Comparative assessment of microplastics removal in mechanical versus nature-based urban wastewater treatment systems: A multi-plant study in Kermanshah, Iran

• Stabilization Ponds remove microplastics best at 89.31%. • Fibers dominated microplastics in all wastewater samples. • Smaller microplastics (<1000 μm) escape treatment more easily. • No system achieved complete removal of microplastic pollution. • Nature-based systems can outperform mechanical treatment. This study aimed to comparatively assess the occurrence, characteristics, and removal efficiency of microplastics (MPs) across three types of wastewater treatment plants (WWTPs): Extended Aeration Activated Sludge (EAAS) as a mechanical system, and Stabilization Ponds (SPs) and Constructed Wetland with Horizontal Subsurface Flow (CWHSF) as natural systems, in Kermanshah, Iran. Sampling was conducted at various influent, intermediate, and effluent points of the three WWTPs over a period of four months. Following the digestion of organic matter, density separation, and extraction, the MPs were quantified and characterized. The mean MP concentration in the raw influent varied from 30.25 particles/L (EAAS) to 77.50 particles/L (SPs). Fibers were the predominant MP shape in all samples, accounting for more than 80% of the total abundance. The results demonstrated that while all three systems were capable of removing MPs, their efficiencies varied; the SPs system, with an overall removal efficiency of 89.31%, performed significantly better than the EAAS system (67.19%). The CWHSF system also exhibited a favorable efficiency of 71.01%. Furthermore, removal efficiency increased with particle size, with smaller particles (<1000 μm) showing a greater potential to escape the treatment systems. This study suggests that natural treatment systems, particularly SPs, possess a higher potential for MP removal compared to conventional mechanical systems, likely due to effective sedimentation processes. However, none of the examined systems were capable of complete elimination, despite demonstrating partial removal capabilities. Therefore, optimizing existing processes and developing advanced strategies for managing this emerging contaminant are essential.