1. School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China; 2. Key Laboratory of Yellow River Water Environment in Gansu Province, Lanzhou Jiaotong University, Lanzhou 730070, China
Abstract：Extracellular polymer substance (EPS) is an important factor affecting the stabilization of excess sludge. To reveal the relationship between EPS characteristics of sludge and its decomposition of vermicomposting, this study investigated the effect of vermicomposting on structural changes of sludge EPS, by comparing with the control treatment without earthworms. The results showed that the electric conductivity and nitrate nitrogen content of vermicompost significantly increased by 0.64 and 0.22 fold, respectively, compared with control treatment (P<0.05), suggesting that vermicomposting can significantly accelerate the mineralization rate of organic matter in sludge. In addition, total protein, and polysaccharide contents of EPS in each layer of vermicomposting decreased by 32.77% and 31.54% compared with control treatment, respectively. Fluorescence intensity of protein reduced more rapid in vermicompost than control, while humus of vermicompost remained at a higher level in later stages. Compared to control, the tryptophan contents in tightly-bound EPS (TB-EPS) and loosely-bound EPS (LB-EPS) in vermicompost decreased by 7.69% and 13.62%, respectively, while the humic acid content in LB-EPS and soluble EPS (SEPS) increased by 25.1% and 7.82%, respectively. The nitrate nitrogen content and electric conductivity during vermicomposting were significantly and positively correlated with DOC, protein, and polysaccharide contents in the structure of SEPS and LB-EPS (P<0.05), and their total DOC contents was significantly correlated with DOC and polysaccharide contents in each layer of EPS (P<0.05). However, only organic matter and DOC contents were significantly correlated with the polysaccharide contents of EPS layers in control during the experiment (P<0.05). The study indicated that earthworms could destroy the EPS structure of sludge to promote the decomposition of organic matter, thus accelerating the humification and stabilization process of vermicomposting, and the change of EPS structure of sludge could be a key indicator to assess the degree of sludge vermicomposting.
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