Humic Acid Promotes Volatile Fatty Acids Production from Excess Sludge in Anaerobic Fermentation

Excess sludge was used to produce volatile fatty acids (VFAs) by fermentation, which has recently attracted much attention because the sludge-derived VFAs were able to function as a kind of ideal carbon source for wastewater biological treatment. This study specifically targeted the mechanism behind the effects of humic acid (HA) on promoting production of VFAs from excess sludge during acidogenic fermentation. The HA was characterized using Fourier-transform infrared (FTIR) spectrum, 13C nuclear magnetic resonance (13C NMR), and 3D excitation–emission matrix (3D-EEM). When HA was present at 1.20 g/g of total chemical oxygen demand, the content of VFAs produced was maximal (1752 mg COD/L) and was about 2.15 times that of the control without HA (815 mg/L). Mechanism exploration revealed that the dominant population of microbial community in the sample with added HA had shifted to Firmicutes, which was related to the acid production. More importantly, the electron transfer between nicotinamide adenine dinucleotide hydrogen (NADH) and nicotinamide adenine dinucleotide (NAD+) was enhanced by HA, contributing to improve the VFAs' accumulation. In contrast, the activities of the key enzymes, including acetic kinase (AK) and phosphotransferase (PTA), in the acidogenic stage had little change with HA addition, which meant that HA could not stimulate acidification by the way of changing enzymatic activities.