Microplastic in Australian processed organics: Abundance, characteristics and potential transport to soil ecosystem

The extensive use and application of recycled organics, including biosolids and compost, has been considered as an effective waste management approach to reduce waste to landfills. However, concerns have been raised about the presence of microplastics (MPs) in these recycled organics and their subsequent transfer to soils. Although the presence of MPs in biosolids has been widely documented, our knowledge about the occurrence and characteristics of MPs in processed organic waste such as compost is still limited. The present study aimed to investigate the abundance, characteristics and potential sources of MPs (>25 μm) in processed organic waste samples collected across eleven sites with different processing systems in Australia. This includes compost, digestate from anaerobic digestion and rapidly dehydrated food waste. MPs, mainly polyethylene, polypropylene and polyester, were found across all samples with concentrations ranging from 1500 to 16,000 MP/kg dry weight. The majority of these MPs fell within the smaller size range of 25-500 μm. Using the concentration and characteristics (size range, morphology, density) of detected MPs, the mass abundance of MPs was estimated to be between 7 and 760 mg/kg dry weight. We also estimated that between 5.2 × 10¹⁰ to 6.2 × 10¹² MP/year could be transferred to the land via application of processed organic waste in Australia. The findings of this study also showed compostable bags as a potential source of MPs in some samples. This study provides the first evidence of MPs in processed organic waste in Australia and emphasises the need to more comprehensively understand the fate of MPs during the composting processes, their contribution to soil MP contamination, and their impacts on soil biota.