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 delta N-15, delta C-13 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, alpha-, beta- and gamma-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 alpha- and beta-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. (C) 2012 Elsevier B.V. All rights reserved.