From wastewater treatment plants to farmland: Microplastic quantification, transfer, and risk assessment from biosolids use in Tasmania, Australia

Wastewater treatment plants (WWTPs) have been reported to remove microplastics (MPs) from influent wastewater; however, the reuse of biosolids transfers MPs to agricultural soils, posing potential environmental concerns. This study quantifies MPs including particles <20 μm in wastewater and biosolids from Tasmanian WWTPs, estimating their transfer to agricultural soils via biosolids reuse, and assessing its preliminary ecological risks. Using FPA-μFTIR, we detected 163.50-206.5 MPs/L in influent wastewater and 127.67-138.0 MPs/g in biosolids across WWTPs. Fragments were the predominant morphotype among the identified MPs, while the majority (48.4 %-67.1 %) of particles were within the >10-20 μm size range. Given the ∼11 μm detection limit of the FPA-μFTIR, some smaller particles may have gone undetected. Polyethylene (PE) was the most abundant polymer, followed by PVC, PU, PP, and PET. In this study, based on regional biosolids application rates, it was estimated that between 3.43 × 10¹⁰ to 1.30 × 10¹¹ MP particles are transferred annually to Tasmanian agricultural soils. A preliminary risk assessment using the Polymer Hazard Index (PHI) and Potential Ecological Risk Index (PERI) suggests MPs may threaten soil health, though toxicity uncertainties persist. These findings highlight the need for enhanced MP monitoring in biosolids to inform sustainable reuse policies, aligning with emerging global regulations. Further research into MP fate and biological interactions in soils is needed to establish an extensive robust risk threshold.