Nanofiltration

Nanofiltration (NF) is a pressure-driven membrane separation technology used to separate ions from solution. Nanofiltration membranes were widely available beginning in the 1980 s. This technology uses micropo-rous membranes with pore sizes ranging from about 0.001 to 0.01 microns. 

Nanofiltration is closely related to RO in that both technologies are used to separate ions from solution. Both NF and RO primarily use thin-film composite, polyamide membranes with a thin polyamide skin atop a poly-sulfone support (see Chapter 4.2.2.2). 

Species Nanofiltration Rejection (%) Reverse Osmosis Rejection (%) 

Fouling and scaling mechanisms are similar for spiral-wound NF and RO membranes. In general, NF feed water should meet the following characteristics to prevent fouling with suspended solids (refer to Table 7.1 for a more detailed description of spiral-wound RO feed water requirements)  

traditional spiral wound NF membranes require the same level of pretreatment as spiral-wound RO membranes, as well as the same flux and flow rate considerations with respect to feed water quality (see Chapters 9.4 and 9.9). 

See reference 1 and 6 - 8 for more detailed discussions about ultrafiltration. 

Nanofiltration is sometimes called loose RO or leaky RO because of its similarity to RO the exception is that NF membranes allow more ions to pass through than an RO membrane.10 Because of the lower rejection of dissolved solids, the increase in osmotic pressure is not as significant with NF system as it is with RO. Thus, NF 

Nanojiltration ryection depends on solute charge, concentration and si e, as mil as membrane characteristics. Consequent, the membranes used were thoroughly characterised and then tested with salt solutions and a model organic compound. Then organics rejection as a function ofpH, organic type, salt concentration and Itydrodynamic conditions was determined. 

Rejection of four membranes was studied for salt solutions and three organic types. The TFC membranes showed a high rejection of organics, which was determined by siqe exclusion. Salt rejection varied with membrane type. The TFC-SR membrane showed a high selectivity between sodium and calcium, whereas the TFC-S and TFC-UW membranes rejected large amounts f calcium and sodium. The Ca-UF membrane refected little salt, and organics rejection varied with solution chemistry. 

The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can cause severe fouling of nanojittration (NF) membranes. Conditions offouling were studied as a function of solution chemistry, organic type, calcium concentration, hydrodynamic conditions, and membrane type. 

Deposition of organic matter was determined by mass balance in feed and concentrate samples. Electron microscopy and X-ray photoelectron pectroscopy (XPSJ were used to study the morphology and composition of the fouling layer. 

Inorganic colloids (hematite, 75 nm) did not cause irreversible flux decline. Pretreatment of the solutions using ferric chloride not only prevented flux decline under criticalfouling conditions (high calcium concentration and IHSS HA), but also influenced rejection. The latter depends on the charge of the ferric hydroxide precipitates. Cation rejection increased when positive ferric hydroxide colloids were deposited on the membrane, which the organic rjection decreased. 

In simple terms, nanofiltration membranes reject species based on size or charge of the particle, depending on the charge of the membrane. For example, cationic NF membranes have negatively- 

Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, University of Leuven, Leuven, Belgium 

In the second half of the 1990s, research on nanofiltration increased (see Fig. 11.1). As a consequence, scientists and industrialists nowadays feel more confident about what can be expected from a nanofiltration membrane, and more and more applieations proved to be successfiil. 

By 2000, the installed capacity was about 6000 ML/day, which is 10 times higher than in 1990 (Schafer et al., 2005). As research continues, membranes become better defined. 

Advanced Membrane Technology and Applications. Edited by Norman N. Li, Anthony G. Fane, W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

This chapter gives a short overview of process principles in nanofiltrafion, with a focus on the aspects that distinguish NF from RO and UF. This should allow us to determine which applications are theoretically possible, based on the composition of the liquid to be treated. Some of the most important applications of NF that are already realized will be discussed as well. The use of NF for drinking water production, which is historically the first and to date still the most important application, will be described in detail other industrial applications with proven performance will be reviewed, and a glimpse of the growing market of separations in organic solvents will be given. 

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An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

Reverse osmosis models can be divided into three types irreversible thermodynamics models, such as Kedem-Katchalsky and Spiegler-Kedem models nonporous or homogeneous membrane models, such as the solution—diffusion (SD), solution—diffusion—imperfection, and extended solution—diffusion models and pore models, such as the finely porous, preferential sorption—capillary flow, and surface force—pore flow models. Charged RO membrane theories can be used to describe nanofiltration membranes, which are often negatively charged. Models such as Dorman exclusion and the... 

Membrane Filtration. Membrane filtration describes a number of weU-known processes including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, and electro dialysis. The basic principle behind this technology is the use of a driving force (electricity or pressure) to filter... 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

Process Description Reverse osmosis (RO) and nanofiltration (NF) processes utilize a membrane that selectively restricts flow of solutes while permitting flow of the solvent. The processes are closely related, and NF is sometimes called loose RO. They are kinetic processes, not equilibrium processes. The solvent is almost always water. 

Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose hmits the practical concentration of whey to about 20 percent total sohds, about a 4x concentration fac tor. (Membranes do not tolerate sohds forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinaiy whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

The second major membrane type is a composite. Starting with a loose asymmetric membrane, usually a UF membrane, a coating is applied which is polymerized in situ to become the salt rejecflng membrane. This process is used for most high-performance flat-sheet RO membranes, as well as for many commercial nanofiltration membranes. The chemistry of the leading RO membranes is known, but... 

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

Ultrafiltration may be distinguished from other membrane operations by example When reverse osmosis is used to process whey, it passes only the water and some of the lactic acid (due to the solubihty of lactic acid in RO membranes). Nanofiltration used on whey will pass most of the sodium salts while retaining the calcium salts and most of the lactose. Microfiltration will pass everything except the particulates and the bacteria. 

Electrodialysis Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis... 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

Singh, Rajindar (M.A.E. Environmental Technologies). A Review of Membrane Technologies Reverse Osmosis, Nanofiltration and Ultrcfiltration. Ultrapure Water, Tall Oaks Publishing, Inc., USA, March 1997. 

Nanofiltration as Final Treatment Before Specific Industrial Reuse... 

FT Feed Trank, MF Microfiltration module, UV ultraviolet Lamp, NF Nanofiltration module. 

Rosberg R (1997) Ultraflltration (new technology), a viable cost-saving pretreatmen for reverse osmosis and nanofiltration - a new approach to reduce costs. Desalination 110 107-114... 

With a growing scarcity of freshwater available for irrigation, other sources of lower quality like brackish water, saline water, and treated wastewater become more important as additional or substituting inputs for the agricultural sector. At the same time, it is clear that a sophisticated treatment like desalination or nanofiltration under current conditions is still far too expensive to be a major solution to future irrigation water needs. Hence adaptation of farming and irrigation practices to the particular water qualities constitutes a more viable approach. 

Dijkstra, H.P, van Klink, G.P.M, van Koten, G. (2002) The Use of Ultra- and Nanofiltration Techniques in Homogeneous Catalyst Recycling. Acceleration Chemistry Research, 35, 798-810. 

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