Exploring the Potential of Hydrothermal Treatment for Microplastics Removal in Digestate

Microplastics (MPs) are increasingly being detected in soil conditioners derived from organic waste streams. The widespread implementation of anaerobic digestion in the biocircular economy for organic waste management has led to the emergence of digestate as a potential reservoir for the release of MP particles. This further amplifies the range of environmental risks associated with MP contamination. Hydrothermal carbonization (HTC) is an easy-to-operate, low-energy technique for advanced waste utilization, although to date MP remediation via this technique is still in the early stage and remains unclear. This study aims to isolate and characterize MPs in digestate and further investigate the efficiency of using HTC for quantifiable MPs removal. The results show that the quantity of MPs from raw digestate was 1400 ± 510 items/kg and 1.01 ± 0.32 g/kg (dry weight digestate). MP shapes mainly occurred as fibers, films, fragments, flakes, and spheres, predominantly in the 0.5 to 1.0 mm size range. On spectroscopic analysis, three polymer types, polypropylene, polystyrene (PS), and polyethylene terephthalate (PET), were identified. It was observed that HTC up to 200 °C significantly reduced the prevalence of quantifiable MPs by ∼90% (particle number). In addition, the average size of MPs in raw digestate was significantly larger than that in the hydrothermally treated digestate, suggesting a shift in MP size distribution to smaller particles, which indicates susceptibility of the MPs to hydrothermal degradation. HTC-induced strong discoloration of PS particles, while PET was most prone to depolymerization. The findings of this study demonstrate the potential applicability of the HTC technique in the degradation of MPs in digestate to mitigate their adverse impacts upon environmental disposal or agronomic application.