Investigations into the flocculent settling of microplastics

Sewage treatment plants (STPs) act as both significant sinks and sources of microplastics (MPs) in the environment. However, there is a lack of investigations into the flocculent settling characteristics of MPs and their removal trends in secondary clarifiers. Therefore, this study attempted to explore the flocculent settling behaviour of small-sized (15-105 μm) MPs of polymer types: nylon, polystyrene, polypropylene, and low-density polyethylene through controlled lab-scale experiments. Results showed that the MPs' removal with flocculent settling improved from 55 to over 80% as sludge mixed liquor suspended solids (MLSS) concentration increased from 50 to ∼1,000 mg/L, highlighting the importance of MLSS in MPs' retention. Furthermore, a lab-scale prototype of a secondary clarifier was used to study flocculent settling trends of a mixture of rising and settling polymers, and iso-removal plots for MPs' flocculent settling were plotted, which provide an estimation of secondary clarifier removal efficiencies. It was observed that approximately 94% of MPs were effectively removed in secondary clarifiers under optimized conditions. This study provides novel insights into the flocculent settling characteristics of MPs in secondary treatment, improving our understanding of their fate in STPs and strategies to enhance MP removal efficiency.