Changes in the chemical characteristics of water-extracted organic matter from vermicomposting of sewage sludge and cow dung
► Vermicomposting causes an increase in the aromaticity of WEOM from the substrates. ► Vermicomposting homogenizes the molecular weight of WEOM from the substrates. ► The WEOM from the vermicompost is characterized by high O-containing groups. ► The WEOM from the vermicompost includes small aliphatic and protein-like groups. ► The WEOM test is a good way to evaluate the biological maturity of vermicompost. The chemical changes of water-extractable organic matter (WEOM) from five different substrates of sewage sludge enriched with different proportions of cow dung after vermicomposting with Eisenia fetida were investigated using various analytical approaches. Results showed that dissolved organic carbon, chemical oxygen demand, and C/N ratio of the substrates decreased significantly after vermicomposting process. The aromaticity of WEOM from the substrates enhanced considerably, and the amount of volatile fatty acids declined markedly, especially for the cow dung substrate. Gel filtration chromatography analysis showed that the molecular weight fraction between 10 3 and 10 6 Da became the main part of WEOM in the final product. 1 H nuclear magnetic resonance spectra revealed that the proportion of H moieties in the area of 0.00–3.00 ppm decreased, while increasing at 3.00–4.25 ppm after vermicomposting. Fluorescence spectra indicated that vermicomposting caused the degradation of protein-like groups, and the formation of fulvic and humic acid-like compounds in the WEOM of the substrates. Overall results indicate clearly that vermicomposting promoted the degradation and transformation of liable WEOM into biological stable substances in sewage sludge and cow dung alone, as well as in mixtures of both materials, and testing the WEOM might be an effective way to evaluate the biological maturity and chemical stability of vermicompost.