Enhancing denitrification with waste sludge carbon source: the substrate metabolism process and mechanisms.

Using waste sludge internal carbon source for nitrogen removal in wastewater has drawn much attention, due to its economic advantages and sludge reduction. In this study, the performance of enhanced denitrification with waste sludge thermal hydrolysate and fermentation liquid as carbon sources at different SCOD/N (soluble chemical oxygen demand/NO3--N) was investigated. The optimum SCOD/N was 8 for sludge thermal hydrolysate and 7 for fermentation liquid, with NO3--N removal efficiency of 92.3 and 98.9%, respectively, and no NO2--N accumulation. To further understand the fate of sludge carbon source during denitrification, the changes of SCOD, proteins, carbohydrates, and volatile fatty acids (VFAs) were analyzed, and three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis was introduced. The utilization of SCOD was consistent with NO3--N reduction, and the utilization efficiency of different organic matter was as follows: VFAs > proteins > carbohydrates. The soluble organic-like materials (region IV) were the most readily utilized organic matter according to three-dimensional fluorescence EEM spectroscopy. Regarding denitrification mechanisms, the denitrification rate (VDN), denitrification potential (PDN), heterotroph anoxic yield (YH), and the most readily biodegradable COD (SS) were also investigated.