NF membrane

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. 

Membrane Limitations Chemical attack, fouling, and compaction are prominent problems with RO and NF membranes. Compaction is the most straightforward. It is the result of creep, slow cold flow of the polymer resulting in a loss of water permeability. It is measured by the slope of log flux versus log time in seconds. It is independent of the flux units used and is reported as a slope, sometimes with the minus sign omitted. A slope of—0.001, typical for noncelhilosic membranes, means that for every threefold increase in log(time), 10 seconds, a membrane looses 10 percent of its flux. Since membranes are rated assuming that the dramatic early decline in permeability has already occurred, the further decline after the first few weeks is veiy slow. Compaction is specific to pressure, temperature, and envi-... 

Chemical attack is often a result either of fouling prevention or cleaning in response to fouling. Chlorine and hypochlorite damage most RO and NF membranes, as do oxidants generally (see discussion of chlorine tolerance below). 

Membrane Chemistry Three chemical families dominate the RO-NF membrane industry. Many other products are made on a small scale, and the field continues to attract significant R D resources. But three types command most of the market. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Effluent pretreatment is necessary when RO is used as tertiary treatment in order to prevent membranes filters form being blocked or abraded. UF offers a powerful tool for the reduction of fouling potential of RO/NF membranes [57]. A typical pretreatment consist of a MF allowing the removal of the large suspended solids form the WWTP effluent followed by UF unit which removes thoroughly suspended solids, colloidal material, bacteria, viruses and organic compounds from the filtrated water. The UF product is sent to the RO unit where dissolved salts are removed. 

There is a considerable interest in using y-alumina NF membranes. Grib et al. 2000 have reported amino acid retention in such membranes with four amino acids. The charge effects... 

NF membranes have been used to remove pesticides from aqueous streams, as their molecular weight invariably is more then 200. Kiso et al. (2000) have studied the rejection properties of a number of pesticides, and it appears that this method is very promising. 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Fig. 4. Application of dendritic ligand 2 in the continuous allylic alkylation of allyl trifluoroacetate and sodium diethyl 2-methyhnalonate in a membrane reactor (Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da) 18a).

Fig. 9. Hydrovinylation in a CFMR using 6a (a) and 6b (b) Conditions T = 23°C, p = 30 bar, flow rates ethene solution 2.5mLh (lOM), styrene solution 2.5mLh (1.8M), r = 4h, MPF-60 NF membrane (Koch Int., Diisseldorf, Ger.). Y, space time yield (mgL for 6a 0.05mmol Pd,...

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. 

Tang, C. Y., and Leckie, J. O. (2007). Membrane independent limiting flux for RO and NF membranes fouled by humic acid. Environ. Sci. Technol. 41,4767 773. 

These effects were observed for both polymeric and ceramic NF-membranes, showing that differences in rejection are not due to swelling. Nevertheless, swelling effects have been demonstrated by Tarleton et al. [82, 83] and are known to affect transport in polymeric membranes. 

Ultrafiltration and microfiltration membranes produce high porosities and pore sizes in the range of 30-100 nanometers (UF) and higher (MF), which enable the passage of larger dissolved particles and even some suspended particles. The separation-filtration mechanism is based on molecule/particle sizes. The nanofiltration membrane lies between the UF and RO membranes, combining the properties of both so that the two mechanisms coexist. In addition, the NF membrane may be... 

Nanofiltration membranes are used to remove hardness from drinking water [4,5]. They may also be used to remove other unwanted dissolved species, even the partial removal of nitrates from ground water. It was recently shown that RO and NF membranes may be backwashed by direct osmotic pressure to clean membrane surfaces, a simple and very beneficial technique [6, 7]. 

Many treatment facilities at different locations were installed to produce water from wastewater for different uses. In some cases, MF membranes are used directly on strained wastewater to remove suspended particles that are too large for the gap between two membranes [30], Simple wastewater-treatment facilities in Europe exist along all large rivers. Secondary treated waters flowing into the rivers are again pumped at a distance of about 200 meters downstream, treated with active carbon and UF membranes, disinfected and then distributed to the system. This is wastewater treatment without an RO section due to the low salinity of the water. The process cannot handle dissolved medicines, hormones, drugs, and other contaminants that could be removed with RO or NF membranes. In some cases, NF membranes are used for better treatment of the water. Information on wastewater costing may be found in Adham et al. [31]. 

Similar properties are needed for other types of membranes in use such as MF, UF, and NF membranes. Improved membranes may be used in other separation processes, not necessarily related to water [46-48],... 

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