In present study, the characteristics of soluble microbial products (SMP) were evaluated in aerobic granular sludge system during denitrification process under different chemical oxygen demand/nitrogen (C/N) ratios. Batch experiment showed that the effluent nitrate (NO3−–N) concentration were 15.24 ± 1.83 and 1.72 ± 1.53 mg/L at C/N ratio of 1 and 6, respectively. For the release of SMP, the protein (PN) and polysaccharide contents increased from 1.23 ± 0.38 and 7.46 ± 1.13 mg/L to 1.80 ± 0.76 and 10.53 ± 1.24 mg/L with increasing C/N ratios, respectively. Excitation–emission matrix identified four peaks in SMP, including aromatic PN-like, tryptophan PN-like, fulvic acid-like and humic acid-like substances. Fluorescence regional integration suggested that biodegradable PN-like substances occupied the percentage between 53.0 and 61.7% in SMP. Synchronous fluorescence spectra coupled with two-dimensional correlation spectroscopy indicated that the release of SMP fractions in the early stage (0–150 min) changed in the following sequences: PN-like fraction > fulvic acid-like fraction.
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