Microplastic pollution during organic compost production: occurrence and ecological risks

The conversion of the organic fraction of municipal solid waste into compost via mechanical and biological treatments is a widely practiced and cost-effective waste management approach. However, the impact of these processes on the characteristics of microplastics (MPs) remains poorly understood. This study investigated the levels and characteristics of MPs during composting and in the final products, and assessed the associated ecological risks. 54 samples were collected from different stages of the composting process, from incoming waste (IW) to refined compost (RC). After pre-treatment, stereo microscopy was used for preliminary identification, followed by Fourier-transform infrared spectroscopy (FTIR) for polymer identification of particles > 1 mm and micro-Raman spectroscopy for smaller MPs. The average microplastic concentration increased from 22,800 N/kg in IW to 75,928 N/kg in RC. Fibers were the most abundant type of microplastic throughout composting, while black MPs dominated (accounting for 50% of all particles). In IW, polyethylene (PE) and polyethylene terephthalate (PET) were the main polymers, whereas in RC, polystyrene (PS) and PET were most prevalent. Ecological risk assessment revealed a polymer hazard index (PHI) of 491.7 (risk level III) and a pollution load index (PLI) of 59, indicating a level IV (extremely high) category based on PHI and PLI classification schemes. These findings indicate that microplastic abundance increases during the composting process, highlighting the need for effective pre-treatment strategies to reduce plastics and microplastics in the feedstock and minimize their release into the environment.