Developing thin film composite membranes for engineered osmosis processes
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The high demand for clean water resources has generated substantial research interest in terms of sustainable and low energy water purification technologies such as forward osmosis (FO). Compared to other membrane based technologies, the FO process is less energy intensive. However, there are challenges that need solutions to enable the FO to compete with other technologies for desalination. Suitable draw solution and a proper membrane are required to overcome the FO process challenges. Enormous effort has been expended to find a new material and better membrane design in order to develop a novel FO membrane that can meet high performance demands in relation to water flux, salt rejection and mechanical strength. This is of particular importance for the newly introduced concept of pressure assisted osmosis (PAO). The objectives of this dissertation are to understand the fundamentals of the FO and PAO as a basis for fabricating a suitable membrane for the FO and PAO process. In the first part of the work, PAO and its potential application to overcome the limitations of osmotic equilibrium in the FO process is investigated. One of the practical applications of the FO process is desalination for irrigation purposes through the means of hybrid desalination units such as fertiliser drawn forward osmosis (FDFO). The utilisation of PAO in FDFO desalination is assessed .By integrating the PAO process into the FDFO desalination unit, water flux can be generated beyond the point of osmotic equilibrium. As a result, diluted fertilizer as DS in the FDFO unit can be applied for direct fertigation without the need for an additional post-treatment process such as nanofiltration to recover the fertiliser draw solution (DS). Integration of the PAO process has proved to be very effective in generating extra water flux. This can serve to reduce the capital costs since no separate post-treatment process such as the NF is necessary. In the second part of the work, a thin film composite membrane for the FO and PAO process is fabricated through Polyethersulfone as a polymer materials base. Phase inversion in the precipitation bath and membrane formation mechanism of these polymers, both with and without backing fabric support, is investigated. The membrane chemical properties and hydrophilicity have been found to play a key role in the mass transfer of water flux during the FO process. Therefore, attention has been directed at increasing the hydrophilicity of the membrane through blending sulphonated materials. It has been found that sulphonation not only affects the membrane performance but also the membrane structure and morphology. Through sulphonation, porosity and hydrophilicity of the substrate increases while the finger like structure disappears. This leads one to suppose that the high water flux does not have a direct relationship with the finger like membrane structure. Regardless of membrane morphology, substrate hydrophilicity is the key to achieving a high performance membrane. Sulphonation has been found to have a tremendous effect on the physical and chemical properties of the membrane. While sulphonation dramatically increases the hydrophilicity of the substrate, it decreases the membrane mechanical strength. Due to higher hydrophilicity and lower ICP as a result of blending the sulphonation polymer, a membrane with better performance in terms of water flux and selectivity has been developed for the FO process. In the last part of the work, a special thin film composite (TFC) flat sheet membrane on a backing fabric is developed for the PAO application. The newly developed concept of PAO has introduced a hydraulic pressure to the feed side to overcome osmotic equilibrium and the extraction of more water. Accordingly, under the PAO process, a membrane with considerable mechanical strength is required. A thin film composite membrane supported on woven mesh fabric is designed to specifically solve the problem by embedding a woven mesh fabric support. An earlier part of this study reveals that the mechanical stability and special physical properties of the support layer are critical for the PAO process.
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