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Reduction performance of microplastics and their behavior in a vermi-wetland during the recycling of excess sludge: A quantitative assessment for fluorescent polymethyl methacrylate
Summary
Researchers quantitatively assessed the removal of fluorescent polymethyl methacrylate microplastic particles (1 µm, 100 µm, and 500 µm) from excess sludge in a vermi-wetland treatment system using Nile red staining, finding removal rates of 86-96%, 95-99.5%, and 100% respectively across the three size classes. The study found the vermi-wetland simultaneously achieved 63-91% chemical oxygen demand removal and 38-52% total solids reduction from the sludge.
Large amounts of microplastics (MPs) that have accumulated in excess sludge may increase the environmental risk for its subsequent treatment. This study aimed to investigate the performance and mechanism of the reduction of MPs in excess sludge in a vermi-wetland. For this, 1 μm, 100 μm, and 500 μm of fluorescent MPs stained with Nile red were added to raw sludge, and their decreased numbers were quantified during the treatment of sludge. The results showed that the removal rates of chemical oxygen demand and total solids from the excess sludge were 63.44%-90.98% and 37.61%-51.56% in the vermi-wetland, respectively. The numbers of 1 μm, 100 μm, and 500 μm MPs could be reduced by 86.62%-95.69%, 95.44%-99.52%, and 100% in the vermi-wetland, respectively. These results indicate that the vermi-wetland is more effective at eliminating MPs. Further insight into the vermi-wetland stratification was obtained, and more than 74.87% of the MPs were intercepted in the vermicompost layer. Moreover, all the particle sizes of MPs were found in the excrement of earthworms. However, only 1 μm MPs were detected in their digestive organs. This study suggests that the interception effect is primarily responsible for elimination of MPs in excess sludge, and the bioturbation of earthworms plays an important role in the mobilization of MPs in vermi-wetlands.