Performance and mechanism of vermifiltration system for liquid-state sewage sludge treatment using molecular and stable isotopic techniques

Performance and mechanism of a conventional biofilter (BF) and a vermifilter (VF) containing earthworms were studied for liquid-state sewage sludge treatment. Results showed that removal rates of the VF for suspended solid (SS), volatile SS, total chemical oxygen demand (TCOD) and 5-day biological oxygen demand (BOD5) were higher than that of the BF. The effluent sludge of VF (ESVF) had lower N, C and H contents, with higher quantities of δ15N, δ13C and aromatic groups than that of BF (ESBF). Compared to ESBF, ESVF had higher activities of protease, glucosidase, lipase and amylase, and less densities of total bacteria, α-, β- and γ-proteobacteria. These indicated the presence of earthworm improved the treatment performance of the VF and promoted the stabilization of ESVF. Additionally, passage through the intestinal tract of earthworms caused a decrease in the densities of α- and β-proteobacteria, and an increase in the density of Nitrospira of the VF biofilms. Stable isotopic analyses implied that earthworm adults, hatchlings and cocoons appeared to be found in the highest position; hirudinea in the second highest position; gastropoda adults and larvae, diptera adults at intermediate level; diptera larvae in the lower position of food web in VF system.

► Compared to BF, VF had better performance for liquid-state sewage sludge treatment. ► Compared to ESBF, ESVF had less C, H and N contents, and higher aromatic groups. ► Compared to ESBF, ESVF had higher microbial enzyme activity and diversity. ► Passage through earthworm intestinal tract changed microbial population density. ► Earthworm seemed to be found in the highest position of trophic level in VF system.