Synergistic removal of microplastic fibres using hybrid pre-treatment: evaluation of Chitosan as a green coagulant

Graphical Abstract

Research Summary

Microplastic fibres (MPFs) are the most dominant type of microplastics detected in water and wastewater treatment plant influents. Due to their shape, they often escape through tertiary treatment stages such as membrane filtration, making them challenging to remove. Pre-treatment is a simple and widely used process in water and wastewater treatment processes. As chemical pre-treatment (coagulation-flocculation) is a low-cost operation that, if optimised, could reduce the load for subsequent treatment processes like membrane filtration, investigating its ability to capture MPFs is both economically, and environmentally important. Further, due to the low-density characteristics of MPF, combining coagulation-flocculation with microbubble introduction presents a promising hybrid approach worth investigating. Although traditional coagulants perform well for removing microplastics, they suffer high ion residue after coagulation and need high dosage for efficient contaminant removal. Further, given the large-scale use of chemical-based coagulants, their environmental impacts can be significant. Therefore, investigating the green and emerging sustainable coagulants could revolutionise current water pre-treatment by reducing coagulant consumption and environmental impacts while achieving optimal results. This will lead to high overall plant performance and accelerate the goals of the circular economy.

The key objectives of this research are to investigate the capacity of existing pre-treatment methods in water treatment to remove MPFs and investigate the potential of a green coagulant; Chitosan (Low molecular weight, Deacetylation 75 - 85%) on the MPF removal efficiency. Two hybrid treatment trains including coagulation-flocculation-sedimentation  (CFS) and coagulation-flocculation-microbubble introduction (CFm) have been tested.  The study evaluated the performance of each hybrid method combination using AlCl₃·6H₂O and Chitosan in the CF step, comparing MPF removal across different scenarios. Further, to represent the MPF removal, a simple, yet efficient approach of measuring the turbidity after each treatment (i.e., Residual turbidity) has been employed. The study was conducted in two water matrices: one in which MPF was present in deionised (DI) water with a surfactant, and another where MPF was in DI water with the surfactant and Humic acid (HA).

The results confirmed that the coagulants exhibited varying performances. In the DI water matrix, Chitosan achieved higher MPF removal in both the CFS and CFm hybrid treatment methods. In the HA-containing matrix,  AlCl₃.6H₂O was more effective in the conventional CFS method for turbidity removal, addressing turbidity caused by both MPF and HA, with an 88.46% turbidity removal efficiency. Meanwhile, Chitosan outperformed in the CFm process, achieving a 78.30% turbidity removal efficiency. These findings highlight the importance of selecting the appropriate hybrid treatment method tailored to specific coagulants, as this can significantly enhance treatment efficiency. The CFm method is particularly recognised for achieving high colloidal particle removal with a lower coagulant dosage. When Chitosan was used as the coagulant in CFm, it demonstrated superior performance compared to AlCl₃.6H₂O. Notably, even in its commercially available form without further chemical modification, Chitosan achieved higher MPF and turbidity removal while requiring less coagulant. This underscores its significant potential as a sustainable and eco-friendly coagulant. Further development and optimisation of Chitosan-based treatments not only aligns with the objectives of sustainable water treatment but also contribute to the broader transition toward a circular economy by reducing reliance on traditional chemical coagulants and promoting the use of green, biodegradable materials.

Proposed date of completion
May 2025

Papers

Thathsarani, N., Khiadani, M., Azhar, M.R., & Zargar, M.,(2024) “Enhanced Removal of Microplastic Fibres Using Aluminium and Chitosan-Based Coagulants Assisted with Microbubble Technology” (Under review in Journal of Environmental Chemical Engineering).

Author

Student: Nimesha Thathsarani, 
School of Engineering, Edith Cowan University
Phone: 0435763423
Email: j.thathsarani@ecu.edu.au

Supervisors: Dr. Masoumeh Zargar (Principal supervisor), Prof. Mehdi Khiadani (Co-supervisor), Dr. Muhammad Rizwan Azhar (Co-supervisor)