Using fluorescence excitation–emission matrix spectroscopy to monitor the conversion of organic matter during anaerobic co-digestion of cattle dung and duck manure

► Spectroscopic characteristics of dissolved organic matter in anaerobic digestion. ► Co-digestion obtains remarkable organic matters removal rate. ► Tyrosine-like/fulvic-like fluorescence intensity indicates nitrogen form conversion. ► Tryptophan fluorescence intensity variations reflect microbial activity. ► Co-digestion corrects the limitation of single manure digestion. In this study, the removal of volatile solids (VSs) and soluble chemical oxygen demand (SCOD) by co-digesting cattle dung (CD) and duck manure (DM) was determined and compared with the reduction achieved with CD or DM digestion alone. Moreover, fluorescence excitation–emission matrix spectroscopy was utilised to characterise the conversion mechanisms of organic nitrogen. It was found that the co-digestion provided 71% VS reduction compared with 58% for CD and 61% for DM. The amounts of COD removed were 28%, 23% and 31% for CD, DM and the mixture, respectively. Tyrosine-like/fulvic-like fluorescence intensity (FI) ratios increased during the initial 15days of co-digestion and were associated with an increase in total nitrogen in the supernatant. After 15days, CD and DM exhibited a lower tryptophan-like/fulvic-like FI ratio (0.8–1.6), whereas the co-digestion remained stable at a high level (3.0–3.6), rendering an improved microbial population and biochemical activity.