Development of Anti-fouling Piezoelectric Polyvinylidene Fluoride Membrane

• Main Author: Su, Yu Ping
• Other Authors: Chong Tzyy Haur
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/155460
• Institution: Nanyang Technological University

Ultrafiltration (UF) is widely used in water treatment to remove turbidity and pathogens; in wastewater membrane bioreactor to retain biomass; in desalination for pre-treatment of seawater. One of the main challenges during UF operation is membrane fouling which limits the potential of UF technology. The occurrence of fouling is inevitable and usually results in an increase in operational costs due to several factors such as reduced membrane lifespan, increased energy demand, and increased chemical cleaning. Many fouling control methods are available for UF operation. An example would be commercially available vibratory shear-enhanced process (VSEP), a type of unsteady-shear methods. In comparison with VSEP, piezoelectric membrane serves as the source of agitation and potentially offers lower energy consumption as an anti-fouling strategy. However, previous studies on piezoelectric membranes mainly focused on enhancing the piezoelectricity of commercial membranes through electrical poling, and the optimal operating conditions for piezoelectric membrane to improve filtration performance. It is important to extend the research to cover the fundamental understanding between membrane properties and piezoelectricity and its impact on membrane fouling, as well as the long-term application of piezoelectric membrane. In the first part of this study, piezoelectric flat sheet polyvinylidene fluoride (PVDF) membranes with different morphologies were fabricated by using different solvents via non-solvent induced phase separation (NIPS). The fabricated membranes were then electrically poled to enhance piezoelectricity and subsequently characterised in terms of morphology, piezoelectric properties, and mechanical properties. The effects of morphology on the filtration performance of piezoelectric PVDF membranes were also analysed. PVDF membranes, with finger-like morphology, had better dielectric and piezoelectric properties among all the fabricated membranes. Electrical poling aligned the dipoles of PVDF which enhanced piezoelectric properties of PVDF membranes and reduced finger-like cavities to drop-like cavities. For poled piezoelectric membranes under electrical signals, 25 to 46% significant improvement in critical flux, 66% reduction in rate of transmembrane pressure increase and approximately 3 times increase in filtration duration were observed. In the second part of this study, different quantities of piezoelectric barium titanate (BaTiO3) nanoparticles (NPs) were added during the fabrication of PVDF membranes to enhance piezoelectricity of the membranes. Some of the BaTiO3 NPs were modified by silane coupling agent to enhance dispersion and compatibility in PVDF membranes. The modified BaTiO3 NPs, particularly 0.1 wt.%, were well-dispersed in the PVDF membranes than unmodified BaTiO3 NPs, which had similar finger-like structures to neat PVDF membranes. The addition of BaTiO3 NPs to PVDF membranes improved its dielectric strength, piezoelectric coefficient, and mechanical properties. Electrical poling of the membranes further improved their piezoelectricity. A linear correlation between piezoelectric d_33 coefficient and critical flux was observed for poled piezoelectric membranes under the influence of applied electrical signals when colloidal silica was used as model foulant. In comparison with neat PVDF membranes, improvement in critical flux by up to 51% and extended duration of multiple filtration cycles by up to factor of 2 to 4 were observed for poled BaTiO3-PVDF membranes under application of electrical signals In the final part of this study, piezoelectric membrane was then implemented to filtration of real wastewater to evaluate long-term performance. The energy consumption to agitate the piezoelectric membrane through electrical AC signals and energy consumption to sustain transmembrane pressure were computed to assess the potential cost savings as compared with conventional VSEP method. The advancements of reversible, irreversible, and irremovable fouling were significantly lowered for piezoelectric membranes. The fouling pattern shifted from irremovable dominance of non-piezoelectric membrane to reversible dominance of piezoelectric membrane. The impacts of alternating current (AC) voltages and cross-flow velocities on fouling mitigation were also examined which demonstrated piezoelectric membrane under AC signals is more favourable as a fouling control strategy than increasing cross-flow due to better fouling mitigation for equivalent energy consumption. A possible synergy between piezoelectric effect and chemical cleaning was also noticed as less foulants were detected on the piezoelectric membranes at the end of the filtration cycles. The findings of this study would provide a better understanding on the fundamental relationships among membrane properties, piezoelectric properties and filtration performance and offer valuable information for the future development of piezoelectric membrane as an energy efficient anti-fouling method.