Insight into the roles of earthworm in vermicomposting of sewage sludge by determining the water-extracts through chemical and spectroscopic methods

•Water extracts can be used to assess the roles of earthworm in vermicomposting.•The activities of earthworms promote the nitrification in the vermicomposting.•Earthworms can accelerate the stabilization process in vermicomposting. This work illustrated the effects of earthworm in vermicomposting (Eisenia fetida) by determining the water-extracts through chemical and spectroscopic methods. A field experiment with sludge as the only feed was subjected to vermicomposting and the control (without worms) for three weeks. Compared to the control, vermicomposting resulted in lower pH and water-extractable organic carbon (WEOC) along with higher electrical conductivity (EC). Moreover, vermicomposting caused nearly two times higher content of water-extractable nitrate (WEN-NO3-) than the control. Furthermore, fourier transform infrared spectra (FT-IR) revealed that vermicomposting promoted the hydrolysis/transformation of macromolecular organic matters and accelerated the degradation of polysaccharide-like and protein-like materials. Fluorescence spectroscopy also reflected vermicomposting led to higher humification degree than the control. In all, this study supplies a new view to assess the roles of earthworm in vermicomposting of sewage sludge by evaluating the water extracts.