Basic Principles Of Forward Osmosis And Its Membrane Fouling Behavior

Forward osmosis （FO） is an emerging green membrane separation technologywhich depends on the osmotic pressure difference on both sides of the selectivemembrane to realize water transfer spontaneously instead of impressed pressure.Like all other membrane separation technologies, membrane fouling behavior is aquite important research aspect in FO. Lower membrane fouling means moreproduced water, less membrane cleaning, longer membrane lifetime thus leading toless total operation and investment cost. However, unlike other pressure-drivenmembrane processes, the membrane fouling behavior of FO is quite differentbecause of the osmotic driving force. It is essential to study the membrane foulingbehavior of FO in order to provide a reference in the aspects of FO application andmembrane preparation in the future.This study first focused on the basic principles of FO and the determination ofgeneral experimental conditions.The flat-sheet forward osmosis membrane used inthis study was provided by Hydration Technologies, Inc.（Hydrowell Filter, HTI,Albany, OR）. The morphology, structure, water flux and reverse solute flux of thetwo HTI FO membranes were characterized and compared. The effects of themembrane orientation modes, cross-flow velocity and osmotic driving force on FOperformance parameter such as water flux, reverse solute flux and cleaningefficiency were discussed respectively. The roles of operating conditions such as thecleaning velocity and time in membrane cleaning were also studied. The generalexperimental conditions in the following research have been assured in this period.The fouling behavior of bovine serum albumin （BSA）, sodium alginate （SA）,humic acid （HA）, Fulvic acid （FA） and nano-TiO₂in FO membrane process werecompared respectively. FA had the highest flux decline as well as the most severemembrane fouling followed by BSA, HA and SA. Meanwhile, nano–TiO₂had thelowest flux decline during FO fouling compared to other foulants which can benearly negligible.The FO membrane fouling by BSA, HA and SA including individual fouling,combined fouling and the role of calcium ions in organic fouling had been studied.For alginate fouling, a much more severe flux decline is observed in the presence ofcalcium ions compared to that in the absence of calcium ions. The presence ofcalcium ions enhances the intermolecular adhesion between alginate moleculesinduced by intermolecular bridging by calcium ions, resulting in more severemembrane fouling. Unlike alginate, BSA fouling slightly weakened instead of aggravating in the presence of calcium ions. Like alginate, HA fouling was enhancedby the presence of calcium ions. The next work aimed to investigate combinedorganic-organic fouling of FO membranes. The flux drop of feed containing BSAand SA was the highest followed by feed containing HA and SA, then feedcontaining BSA and HA. For the size distribution of feed, the number of foulantswith small size in FS became less and the foulants with larger size became more,and the main size increased from the beginning to the end during fouling period.Combined organic-inorganic membrane fouling behavior was investigated andthe effects of calcium ions and the concentration of nano-TiO₂were also discussed.The combination of nano-TiO₂and organic foulants such as BSA, HA and SAdecreased the flux decline during fouling compared to the individual organic fouling.A much more severe flux decline is observed during organic-inorganic combinedfouling in the presence of calcium ions compared to that in the absence of calciumions.