Performance of different aeration strategies for biological nutrient removal and the mitigation of N2O emissions

Aerobic granular sludge (AGS) represents a sustainable alternative to conventional biological wastewater treatment, as it allows simultaneous nitrogen (N) and phosphorus (P) removal with a reduced dissolved oxygen (DO) and carbon demand, and improved settleability. Emissions of the greenhouse gas nitrous oxide (N2O) are a major challenge for the sustainability of this technology. The aeration is a key operational parameter with a strong influence on the dynamics of N2O formation, reduction, and emission. This study aims to evaluate the long-term performance and stability of two different aeration strategies. Under alternating aerobic and anoxic conditions, a DO of 2 – 4 mg O2∙L-1 was found unfavorable for a stable PAO community, while 1 – 3 mg O2∙L-1 resulted in rapid stabilization. This strategy was characterized by pronounced N2O formation and stripping during aerobic steps. Continuous low aeration with an average DO of 0.5 mg O2∙L-1 was found beneficial for stable P removal, high efficiency of simultaneous nitrification and (shortcut) denitrification (SND), and low N2O levels. The two aeration strategies resulted in notable differences in the microbial communities of nitrifiers and denitrifiers, which are decisive for N2O production.