Distribution and Transformation of Organic Matter During Liquid-State Vermiconversion of Activated Sludge Using Elemental Analysis and Spectroscopic Evaluation

Liquid-state vermiconversion (LSV) of excess activated sludge by vermifilter (VF) is a low-cost and ecologically sound process. A VF (with earthworms) and a conventional biofilter (BF; no earthworms) were steadily performed in parallel to treat sewage sludge for 200 days. Elemental analysis, Fourier transforms infrared spectroscopy, gel filtration chromatography, and three-dimensional excitation-emission matrix fluorescence spectroscopy were used to identify changes in organic substances through LSV. Results demonstrated that after BF and VF treatment, the C/N and H/C ratios increased in the VF treatment and decreased in the BF treatment. On the basis of elemental analysis and Fourier transforms infrared spectra, the vermicast in the VF treatment appeared increasingly similar to native excrement expelled by earthworms. Additionally, the VF played a crucial role in converting large molecules (160 kDa) into intermediate molecules (141.9 kDa), while the BF had no effect on the conversion of the large molecules (169.5 kDa). Excitation-emission matrix fluorescence contours described that humic-like substances were detected in sludge treated with VF, while humic-like substances were not detected in BF, suggesting that the VF provides more adequate stability on activated sludge. Consequently, vermifiltration is adaptable for LSV of excess activated sludge, especially in small towns where the economic and technical aspects of treatment are of concern.