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he forward osmosis (FO) process is a novel membrane technology that has recently shown great potential for water reuse and desalination. The FO process makes use of the natural osmosis phenomenon for the transport of water from a low solute concentration feed solution to a high solute concentration draw solution across a partially permeable membrane,with the osmotic |
pressure difference between the feed and the draw solution as the driving force. Usually, the feed solution is the target water to be treated whereas the concentrated draw solution contains a draw solute that is capable of generating a large amount of osmotic pressure as the driving force. After water permeates into the draw solution due to osmosis, the diluted draw solution can then be re-concentrated using a suitable post-treatment process for reuse in the FO process while high quality water can be produced.
The FO process has various advantages over current membrane technology, especially for the reverse osmosis (RO) process. These include lower fouling potential and lower energy consumption. One of the most important challenges in utilizing the FO process for actual application is the lack of an ideal FO membrane. Currently, there are only two types of FO membranes that are commercially available. In a number of reported FO studies including our own, the water fluxes obtained were lower than expected. A common explanation to account for the low flux attributes it to the severe impact of the internal concentration polarization (ICP) effect occurring in the porous substrate of the FO membrane. Figure 1 shows scanning electron microscope (SEM) images of the magnified cross-sections of the various possible membranes used in the FO process in our study.
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Figure
1: SEM images of the cross-section of the (a) Cellulose acetate RO membrane, (b) Polyamide RO membrane, (c) First generation FO membrane and (d) Second generation FO membrane. (1) is the dense selective layer and (2) is the porous substrate layer. |
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Due to the limitations posed by the severe ICP effect on water permeation for current FO membranes, new FO membranes with superior water permeation, high salt rejection and sufficient mechanical strength need to be developed. To achieve this, we developed a nano-composite FO membrane that contains dispersed multi-walled carbon nanotubes (MWNTs) within the cellulose acetate polymer matrix using the phase inversion method. The permeation and morphology of the MWNTs FO membranes were found to be dependent on the amount of MWNTs used. Compared with the FO membrane fabricated without MWNTs, a breakthrough in the water permeability of the MWNTs FO membranes was achieved, whereby as much as 40% increase in water flux was achieved with almost unchanged solute rejection performance. The FO test results are given in Figure 2. The optimum MWNTs content was determined to be 0.2 wt% and its separation performance as well as other morphological aspects were found to be optimized.
A breakthrough in the development of a new FO membrane with excellent flux performance has been achieved by the application of MWNTs to enhance the water permeation across the membrane. The new MWNTs FO membrane can facilitate the development of the FO process for water reclamation and desalination, with potential to be realized in the very near future.
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2: Comparisons of water fluxes for different MWNTs wt% membranes and a commercial FO membrane. Operating conditions are, feed: 0.5 M NaCl; draw: 2 M NaCl; crossflow rate of 2 L/min (8.34 cm•s-1); temperature: 25 °C; Orientation: normal (FO) mode. |
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Dr Ng How Yong is currently an Associate Professor and the Deputy Head (Research) of the Division of Environmental Science and Engineering. His research interests are in water reclamation, focusing on fouling in membrane processes, biological processes for wastewater treatment, and energy reduction and recovery from waste and wastewater. He is the recipient of the International Water Association’s Young Professional Award 2006, the Singapore Young Scientist Award 2007, the Singapore Youth Award 2008 and the NUS Young Researcher Award 2009.
Email: esenghy@nus.edu.sg |
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