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N a n o f i l t r a t i o n : Nanofiltration: T h e U p - A n d - C o m i n g M e m b r a n e P r o c e s s The Up-And-Coming Membrane Process The forgotten child of the membrane family has plenty of capabilities and potential. Learn the factors and applications that are increasing the popularity of nanofiltration. By David Paulson N anofiltration, a membrane-mediated process, is enjoying a resurgence of attention throughout the marketplace, including the potable, water reuse, and industrial process sectors. But misconceptions frequently appear about where it came from, how long it has been around, and even what the definition of nanofiltration is. A better understanding of how nanofiltration relates to its sibling processes of reverse osmosis and ultrafiltration, its history, and the potential that its current successful applications imply will make it even more useful. The pressure-driven membrane processes are divided into four classes: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). Probably because its separation characteristics bridge the salt-rejecting and salt-passing classes, nanofiltration seems to have the most confusion in the literature and among application engineers and is inconsistently defined even in written standards. Understanding how NF was developed helps understand what it is. Brief History Of Membrane Filtration The modern microfiltration membrane was conceived in the early 1900s, but was consistently manufactured as the artificial, polymeric membrane we know today following World War II and has become increasingly essential in medicine, pharmaceutical production, and microbiology. RO was the next class of membranes developed and was conceived in the 1950s, developed in the '60s, and commercialized in the early '70s. Its target use was making drinking water from brackish water and seawater. Soon after, UF was developed and commercialized, and UF fit nicely between the salt-rejecting RO and salt-passing, particle-retaining MF. Both RO and UF needed to run in the crossflow mode to be economically viable, which created a processing liability in some cases, but the technology was a major advance. Historically, it has been understood that UF class membranes allow complete passage of ionic species, but retain uncharged solutes above 5,000 or 10,000 molecular weight through simple sieving. It was also accepted that RO membranes remove high levels of ionic species using a more complex mechanism. So, both classes of membranes removed uncharged solutes, with the "tighter" RO removing down to below 150 molecular weight (MW) while UF membranes could reliably remove solutes down to 10,000 MW for the tightest versions, and "looser" (larger pore) versions would pass such molecules but remove "macromolecular" solutes and colloids (as well as virus and bacteria). Need For Nanofiltration There was a tight membrane class (RO) that would remove (reject) essentially all salt ions and most uncharged organic solutes, and a group of looser membranes (UF) with smallest pore sizes from 10,000 up to 300,000-plus molecular weight cutoff (MWCO), and they worked well for many applications. But what about all those uses that both industry and science envisioned for separating one solute molecule from another in the common and valuable 500 to 10,000 MW range? And what if the purpose was not just to purify water? What membrane could a company use to desalt a protein or dye broth, to remove sugar from protein or plant matter, or to separate a mono- from a trisaccharide, etc.? An increasing need pushed the membrane manufacturers to modify their membranes to fill this gap. A few pioneering membrane companies, especially those that focused on industrial and process applications, reacted by developing and testing what were first called "loose RO" and "RO/UF hybrid" membranes. Hence, a new category of membrane performance was realized. Further, what if the ions (dissolved salts) that a municipality wanted removed for potable water were only the hardness ions, and the added pressure that the tight RO membranes required was an unnecessary cost? The earliest documented application of NF was a potable water application in Florida in the late 1970s and was probably the first commercial, intentional use of 8 The filtration spectrum (credit: Koch Membrane Systems) wateronline.com ■ Water Innovations