Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/795216
The most common method to keep membrane surfaces clean and active is to sparge air across the membrane, allowing the resulting air bubbles to dislodge particulates from the surface and thin the foulant layer. It's possible to manipulate the size of the bubbles, and one industry manufacturer has determined that smaller bubbles are more advantageous for its systems. Introducing a continuous barrage of air requires energy, however, and the resulting cost can become significant in the overall operating cost. To help mitigate the energy cost, another MBR supplier has developed a process that generates random bursts or pulses of air, so that the resulting bubbles are not applied in a continuous stream. The MemPulse® MBR offers a random, rapid pulse of air up the membrane fiber column without any moving parts or valves to control the pulse, providing a constant flow of air to the equipment. In addition to scouring the membrane surface, the subsequent turbulence creates a flotation effect that helps move grease, oil, and other floating particulates to the top of the fiber bundles where they can be minimized or removed. Beyond "Hairy" Hollow Fibers: Evolving Membrane Technology Human hair has proven to be particularly challenging for wastewater treatment systems, especially those that employ hollow fiber membranes. Hair does not readily dissolve, and lengthy strands are strong enough to wind around and damage membrane fibers. Recent system designs have addressed this challenge with components that overcome the buildup of hair and other fibrous materials that plagued early MBRs. Eliminating this unique risk helps reduce operating costs and mitigates the unsavory task of disconnecting membrane cassettes to remove materials by hand. The evolution of hollow fiber technology began with manufacturers potting fibers at both ends of the MF/UF module to hold them in place. Other designs allowed the feed, filtrate, and backwash to be collected at different points, and air was introduced at the base of the module, dislodging solids as it rose up through the fiber bundle. Now some manufacturers have introduced configurations where the top end of the fiber is sealed but is no longer potted in place. This allows the fibers to move freely within the module or cassette in a continuous motion that helps dislodge particulates from the membrane's surfaces. This open configuration also allows aeration to penetrate deeper into the fiber bundle and release solids during air scouring. Another supplier developed an MBR system with submerged cassettes that gently rock back and forth in a reciprocal motion. The process claims to discourage deep particulate caking, keeping the foulant layer thin, so that longer treatment cycles can be achieved. Low energy, no aeration membrane bioreactors (LENA MBR) allow the fibers to move gently back and forth, and the resulting motion continuously shakes foulants off the membrane's surface. Sludge accumulation between the fibers is also minimized by the reciprocal motion of the cassettes and the pulsating fibers. Eliminating aeration reduces operating costs related to air scouring and recirculation of the streams. There are a number of demonstration plants in the U.S. that are using these operational and design technologies, and several full-sized plants should be installed in the near future, so we can all evaluate their efficacy and advantages. While only hollow fiber systems were specifically addressed here, both flat sheet and ceramic sheet membrane systems also use similar methods of minimizing the depth of solids buildup on membrane surfaces. As with hollow fiber membranes, surface air scouring helps extend the hours of operation and maintain an acceptable thickness of solids on the surface of flat sheet membranes. We've recently heard that small plastic balls are being added to the feedwaters of some tubular membrane systems to assist in the scouring process and help dislodge foulants. Adding an innocuous physical component to create turbulence is certainly an interesting concept and may be of value to future MBR developers. Microfiltration and ultrafiltration processes have proven to be extremely effective in removing solids from high-particulate waters and wastewaters. The ongoing challenge for system designers is developing effective ways to mitigate the accumulation of those same solids on membrane surfaces. As illustrated here, methods exist and continue to evolve, and there can be no doubt that achieving a competitive advantage in longer run times and reduced operational costs will ensure continued innovation. The American Membrane Technology Association (AMTA) has prepared membrane technical fact sheets on a variety of membrane and related technologies and applications including MBR, MF/UF, reverse osmosis, and more. AMTA fact sheets can be found at www. amtaorg.com. n wateronline.com n Water Innovations Harold G. Fravel, Jr. accepted the position of executive director for the American Membrane Technology Association (AMTA) after working for Dow Chemical /FilmTec Corporation for 36 years. He has a Ph.D. in organic chemistry from the University of North Carolina and a B.S. in chemistry from Florida State University. He resides in Jupiter, FL. Karen Lindsey is an executive member of the American Membrane Technology Association (AMTA) board of directors. She is the VP and co-founder of Avista Technologies and has 30 years' experience in the water treatment industry, working with companies that cast cellulose acetate membrane, produced polyamide elements, and formulated specialty chemicals. About The Authors FILTRATION 26 A cassette is lifted from the MBR process to clean off solids.