano-science studies the special properties of matter at 0.1–100 nm and how to use them. Broadly, nanomaterials are those with at least one dimension in the nano range or built from such units. Nanomaterials differ greatly from ordinary materials in mechanical, magnetic, optical, electrical and thermal properties, possessing new characteristics of radiation, absorption, catalysis and adsorption. Many scientists have studied these characteristics and their effect on certain water pollutants, indicating nano-science may bring breakthrough changes to water-treatment technology.
