Microplastics in Mediterranean coastal wastewater treatment plants: Seasonal trends driven by tourism and weather conditions

Microplastics (MPs) are emerging pollutants of global concern due to their persistence, ubiquity, and ecological risks. Wastewater treatment plants (WWTPs) are critical pathways for MP discharge into aquatic environments, especially in densely populated areas. This study assessed MP concentrations, seasonal variability, and removal efficiencies in two WWTPs located in Cambrils and Palamós (Catalonia, Spain). Composite 24-h samples were collected from influent, primary and secondary effluents in January, May, and September 2023. MPs were isolated using oxidative digestion and density separation, followed by stereomicroscopy quantification and FTIR-based polymer identification. Concentrations of MPs ≥25 μm ranged from 30.0 ± 8.0 to 75.0 ± 18.0 particles/L in influent and from 6.7 ± 2.0 to 20 ± 5.0 particles/L in secondary effluents, with the highest values recorded during warmer months. Removal efficiencies ranged from 73.3% to 86%, with seasonal peaks in effluent coinciding with increased temperature, precipitation, and tourism. Statistical analysis revealed significant correlations (p < 0.05) between MP concentrations and both temperature (at both sites) and precipitation (notably in Palamós). Polymer composition, shape, and color profiles indicated dominant sources linked to domestic wastewater and marine-related activities, especially in Palamós. The most frequently detected polymers were polypropylene (PP) and polyethylene (PE) in both WWTPs, reflecting inputs from packaging, textiles, and fishing gear. Estimated emissions reached up to 131 billion MPs/year in Cambrils and 91 billion MPs/year in Palamós, highlighting the scale of WWTP-derived MP pollution These findings highlight the need for seasonally resolved monitoring and source-targeted mitigation strategies to reduce MP emissions from WWTPs in tourism-impacted coastal regions.