Wastewater Treatment Glossary: Nanofiltration
Definition 1: a membrane separation technology driven by a pressure differential which sits between reverse osmosis and ultrafiltration and retains nanometer-sized particles from water.
It has the following two characteristics:
Beyond this, a range of additional factors must be taken into account in actual engineering applications.
Nanofiltration is mainly used to purify drinking water and industrial water, to purify wastewater, and to concentrate valuable components in process fluids. Most nanofiltration membranes are derived from reverse osmosis membranes, such as CA and CTA membranes, aromatic polyamide composite membranes and sulfonated polyethersulfone membranes. Compared with reverse osmosis, however, the operating pressure is lower, which is why nanofiltration is also called low-pressure reverse osmosis or loose RO.
Nanofiltration membranes are charged membranes capable of electrostatic adsorption. Producing water under the same water quality and environmental conditions, a nanofiltration membrane requires lower pressure than a reverse osmosis membrane. In terms of separation principle, nanofiltration therefore resembles reverse osmosis in some respects and differs from it in others. The pore size and surface characteristics of nanofiltration membranes determine their distinctive performance, and they exhibit different Donnan potentials toward ions of different charge and valence. Their separation mechanism combines sieving with solution-diffusion while also involving charge repulsion, allowing them to remove divalent and multivalent ions effectively and to remove substances with a molecular weight above 200, and to partially remove monovalent ions and substances with a molecular weight below 200. The separation performance of nanofiltration membranes is clearly superior to ultrafiltration and microfiltration, while compared with reverse osmosis membranes they offer partial removal of monovalent ions, low process osmotic pressure, low operating pressure and energy savings [2].
As a new membrane separation technology, nanofiltration works on a principle close to mechanical sieving. However, the nanofiltration membrane itself carries a charge. This is the important reason why it still delivers high desalination performance at very low pressure, and why membranes with a molecular weight cut-off of only a few hundred can also remove inorganic salts.
Nanofiltration is being applied ever more widely in industries such as electronics, food and pharmaceuticals, in practical separation processes including ultrapure water preparation, high-level juice concentration, peptide and amino acid separation, antibiotic concentration and purification, whey protein concentration, and nanofiltration membrane coupled with biochemical reactors. Compared with ultrafiltration or reverse osmosis, nanofiltration retains monovalent ions and organics with a molecular weight below 200 relatively poorly, but achieves high rejection of divalent and multivalent ions and of organics with molecular weights between 200 and 500. Based on this characteristic, nanofiltration is mainly applied to water softening and purification; to the separation, fractionation and concentration of substances with relative molecular masses in the hundreds, such as the fractionation and concentration of dyes, antibiotics, peptides and polysaccharides and other chemical and bioengineering products; and to decolorization and deodorization. In drinking water it is chiefly used to remove hardness components such as Ca and Mg ions, trihalomethane intermediates, off-flavors, color, pesticides, synthetic detergents, soluble organics and evaporation residues.
As awareness of environmental protection and comprehensive resource utilization continues to rise, people hope to recover valuable substances while treating wastewater. For example, soybean whey wastewater contains about 1% oligosaccharides and a small amount of salt; the calcium sulfite waste liquid produced when making chemical fiber pulp and paper pulp by the sulfite process contains 2% to 2.5% hexoses and pentoses; and the waste molasses generated in the sugar industry contains a small amount of salt.
NF separation is a green water treatment technology that can in certain respects replace conventional wastewater treatment methods that are costly and procedurally complex. Its technical features are: it retains organics with a molecular weight above 100 as well as multivalent ions while allowing small organic molecules and monovalent ions to pass; it can run under harsh conditions such as high temperature, acid and alkali and resists fouling; it operates at low pressure with high membrane flux and low plant operating costs; and it can be combined with other wastewater treatment processes to further reduce cost and improve results. In water treatment, NF membranes are mainly used to treat solvent-bearing wastewater and can effectively remove color, hardness and off-flavors from water. Thanks to its special separation performance, NF has been applied successfully to wastewater treatment in sugar refining, pulp and paper, electroplating, machining and the recovery of chemical reaction catalysts.
Nanofiltration is a green water treatment technology and represents the latest development in membrane separation internationally; in certain respects it can replace conventional wastewater treatment methods that are costly and procedurally complex. The special filtration performance of its nanometer-scale, charged pores means that it retains organics with a molecular weight above 200 as well as multivalent ions while allowing small organic molecules and monovalent ions to pass; that it can run under harsh conditions such as high temperature, acid and alkali, tolerating a wide range of conditions, achieving high concentration ratios and resisting fouling; and that it operates at low pressure with high membrane flux, low plant operating costs and extremely low energy consumption, pressure being the only driving force.
Nanofiltration membranes possess excellent properties such as thermal stability and resistance to acid, alkali and solvents, and play an invaluable role in recovering valuable substances from wastewater. They are widely applied in the recovery and treatment of all kinds of organic wastewater, for example pesticide waste liquid treatment, desalination of whey and antibiotics, metal recovery from electroplating waste liquid, and treatment of various petrochemical wastewaters. In water supply treatment, nanofiltration membranes are mainly used to prepare softened water and purified drinking water, effectively removing color, hardness and off-flavors from water [2].
Sibille and colleagues studied the groundwater of Auvers-sur-Oise in France, comparing nanofiltration with biological drinking water treatment using ozone and biological activated carbon filtration. The results showed that nanofiltration can significantly improve drinking water quality, reducing bacterial counts and organic matter concentrations and thereby making subsequent disinfection more effective while also reducing the formation of chloroform. However, the study also noted that small amounts of readily assimilated biodegradable organic matter (BOM) and assimilable organic carbon (AOC) can also pass through the nanofiltration membrane.
(2) Petroleum industry wastewater treatment.
Petroleum industry wastewater mainly comprises wastewater containing various inorganic salts and organics generated during crude oil extraction and refining. Its composition is extremely complex and it is difficult to treat. Combining membrane methods, NF in particular, with other methods can both treat the wastewater effectively and recover useful substances. For instance, an NF membrane can first separate crude oil wastewater into an oil-rich aqueous phase and an oil-free brine phase; the oil-rich phase is then added to fresh feed water and returned to the oil-washing step, so that crude oil is recovered and water is saved at the same time. Previously, reverse osmosis combined with phase separation was often used for petroleum industry wastewater, but severe membrane fouling was a problem; adding an NF membrane ahead of reverse osmosis solves it. Phenolic wastewater from the petroleum industry mainly contains phenol, cresol, nitrophenol and various substituted phenols; these substances are highly toxic and must be removed before discharge. With NF technology, phenol removal can exceed 95%, and high-valence heavy metal ions such as cadmium, nickel, mercury and titanium can be removed efficiently at relatively low pressure, at a far lower cost than reverse osmosis and similar methods.
(3) Pesticide wastewater treatment. Ordinary water treatment methods cannot remove low-molecular-weight organic pesticides from contaminated water. Studies of the rejection performance of NF membranes toward non-phenolic pesticides found that, apart from dichlorides, rejection rates for all pesticides exceeded 96.7%, and the adsorption capacity of every pesticide on the NF membrane was influenced by its hydrophobicity. NF is also highly effective for treating wastewater containing phenolic pesticides.
(4) Chemical fiber and dyeing wastewater treatment. NF can be used to remove and reuse dyes and auxiliaries from dyeing process effluent. When treating dye polymerization slurries, since most dyes have molecular weights from several hundred to several thousand, the NF membrane lets some inorganic salts or small molecules pass while retaining the larger dye molecules. After a crude dye slurry passes through an NF system the dye is concentrated while the inorganic salt concentration falls: desalination exceeds 98% and dye loss is below 0.1%, and the system can operate at high temperature. In addition, NF can be used to treat and recycle oily wastewater from fiber processing.
(5) Domestic sewage treatment. When domestic sewage is treated by the common combination of biodegradation and chemical oxidation, oxidant consumption is high and residues are numerous. Inserting an NF system between the two steps allows small molecules that microorganisms can degrade, with a molecular weight below 100, to pass, while non-biodegradable large organic molecules with a molecular weight above 100 are retained, chemically oxidized and then biodegraded. This gives full play to biodegradation, saves oxidant or activated carbon, and lowers the final residue content.
(6) Treatment and reuse of secondary wastewater from thermal power plants. Secondary wastewater from thermal power plants comes mainly from ash flushing, dust removal and cooling systems, and contains large amounts of suspended solids and ash together with high salt content and some organics. NF can turn this wastewater into industrial reuse water. Microfiltration first removes all suspended particles from the water, along with 99% of BOD, 98% of COD, 73% of total nitrogen and 17% of total phosphorus, while reducing the total bacterial count to 3 to 4 per liter; acid is then added to lower the pH and remove CO2; finally NF desalination brings the water up to boiler feedwater quality. The Eraring power station of Australia's Pacific Power has used NF to treat such wastewater, processing 1,000 to 15,000 m3 per day. This both eases the load on the municipal water supply system and saves the plant US$800,000 in operating costs each year. The plant planned to expand its generating capacity with a corresponding increase in water use; by 2010 the volume of such wastewater treated was estimated to reach 5,000 m3/d, with extremely considerable benefits.
(7) Pickling waste liquid treatment. In a steel mill's pickling process, steel is immersed in a sulfuric acid pickling tank with a mass fraction of about 20%. As pickling proceeds the sulfuric acid concentration gradually falls while the ferrous sulfate concentration keeps rising. When the sulfuric acid mass fraction drops to 6% to 8% and the ferrous sulfate produced exceeds 200 to 250 g/L, the pickling rate declines and the pickling liquid must be replaced and the spent liquid discharged. The pickled steel must then be rinsed with clean water to remove surface acid, producing further acidic wastewater discharge. To protect the environment and conserve resources, an NF process can be used to treat pickling waste liquid. Exploiting the difference in NF membrane rejection between sulfuric acid and ferrous sulfate, ferrous sulfate is first retained in the concentrate, which is then sent to a cooling crystallization tank to crystallize FeSO4-7H2O; the permeate passes through another NF module capable of retaining sulfuric acid and is concentrated to 20% sulfuric acid for reuse, while that permeate goes to the waste acid water station for further treatment before discharge or recovery. This process recovers both sulfuric acid and ferrous sulfate while achieving comprehensive reuse of the pickling waste liquid and compliant discharge of waste acid water.
(8) Papermaking wastewater treatment. Replacing conventional chemical treatment with NF membrane technology removes dark-colored lignin more effectively. The chlorinated lignin produced during wood pulp bleaching is negatively charged and is easily retained by the negatively charged NF membrane without fouling it. Moreover, since there is no strict requirement for cation (Na+) removal throughout the treatment process, reverse osmosis is unnecessary. Ultrafiltration combined with nanofiltration works very well for treating kraft paper manufacturing wastewater.
The pore size of nanofiltration membranes lies between that of reverse osmosis and ultrafiltration membranes. They achieve high rejection of divalent and multivalent ions and of organics with molecular weights between 200 and 1,000, but lower rejection of monovalent ions and small molecules. Compared with reverse osmosis, the operating pressure of nanofiltration is lower, generally around 1.0 MPa; at the same time, because nanofiltration membranes reject monovalent ions and small molecules poorly, the process osmotic pressure is small, so under identical conditions nanofiltration saves about 15% energy compared with reverse osmosis [3]. In water treatment, nanofiltration is therefore widely applied to the concentration and purification of drinking water, water softening, desalination and concentration of organics and bioactive substances, removal of trihalomethane precursors from water, fractionation and concentration of organics of different molecular weights, and wastewater decolorization.
(1) Household chemical wastewater treatment. Applied research on treating household chemical wastewater with NF membranes shows that NF membranes resist acid and alkali, offer excellent rejection and remove heavy metals very well, with no membrane fouling problem. It is estimated that, because NF operating costs are lower than those of reverse osmosis and rejection of small organic molecules is good, NF may cover more than 90% of household chemical wastewater treatment.
Although the engineering application of nanofiltration has already been rolled out on a large scale in the water supply industry of countries such as the United States and Japan, in China the conditions for applying nanofiltration widely in engineering practice are not yet mature and it remains at the trial stage; the main problem is that the performance indicators of domestically produced nanofiltration membranes are not yet good enough. Engineering examples have nevertheless been reported, such as China's first industrial large-scale membrane softening system, the Nanhuangcheng nanofiltration demonstration project on Changdao in Shandong, a practical application of nanofiltration to purifying high-hardness brackish island water. Designed by the Hangzhou Water Treatment Center of the State Oceanic Administration, the project formally began producing fresh water in April 1997, and the system ran normally and continuously for 27 months, with the desalinated water meeting the national hygienic standard for drinking water.
Researchers once ran advanced treatment trials using nanofiltration membranes on the tap water of a city, with raw water taken from the heavily polluted Huai River, studying the effectiveness of a recirculating nanofiltration water production process. The results showed that, compared with a single-pass nanofiltration process, the recirculating process achieved a higher water yield at the same low pressure while lowering energy consumption and reducing concentrate discharge. Even at a high recovery rate of 80%, the total organic carbon (TOC) in the membrane effluent was still 50% lower than in tap water; removal of mutagenic substances was very marked, turning water that tested positive in the Ames test negative.
Nanofiltration membranes offer high membrane flux and can retain organic and inorganic pollutants while allowing relatively high passage of certain ions essential to the human body. Applying nanofiltration membranes to the advanced purification of drinking water therefore has considerable advantages over other membrane separation technologies. The main problems in applying nanofiltration membranes to water supply treatment are:
These three problems are the fundamental problems of membrane separation and the main reason why nanofiltration-based water treatment technology is difficult to apply widely. Water treatment specialists around the world are conducting extensive research to find ways of solving them. Wider application of nanofiltration in water supply treatment still depends on further progress on these issues.
NF membranes can separate small organic molecules with molecular weights of a few hundred from water and remove color, hardness and off-flavors very effectively, and they operate at low pressure with high water flux, so they will play a major role in water treatment. Many technical problems still need to be solved in the preparation, characterization and separation mechanisms of NF membranes; inexpensive membranes with excellent performance still need to be developed, and users must be supplied with accurate membrane performance parameters. All of this is key to nanofiltration in wastewater treatment and other applications.
Nano-TiO2 photocatalytic oxidation technology: this section introduces the principles of nanotechnology, particularly nano-TiO2 photocatalytic oxidation and nanofiltration membrane technology, together with their role and methods of application in water treatment, and argues that the application of brand-new nanoscale water treatment technologies is not far off. Nanotechnology research concerns the special properties of matter within the 0.1 to 100 nm scale range and how to exploit them. Broadly speaking, nanomaterials are materials in which at least one dimension in three-dimensional space falls within the nanometer scale, or which are built from such units. Nanomaterials differ greatly from ordinary materials in mechanical, magnetic, optical, electrical and thermal properties, exhibiting new characteristics such as radiation, absorption, catalysis and adsorption. Many scientists have studied these properties of nanomaterials and their effect on certain pollutants in water bodies, indicating that nanotechnology may bring about breakthrough changes in water treatment technology.
