Macro- and microplastics in composts from municipal solid waste industrial composting Plants in Uganda

Organic fertilizer use constitutes one of the main pathways through which (micro)-plastics enter the environment. However, little is known about the extent of plastic contamination in biowaste composts from Sub-Saharan Africa, where municipal biowaste collection involves minimal to no source separation. This study assessed macro- and microplastic contamination in composts from two Clean Development Mechanism composting facilities in Uganda processing mixed municipal solid waste for agricultural use. Finished composts were sampled per Plant at monthly intervals for four months (September 2023 to January 2024), and sieved into different size fractions: >5, 1.18-5, 0.6-1.18, 0.212-0.6, and 0.063-0.212 mm. Microplastics in each fraction (<5 mm) and macroplastics (>5 mm) in composts were analyzed. The results indicated heavy macro- and microplastic contamination in composts from both Plants. Macroplastics (>5 mm) ranged from 0.11 to 4.70 (1.50 ± 1.24) g/kg dry weight in Plant 1 and 0.36-4.93 (1.78 ± 1.27) g/kg dry weight in Plant 2. Total microplastics in composts exceeded all previous literature studies, averaging about 49,000 ± 7,000 and 62,000 ± 6,000 items/kg dry weight, respectively. For both facilities, compost fraction 0.212-0.6 mm showed the highest microplastic contamination, followed by 0.063-0.212, 0.6-1.18, and 1.18-5 mm. Total microplastics in composts depended on facility pile turning, feedstock contamination, and local population density. Polyethylene and polypropylene were the most dominant polymers in macro- and microplastics from composts, with fragments and fibers most dominant microplastic shapes. Estimations of plastic load show that such MSW compost can input into amended soils macroplastics of 2.0-106.7 kg/ha and 2.9×10⁸ - 2.4×10⁹ microplastic items/ha per recommended application.