Novel insights into integrated fermentation and nitrogen removal by free nitrous acid (FNA) serving as treatment...

Free nitrous acid (FNA) has only been studied as the pretreatment of waste activated sludge (WAS). Integrated fermentation and nitrogen removal using FNA as a primary means of treatment are seldom investigated. WAS fermentation was characterized under various FNA concentration. The production of COD, protein, and carbohydrate increased with FNA concentration (in the range of 0.197-1.97 mg/L) before the denitrification process. Volatile fatty acids (VFA) were only produced after complete denitrification. Potential FNA impact on fermentation step found FNA facilitated both solubilization and hydrolysis but inhibited acidification, acetogenesis, and methanogenesis processes. The types of fermentation were determined using threedimensional excitation-emission matrix (EEM) fluorescence spectroscopy. Protein-like substances and Tyrosine/Tryptophan were the most dominant dissolved organic matters (DOMs). The cell decay rate increased from 0.044 to 0.102/d based on the nonlinear fitting for the FNA concentration of 0.197-1.97 mg/L. The microbial biomass mortality reached 92.7% when the FNA in tight extracellular polymeric substances (T-EPS) exceeded 0.04 mg/L. In addition, the microbial diversity and microbial structure were substantially reduced by FNA during long-term operation, while the bacterial abundance associated with hydrolysis and acidification increased significantly.