Membrane Fabrication and Modification for the Treatment of Wastewater Using Membrane Distillation

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Owing to both water shortage and environmental protection, the government around the world is creating policies to regulate the release of wastewater into natural bodies of water. Therefore, there is a growing requirement of highly efficient wastewater treatment technologies. Currently, centralized wastewater treatment plants were used in most cities and towns globally. The on-site treatment of domestic wastewater is consistently gaining interests because it reduces the needs of central treatment plants, which require large footprint and high maintenance. In addition, the potential use of the reclaimed water for non-potable purposes reduces the stress in local water supply, and hence the cost of freshwater production and transportation. However, conventional wastewater treatment technologies used in the centralized treatment plant are not feasible to be implemented in the decentralized treatment facilities owing to its large footprint and long retention time. Currently membrane bioreactor (MBR) has been developed for wastewater treatment facilities with small footprint. However, the quality of permeate discharged from MBR system is not high enough for direct use. Therefore, additional treatment such as UV and reverse osmosis (RO) are essential, which requiring more footprint and energy consumption. To address these issues, the overall aim of my PhD study is to develop highly efficient membrane system to treat wastewater with special designed membranes. Membrane distillation (MD) has many advantages over conventional microfiltration (MF) such as high quality permeates, leading to potentially high energy efficiency for the whole wastewater treatment plant. So, it is critical to evaluate the possibility of MD to replace MF in MBR, as MD has unique mechanism which provides several advantages over MF. Hereby, following research objectives have been developed to meet the aim: 1. to develop the first thermophilic anaerobic membrane distillation bioreactor (anMDBR) for the treatment of municipal wastewater. 2. to improve the permeation performance of hollow fiber membranes. 3. to investigate the behavior (mass transport, fouling, and rejection) of organic compounds in MD processes regarding fouling and mass transfer. During my PhD studies, I have successfully developed the first anMDBR system in lab-scale. High inorganic and organic compounds rejection of 99.99% could be achieved using this system. Based on the results, I recommend 45 °C as optimal bioreactor temperatures for its stable flux performance and resource recovery. It was found that salinity of the mixed liquor had minimal impact on anMDBR. The potential recovery of biogas and volatile fatty acids offered economic benefits. Some challenges were realized when developing anMDBR system: 1. Permeation flux was very low (~2 LMH). 2. Impact of volatile organic compounds (VOCs). 3. Membrane fouling issues. Hence, following solutions have been suggested: 1. Development of membrane with high permeability. 2. Study of VOCs behavior. 3. Development of membranes with anti-fouling properties. Therefore, in next stage of my study, I developed hollow fiber membranes with bicontinuous structure and porous skin without using weak solvent in coagulation bath. The open pore morphology improved permeation performance in membrane distillation due to high porosity and mean pore size. Also, I found that non-solvents in the dope solution affect the morphology of hollow fiber membranes. It can be concluded that this is a simple and cost-effective technique, which is suitable for mass production. Then I investigated the behavior of VOCs in feed tank. It was found that VOCs with surfactant properties existing in the feed have high mobility and could cause rapid wetting. Also, the total flux of VMD was increased with the addition of VOCs into the feed. And VOCs showed negative impacts on the permeate quality as they could penetrate membranes. I also conducted some research regarding permeation performance improvement using heat-treatment and optimal conditions had been determined. Recommendations for future study have also been addressed in this thesis.
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