Coupling of endogenous/exogenous nitrification and denitrification in an aerobic granular sequencing batch reactor

The performance of endogenous/exogenous nitrification and denitrification in an aerobic granular sequencing batch reactor was investigated for treating inorganic wastewater with ammonia nitrogen of 250 mg/L. The sequencing batch reactor with an effective volume of 120.5 L was started by seeding autotrophic nitrifying granular sludge (ANGS) and operated under oxic (110 min)/anoxic (120 min)/oxic (110 min) aeration mode. The total inorganic nitrogen (TIN) removal efficiency of ANGS was between 60% and 70% without external carbon sources in days 1-25. However, the operation mode was unsustainable as endogenous nitrification and denitrification would lead to an obvious decrease of sludge concentration. After sodium acetate (the contributed chemical oxygen demand in the reactor was 250-300 mg/L) was added at the beginning of the anaerobic/anoxic stage from day 26, aerobic granules were inadaptable in a few days, which resulted in particle disintegration and SVI increase. As microbes gradually acclimated to the new environment, the aerobic granular sludge became smoother and denser, the relative abundance of denitrifying bacteria increased to 66.07%, and the removal efficiency of TIN gradually increased to more than 90% from day 89. Contributions of endogenous/exogenous nitrification and denitrification to TIN removal were 54.09% and 46.01%, respectively. The coupling of endogenous/exogenous nitrification and denitrification could reduce the aeration consumption, save the external carbon source dosage and decrease the alkalinity consumption, which provided another option for treating wastewater from ionic rare earth mine.