FOULING IN FORWARD OSMOSIS MEMBRANE PROCESSES: CHRACTERIZATION, MECHANISMS, AND MITIGATION
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As a potential solution to the increasingly severe global water crisis and energy shortage, forward osmosis (FO) membrane process has attracted growing attention in many applications such as desalination, water purification, wastewater reuse, food processing, and sustainable power generation. However, the advancement of the FO membrane process is greatly hampered by a long-standing problem of membrane fouling. Membrane fouling is caused by the accumulation of foreign substances on the surface or within pores of the membrane. Membrane performance, such as energy consumption, water flux, and effluent quality, can be severely deteriorated by fouling. Therefore, developing fouling-resistant membranes is key to more efficient use of FO membrane technologies. The objectives of this research are to fundamentally understand membrane fouling mechanisms at the molecular level and to develop novel antifouling materials for FO membrane process. Three major research tasks were performed in this study. The first task was to systematically characterize FO membrane fouling behavior by performing microscope-assisted online monitoring experiments to study the kinetics of fouling layer formation and flux decline. The second task was to combine nanoscale characterization experiments (e.g. interfacial force measurement by atomic force microscope, quartz crystal microbalance with dissipation) and molecular simulation to understand membrane fouling mechanisms. The third task was to develop novel membrane modification strategies by using hydrophilic materials, such as polydopamine and zeolite nanoparticles, to improve the membrane's antifouling properties in FO membrane process. Major research achievements are summarized below. (1) A microscope-assisted direct observation FO system was developed to provide critical information on the morphology and formation kinetics of fouling layer for various types of fouling, including organic fouling, scaling, biofouling, and combined fouling. (2) The successful combination of experimental characterization and molecular simulation gave insights into the role of membrane surface characteristics (such as functionality and charge) in FO membrane fouling, thus providing critical information to develop new antifouling FO membranes. (3) Polydopamine and zeolite nanoparticles were successfully grafted onto FO membrane surface. The surface modification proved to greatly increase membrane surface hydrophilicity and to reduce fouling propensity in FO membrane process.