Reduction performance of microplastics and their behavior in a vermi-wetland during the recycling of excess sludge: A quantitative assessment for fluorescent polymethyl methacrylate.

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.

Performance and stratified microbial community of vermi-filter affected by Acorus calamus and Epipremnum aureum during recycling of concentrated excess sludge.

Vermi-filter combined with wetland plant (VFP) is an eco-friendly and sustainable approach for recycling of excess sludge by joint action of earthworms, wetland plants and microorganisms. However, the effects of wetland plants on sludge decomposition and involved microorganisms are remain unclear. This study investigated the effects of wetland plants on the treatment performance of VFP for concentrated excess sludge and microbial community distribution inside the VFP. The wetland plants' species of Acorus calamus and Epipremnum aureum were separately planted on the surface layer of vermi-reactors by earthworms Eisenia fetida. Results showed that the growth rate of E. fetida in VFPs significantly (P < 0.001) increased by 75%, compared to VF. In addition, the removal rates of total solids and chemical oxygen demand in VFPs could reach 61%-79% and 36%-68%, respectively, displaying a better performance of sludge reduction than in the VF. The surface layer of VFPs was a hotspot for degradation of organic matter, where bacteria played an important role in bio-decomposition rather than eukaryotes. Moreover, wetland plants could significantly (P < 0.001) enrich the eukaryotic population, rather than bacterial population. Compared to the VF, the wetland plants could promote the diversities of bacterial community in VFPs, showing specific functioned genus in different layers. In contrast, A. calamus could be a better candidate than E. aureum in the VFP. This study suggests that the inoculation of wetland plants can improve the performance for treating concentrated excess sludge by changing the biomass of earthworms and the structure of microbial community within the VFP.