Synergistic removal of microplastic fibres: Integrating Chitosan coagulation in hybrid water pre-treatment systems

Microplastic fibres (MPFs) are the most prevalent form of microplastics detected in the influents of water and wastewater treatment plants. Owing to their elongated shape and low density, MPFs frequently evade removal even during tertiary processes such as membrane filtration. Enhancing pretreatment, particularly through cost-effective coagulation-flocculation can reduce the treatment burden on downstream systems. However, conventional inorganic coagulants often necessitate high dosages and can leave residual ions, posing environmental concerns. As a sustainable alternative, the use of environmentally friendly coagulants such as Chitosan has gained attention for MPF removal. While Chitosan exhibits favourable qualities, its practical application is hindered by poor solubility and a narrow effective pH range. Enhancing its solubility across a broader pH spectrum and increasing its cationic charge density could address these limitations and improve its coagulation performance. This thesis investigates hybrid pretreatment approaches combining coagulation-flocculation sedimentation (CFS) and coagulation-flocculation and microbubble introduction (CFm) with a novel integration of Chitosan, as a green coagulant for enhanced MPF removal. In the initial phase, the effectiveness of hybrid pretreatment methods and Chitosan were evaluated across different water matrices containing humic acid and surfactants to mimic drinking water treatment plant influent and laundry wastewater. The results were then compared against the conventional coagulant AlCl₃.6H₂O. Both AlCl₃.6H₂O and Chitosan were tested in CFS and CFm systems, with MPF removal assessed via residual turbidity measurements. In the final phase, to address Chitosan’s limitations, low solubility and pH sensitivity, two amphoteric derivatives were synthesised including: CMC-CTA (CTA-modified carboxymethyl Chitosan) and CMC-g-PDMC (carboxymethyl Chitosan grafted with poly[(2-methacryloyloxyethyl) trimethyl ammonium chloride]). The success of applied Chitosan modification strategies were characterised by Fourier-transform infrared spectroscopy (FTIR) and 1H-nuclear magnetic resonance (1H-NMR). Subsequently, the MPF removal efficiency via residual turbidity measurement was evaluated against Chitosan and its derivatives in both hybrid pre-treatment methods. This work underscores the potential of integrating green coagulants with hybrid pre-treatment methods to achieve more efficient and sustainable MPF removal, contributing to higher plant performance and advancing circular economy objectives.