Novel insights into enzymatic-enhanced anaerobic digestion of waste activated sludge by three-dimensional excitation and emission matrix fluorescence spectroscopy

Abstract In our previous study, it has been proposed that the hydrolysis of waste activated sludge (WAS) can be enhanced by hydrolytic enzymes. In this study, fluorescence spectral characteristics of extracellular polymeric substances (EPSs) and dissolved organic matter (DOM) during anaerobic digestion were investigated using three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy to explore the destruction mechanisms of WAS enhanced by additional enzymes (protease, α-amylase and the mixture). Two individual fluorescence peaks associated with protein-like fluorophores (aromatic and tryptophan protein-like substances) were identified in the EEM fluorescence spectra of the EPS after 1 and 6 d, and only aromatic protein-like substances were observed after 12 d of anaerobic digestion for all treatments. As for the DOM, three individual fluorescence peaks were identified, but the peaks associated with visible humic acid-like fluorophores disappeared after 12 d. The EEM fluorescence intensity of EPS decreased during the entire anaerobic process, whereas that of the DOM increased at 1 d and then decreased till the end. In the EPS, the residual protein-like substances were found to be the lowest during the entire anaerobic process when treated with protease. Correspondingly, the protein-like substances in the DOM increased rapidly from 1 to 6 d, and decreased to the lowest level after 12 d for the protease treatment.