Nanofiltration NF

Physical sieving (steric hindrance) is the dominant rejection mechanism in NF for colloids and large molecules, whereas the chemistries of solute and membrane become increasingly important for ions and lower molecular weight organics. The mechanisms, however, are still poorly understood. Macoun (1998) summarised NF rejection mechanisms as follows 

The normally negatively charged membranes may also function to a limited extent as a cation-exchange membrane (Mallevialle et al. (1996)). 

UF and RO models may all apply to some extent to NF. Charge, however, appears to play a more important role than for other pressure driven membrane processes. The Extended-Nemst Planck Equation (equation (3.28)) is a means of describing NF behaviour. The extended Nernst Planck equation, proposed by Deen et al. (1980), includes the Donnan expression, which describes the partitioning of solutes between solution and membrane. The model can be used to calculate an effective pore size (which does not necessarily mean that pores exist), and to determine thickness and effective charge of the membrane. This information can then be used to predict the separation of mixtures (Bowen and Mukhtar (1996)). No assumptions regarding membrane morphology ate required (Peeters (1997)). The terms represent transport due to diffusion, electric field gradient and convection respectively. Jsi is the flux of an ion i, Di,i is the ion diffusivity in the membane, R the gas constant, F the Faraday constant, y the electrical potential and Ki,c the convective hindrance factor in the membrane. 

The equation predicts solute rejection as a function of feed concentration, ion charge, convection across the membrane, and solute diffusion (Braghetta (1995)). The model has proven to be successful for modellii the solute transport in simple electrolyte solutions, although its applicability in the presence of organics is questionable. 

Wang et al (1995b) developed the model further to account for the transport phenomena of organic electrolytes, thus combining electrostatic and steric hindrance effects. The steric hindrance pore model suggested by Nakao et al. (1982) was incorporated into the modified Nernst Planck equation. 

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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. 

Recently, membrane filtration has become popular for treating industrial effluent. Membrane filtration includes microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse... 

Nanofiltration (NF) pressure difference removal of small organics mono/divalention separation... 

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

Nanofiltration (NF) also seems to be a promising alternative for eliminating pharmaceuticals, as it is able to achieve removal rates greater than 90% [87]. 

M. Pontie, C. Diawara, A. Lhassani, H. Dach, M. Rumeau, H. Buisson, J.C. Schrotter, Water defluoridation processes A review. Application Nanofiltration (NF) for future large-scale pilot plants, in A. Tressaud (Ed.), Advances in Fluorine Science, Vol. 2, Elsevier, Amsterdam, 2006, pp. 49-80. 

Process Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF) Reverse Osmosis (RO)... 

An excellent review of composite RO and nanofiltration (NF) membranes is available (8). These thin-film, 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-film 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-immiscible solvent. 

Vandanjon et al. [37] reported on using a combination of UF and RO, or UF and nanofiltration (NF) to remove both volatiles and non-volatiles from the cooking water of shrimp, buckies, and tuna. The preliminary use of UF is common in that it removes the larger materials that would clog the subsequent membrane steps. NF was not found to be as efficient in the recovery of volatiles as RO. Unfortunately, the authors did not evaluate the use of the recovered materials as flavourings. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

Various membrane operations are available today for a wide spectrum of industrial applications. Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), gas and vapor separation (GS, VS), pervaporation (PV), dialysis (D), electrodialysis (ED) and membrane contactors (MCs) are only some of the best-known membrane unit operations. 

As the quality of drinking water sources gets worse, the methods of water treatment or the traditional water treatment systems need to be modernized. Pressure-driven membrane systems such as reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) and electric-driven membrane system such as... 

Water Defluoridation Processes A Review. Application Nanofiltration (NF) for Future Large-Scale Pilot Plants... 

H. Dach, J. Leparc, H. Suty, C. Diawara, A. Jadas-Hecart, A. Lhassani, M. Pontie, Innovative approach for characterization of nanofiltration (NF) and low pressure reverse osmosis (LPRO) membranes for brackish water desalination, Desalination, 2006 (submitted). 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

In this chapter, the impact of other membrane technologies on the operation of RO systems is discussed. Technologies considered include microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) as pretreatment to RO, and continuous electrodeionization (CEDI) as post-treatment to RO. This chapter also describes the HERO (high efficiency RO—Debasish Mukhopadhyay patent holder, 1999) process used to generate high purity water from water that is difficult to treat, such as water containing high concentrations of silica. 

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 microporous 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 polysulfone support (see Chapter 4.2.2). 

Nanofiltration (NF) and RO are closely related in that both share the same composite membrane structure and are generally used to remove ions from solution. However, NF membranes use both size and charge of the ion to remove it from solution whereas RO membranes rely only on "solution-diffusion" transport to affect a separation (see Chapters 16.2 and 4.1, respectively). Nanofiltration membranes have pore sizes ranging from about 0.001 to 0.01 microns, and therefore,... 

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