The continuous increase in plastic production and its subsequent release into natural ecosystems represents a major concern for both environmental and human health. Wastewater Treatment Plants (WWTPs) have been identified as key pathways for the transfer of plastic particles, particularly microplastics, into aquatic environments [1]. Although previous studies have demonstrated the effectiveness of coagulation–flocculation processes in removing plastics under controlled laboratory conditions, their chemical performance and scalability in real wastewater matrices remain insufficiently investigated [2, 3].

In this study, the removal of plastic particles was evaluated through coagulation–flocculation processes, with particular attention to the physicochemical interactions between plastic surfaces and polymeric flocculants. Four commercial flocculants, including two bio-based polymers and two polyacrylamide-based synthetic polymers, were tested at concentrations close to those typically applied for suspended solids removal (0.10 g/L). Jar tests were performed using real wastewater samples spiked with a defined mixture of plastic particles fibers, which represent the most abundant form of microplastics in aquatic systems (35%) [4].

The removal mechanisms were interpreted by examining the interactions between flocculants and plastic surfaces, with particular emphasis on floc formation, including its structural, dimensional, and stability characteristics. Plastic removal efficiency was quantified by comparing residual particle concentrations after treatment with initial levels. In parallel, key chemical parameters were monitored to assess compliance with regulatory standards, including those defined by the European Directive (EU) 2024/3019. The results highlighted the critical role of the coagulation step in destabilizing colloidal systems and enhancing subsequent floc formation.

Among the tested coagulants, polyaluminum chloride (PAC), applied at 0.08 g/L, provided optimal performance while maintaining residual concentrations within regulatory limits. Under these optimized conditions, flocculation tests yielded high plastic removal efficiencies, reaching 96.25% ± 0.72 for bio-based polymers and 99.58% ± 1.65 for polyacrylamide-based polymers. Overall, the findings demonstrate that the efficiency of plastic removal is strongly governed by the chemical interactions between coagulants, flocculants, and plastic particles. Moreover, while synthetic polymers exhibited slightly higher removal efficiencies, bio-based flocculants showed comparable performance, suggesting their potential as more sustainable alternatives. This study provides a chemically grounded assessment of the mechanisms of action and interaction of coagulants and flocculants with plastics, supporting their possible implementation at full scale for mitigation in WWTPs.

References

1- J. C. Prata Mar. Pollut. Bull. 2018, 129, 262.

2- B. Ma, W. Xue, Y. Ding, C. Hu, H. Liu, J. Qu Journal of Environmental Sciences. J. Environ. Sci. 2019, 78, 267.

3- Y. Zhang, X. Wang, Y. Li, H. Wang, Y. Shi, Y. Li, Y. Zhang J. Hazard. Mater. 2022, 425, 127.

4- S. Mishra, R. P. Singh, C. C. Rath, A. P. Das J. Water Process Eng. 2020, 38, 101.