Reduction of microplastics in sewage sludge by vermicomposting

Sewage sludge application in agricultural soils represents a salient route for microplastics (MPs) to enter the environment. The aim of this study was to assess the bioremediation of dewatered anaerobically digested sewage sludge (D-AD-SS) contaminated with MPs through vermicomposting using Eudrilus eugeniae earthworm. The specific objectives were to determine the effects of polypropylene (PP) and high-density polyethylene (HDPE) MPs of sizes 1650-2000 mu m and 146-500 mu m on the physical activities of the earthworms and the chemical properties of the vermicomposting process. Results showed that the presence of MPs had no influence on the percentage mortality of the earthworms compared to the blank. The abundance of PP (146-500 mu m), HDPE (146-500 mu m), HDPE (1650-2000 mu m) and PP (1650-2000 mu m) decreased significantly by 78 %, 56 %, 31 % and 22 % respectively after 14 weeks of vermicomposting. The FTIR spectra for 146-500 mu m showed a reduction in the absorbance band for alkane group by 11 % for PP and 34 % for HDPE, indicating possible biodegradation. Moreover, the presence of MPs had no effect on chemical properties during vermicomposting process in terms of pH, electrical conductivity and C:N ratio. Interestingly, the presence of MPs reduced the cadmium content in the vermicompost compared with the blank. Furthermore, the cluster analysis showed that significant differences existed between PP-MPs (1650-2000 mu m) and the other MPs with respect to the overall impact on vermi-composting process. This study demonstrates that sewage sludge, contaminated with MPs of different types and size ranges, may safely be reused and applied to agricultural soils, once it has been bio-remediated through vermicomposting.