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will undoubtedly be more applications since the interest in using membranes is growing even faster now than in the 1980s. Many of these successful fractionation applications are kept silent as trade secrets or only show up in issued patents or patent applications, reducing the public's understanding of how widely nanofiltration may be used in their everyday lives. Nanofiltration Today N ew and creative applications are driving the further development of NF membranes. While its siblings RO and UF continue to grow in importance for treating the world's potable water and wastewater, NF is helping in those areas and many others as well. The need to separate related chemicals in various production streams such as pesticides, biochemicals, nutraceuticals, flavorings, and pharmaceuticals is well understood in these industries; as a result, NF is increasingly brought into play. Today several important applications for producing, refining, and recovering fine chemicals, sugars, amino acids, food, feed additives, and medicines are well established (one illogical application, making clear beer, has gone away). The use of NF to replace RO or develop previously infeasible applications in water reclamation and reuse in both municipal applications and in the extraction industries (oil, gas, and mining) is now economically important. The reasons include reduced energy costs; reclamation of valuable minerals, elements, and chemicals; reduced volumes of toxic pollutants for disposal or further treatment; and lower-cost process streams suitable for direct disposal. Nanofiltration Tomorrow Although it is unlikely to reach the total area of global, installed membrane for either RO or UF, nanofiltration may well eclipse both of these classes in the number of different applications, if it has not already. The range of separations possible is not the only reason for this. Unlike the RO membrane class, comprised essentially of only two polymers, commercially available NF membranes include those of RO (cellulose acetate and polyamide-polyimides) and also more chemically resistant polymers. And more recently, ceramics companies are claiming products in the NF range. Both these material categories extend the range of NF applications significantly. For the acid, base, and solvent stable polymeric membranes, both published lab tests and proprietary industrial applications have demonstrated that these NF class membranes can economically recover both acids and metals from acidic solutions in reuse from mining and refining streams. Closer to the economic cusp are those used for recovering acids and especially bases from food processing, industrial and commercial laundry, and other cleaning solutions, thanks to the longer life they have in these harsh environments. For the emerging ceramic NF membranes, the same and better resistance (and therefore lifetime) can be expected — and, given the higher probable cost, is required to be competitive. To the extent that ceramic materials can withstand higher temperatures, stronger chemicals, and higher pressure, they will eclipse the polymers as the preferred membrane in high-value applications that can afford their higher cost. Even more and potentially bigger developments are likely in the next five to 10 years. These include improved fouling resistance and customizable separation selectivity due to the incorporation of nanomaterials, for which efforts are under way in all the membrane classes (therefore nanofiltration will still not be a term properly related to the NF membranes material of construction). Just how economically feasible and how much the benefits of this technique will be are still speculative. But one breakthrough would be of enormous benefit, and its value is assured — the development of a hollow-fiber-style NF that can be back-flushed to clean it and therefore allowed to run in direct-flow mode instead of crossflow, as with current hollow-fiber UF. This is perhaps the chief advantage ceramic NF has now, and it is probably closer to commercialization due to more favorable existing manufacturing techniques. However, regardless of any breakthrough in materials or configuration of the NF membrane itself, nanofiltration applications are destined to multiply at a rate equal to, or probably greater than, the other membrane classes. This potential alone justifies the attention that NF has today. 10 wateronline.com ■ Water Innovations Solute rejection model (credit: Tzahi Y. Cath) David Paulson is principal partner in Water Think Tank and Prime Membrane Partners, LLC. Water Think Tank provides a variety of expertise in the membrane filtration and water treatment industries. Prime Membrane Partners aids companies in establishing and expanding media and component manufacturing capabilities, critical component sourcing, strategic partnerships, and optimum positioning in the marketplace. About The Author MEMBRANES