Water Journal : Water Journal April 2011
membranes & desalination technical features 106 APRIL 2011 water elements. However, they may well affect the deposition of bacteria. There are two views on the topic, one that the spacer is the culprit, the other that flux is the key factor, since nutrients in the feed water will be concentrated at the membrane surface and actually enhance biofilm growth. Our studies (by Ziggy Chong, Stani Suwarno and Vera Puspitasari) in lab-scale RO tests with carefully controlled conditions have addressed the spacer vs membrane problem. Figure 5 shows the build-up of biomass, as measured by polysaccharides and protein, in a flat sheet RO test cell with no spacer. There is an obvious effect of the imposed flux. Figure 6 shows that the spacer, by increasing mass transfer, prolongs TMP rise. The superimposed photomicrograph dramatically shows the biofilm growth. Figure 7 demonstrates the effect of such growth on TMP rise and Figure 8 shows the effect of increased flux. Photos of a ladder design of spacer show that the biofilm first develops near the downstream filament. This investigation of the nature of the biofouling in RO requires good collaboration between membrane specialists and microbiologists. Autopsies of fouled membranes, performed by Chen Xi at Nanyang Technological University, show that there are more bacteria on the membrane than on the spacer. The answer to the question: "Is it a spacer or membrane problem?" is probably that biofouling initiates on the spacer but spreads to the nutrient-enriched membrane surface; in other words, both mechanisms operate. Spacers may delay the development of the biofilm and TMP, but clearly influence the pattern of biofouling. How can we control biofouling? Other than limiting flux and/ or nutrient levels, we are examining ways of inactivating the bacteria or disturbing the biofilm development. For example, we have investigated the use of UV pretreatment and have shown that with a suitable dose, we can reduce the rate of biofouling by up to 70%. Our future work will optimise this system and also combine it with other biocides. Collaboration with microbiologists is leading to biofilm disruption strategies. For example, nitric oxide has been shown to act as a dispersant, and in laboratory rigs it has been shown by Kjelleberg et al. (2006) to remove bacterial film very effectively. We are trying this approach, alone and in combination with other agents, in a test rig and on a real spiral-wound element. Membranes of the Future: Biomimicry Membranes of the future could be adapted from biology. Aquaporins are proteins which can be extracted from, say, E. coli, and function in living cells as natural water purification channels. These aquaporins form clusters with 'pores' that are the exact size to achieve water transport and rejection of the larger hydrated salt ions. The target is to prepare them artificially on the surface of a microporous support and achieve high water permeability and very low salt transmission. Work in this exciting area is being done by various groups in the US, Denmark, Israel, Korea and two institutes in Singapore. If this approach succeeds it could revolutionise membrane technology. The Author Professor Tony Fane (email: email@example.com) is Director of the Singapore Membrane Technology Centre (SMTC) at the Nanyang Technological University. The SMTC has 80 researchers dedicated to membranes for water and the environment. He is also Professor (and former Director) at the UNESCO Centre for Membrane Science and Technology at the University of New South Wales and Patron of the Membrane Society of Australasia. His current interests are in membranes applied to environmental applications and the water cycle, with a focus on the sustainability aspects of membrane technology, including membrane bioreactors and reuse. Acknowledgements Professor Fane acknowledges his co-workers: Ziggy Chong, Shuwen Goh, Winson Lay, Diane McDougald, Choon Aun Ng, Vera Puspitasari, Scott Rice, Stanu Suwarno, Rong Wang, Harvey Winters, Fooksin Wong, Chen Xi & Jinsong Zhang. spacer no spacer Flow 0.0 0.2 0.4 0.6 0.8 1.0 X (mm) 0.2 0.4 0.6 0.8 1.0 Y (mm) Spacer Figure 6: Biofouling with and without spacers. • Flux 35 lmh p.aeruginosa, Xﬂow 0.2 m/s. • The presence of the spacer signiﬁcantly reduces the rate of biofouling in these tests. 3 days 30%coverage 6 days 73%coverage 10 days 85%coverage TMP rise Flux 35 l/m2h Velocity 0.17m/s Flow Figure 7: Bioﬁlm development – diamond spacers. • Initial bioﬁlm inﬂuenced by spacer geometry. • Spreads across the membrane and TMP rises. Flux 20 l/m2h Flux 35 l/m2h 6 days 27%coverage 6 days 73%coverage 10 days 67%coverage 10 days 85%coverage Biofilm development and TMP rise increase with flux. Figure 8: Bioﬁlm development – diamond spacers.
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