Water. Desalination + reuse
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TECHNOLOGY August-September 2015 | Desalination & Water Reuse | 31 | wastewater treatment and signals ceramics' growing competitiveness and potential for breakthrough into the wastewater treatment market. EarLY sTaGE iNNOvaTiON While key established producers continue to strive to reduce the cost of conventional ceramics to compete with polymeric membranes, new entrant companies around the world have, over the past few years, METaL MEMbraNEs Metal Membranes takes a thin sheet of aluminum or titanium and exposes it to a plasma arc which forms a metal-oxide – ceramic layer of either aluminum oxide or titanium dioxide. The reverse of the metal plate is then etched to leave just the ceramic layer exposed. A metal plate is then used as a structural support for the ceramic layer. One of the potential advantages of this manufacturing approach is that it creates very thin, layered ceramic membranes. The company says it can produce pore sizes of 1nm (0.001µm) in diameter. NaNOsuN NanoSun has developed a novel, flexible microfiltration membrane based on titanium dioxide nanofibres with a pore size of 0.1µm – 1µm. Laboratory testing has shown the membrane to have key advantages including: • very high flux – up to 960 lmh; • low fouling rates and a high temperature tolerance that make it possible to heat it to 250 o C to burn off organic foulants; and • the capacity for photocatalytic oxidation once the titanium dioxide nanofibres are treated with ultraviolet radiation; • the titanium dioxide nanofibre membrane has inherent biocidal properties. NanoSun's membrane is fabricated using a 3D-printing technology developed at Nanyang Technological University. The nanofibres are made from an electrospinning process. Although there are already titanium dioxide-based ceramic membranes available, there are no commercial titanium dioxide nano-composite membranes on the market. This suggests that if the membrane qualities claimed by NanoSun can be validated, the technology could be highly disruptive to the membrane market. Current testing has been done using flat-sheet plate and frame membranes however the flexible nature of the membrane is enabling NanoSun to explore spiral-wound forms. CEraHELix The Helix NFM is a ceramic nanofiltration membrane with pore sizes of about 1nm. The membranes are based on a non-crystalline titania ceramic that enables Cerahelix to narrow the pore size range. Titania is more expensive than alumina and has relatively high permeability, high hydrophilicity, is resistant to a wide pH range, and is suitable for ultrafiltration, nanofiltration and as a substrate. The membrane layer is produced by exploiting a property of the genetic material, DNA, which causes it to align in a silica or titanium oxide sol gel to form columns. The gel is applied to an alumina substrate and sintered to form the active ceramic membrane layer. During sintering the DNA strands are burned off to leave straight pore channels with diameters in a narrow range. The straight channels and thinness of the active membrane layer holds the promise of better flux rates than other ceramic membranes. Current pilot units consist of tubular or multi- channel membranes operated in cross-flow. Cerahelix has produced flat-sheet membranes as well and, for high-volume applications, will offer monoliths with more channels than its current multi-channel systems. LaNCE ENErGY sErviCEs Lance Energy Services has developed a novel chemical treatment to produce "nano-functionalized ceramic membranes". The majority of testing to date has been with 0.2µm pore size membranes and the company's focus has been entirely on treating oily wastewater in the oil and gas industry. The company says the chemical treatment does not simply coat the ceramic membrane but soaks into its matrix. So the treatment does not wear out or disassociate from the membrane. The chemical treatment makes the membrane strongly hydrophilic and "organo-phobic" which means it prevents oil and bacteria from attaching to the membrane surface. According to the manufacturer this oil-repelling characteristic arises from the formation of an aqueous layer on the surface of the membrane giving it greater anti-fouling properties. The membrane modification can be applied to ceramic membranes of various form factors. developed novel methods for producing ceramic membranes (see box Going forth). Ceramic membranes are typically produced by applying a slurry of oxide particles and solvents onto a supporting substrate and sintering the membrane in a kiln. It is a multi-step process involving successively finer layers on a substrate to produce smaller pore sizes while polymeric membranes can be produced in a single step. This manufacturing difference is the key source of the cost gap between ceramic and polymeric membranes. LONG-TErM suCCEss iNdiCaTEd Growing numbers of installations by major technology providers like Metawater, as well as new innovations that look to fundamentally change the production of ceramic membranes, combine with a growing market competitiveness to indicate a strong future for ceramics. l Going forth Some of the new methods for making ceramic membranes may lead to lower production costs, while others may improve the membrane's performance characteristics enough to swing the economics.