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Abstract
Membrane distillation (MD) is an emerging low energy desalination technology, where low-grade heat source can be utilized to provide heat to the feed, offering an alternative solution to saline concentrate treatment. Yet, MD still suffers prominently from scaling, wetting and low flux. To overcome these problems, methods of fouling control are frequently investigated, such as process optimization by selecting the appropriate operational parameters and cleaning protocols, and refining the intrinsic properties of the membrane to meet the needs of membrane distillation, in terms of enhanced fouling/wetting resistance and reduced mass transfer resistance.
This dissertation examined the fouling mechanisms in MD process by different substances in treating saline concentrates produced from coal seam gas generation, as well as the desalination of brackish groundwater. In particular, the fouling mechanisms of substances such as alkaline scalants, silica, and humic acids were studied. During the treatment of coal seam gas water with the presence of silica, magnesium formed porous structured depositions with silica, which caused a less severe flux decline as compared to the feed without silica. Initially, the precipitation of calcite was spotted, followed by the deposition of magnesium silicate and sodium chloride. The roles of fouling control via intermittent cleaning, mechanical agitations, as well as an integrated thermal crystallizer were investigated. As such, cleaning of the membrane prior to catastrophic membrane degradation is a critical operating protocol as an approach to the maintenance of membrane performance. It should be noted that the approaches to fouling control should be carefully selected according to the specific fouling mechanism by the pollutants present in the feed. While vigorous agitations might be undesirable in MD process for the treatment of inorganic feed, it was discovered that the application of an integrated thermal crystallizer could help sustain membrane performance and harvest crystals. On the other hand, high speed transverse vibration at 500 rpm was found to be effective in fouling control for the treatment of brackish groundwater concentrates with a considerable amount of humic aicds.
This work also explored the effect of surface engineering on membrane performance for different MD processes. To be specific, two approaches to surface functionalization were investigated as the purpose of different MD applications, namely Janus hydrophobic-hydrophilic membrane and superhydrophobic surface. To fabricate a Janus membrane, a facile solution-immersion method in the dopamine solution was applied. Significant increase of flux measured in a submerged direct contact membrane distillation configuration was observed from the modified membrane by this approach. The thickness of the hydrophilic layer determined the flux values and the rejection rate for long-term operation in saline solution; the trade-off caused by these two parameters should be carefully examined during the design of membrane. The maximum flux enhancement achieved by the Janus membranes was 120 % as compared to the nascent membranes. To fabricate the superhydrophobic surface, the membrane was immersed in the dopamine solution, followed by the deposition of silica nano-particles and low surface energy material of 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PDTS). The modified membrane showed enhanced anti-fouling and anti-wetting properties in a submerged vacuum membrane distillation setup for the desalination of brackish groundwater concentrates and sodium chloride solution. Both modified membranes showed superior stability in long-term desalination processes for MD applications.