Effect of aeration regime on N₂O emission from partial nitritation-anammox in a full-scale granular sludge reactor.

N2O emission from wastewater treatment plants is high of concern due to the strong environmental impact of this greenhouse gas. Good understanding of the factors affecting the emission and formation of this gas is crucial to minimize its impact. This study addressed the investigation of the N2O emission dynamics in a full-scale one-stage granular sludge reactor performing partial nitritation-anammox (PNA) operated at a N-loading of 1.75 kg NH4+–N m−3 d−1. A monitoring campaign was conducted, gathering on-line data of the N2O concentration in the off-gas of the reactor as well as of the ammonium and nitrite concentrations in the liquid phase. The N2O formation rate and the liquid N2O concentration profile were calculated from the gas phase measurements. The mean (gaseous) N2O–N emission obtained was 2.0% of the total incoming nitrogen during normal reactor operation. During normal operation of the reactor under variable aeration rate, intense aeration resulted in higher N2O emission and formation than during low aeration periods (mean N2O formation rate of 0.050 kg N m−3 d−1 for high aeration and 0.029 kg N m−3 d−1 for low aeration). Accumulation of N2O in the liquid phase was detected during low aeration periods and was accompanied by a relatively lower ammonium conversion rate, while N2O stripping was observed once the aeration was increased. During a dedicated experiment, gas recirculation without fresh air addition into the reactor led to the consumption of N2O, while accumulation of N2O was not detected. The transition from a prolonged period without fresh air addition and with little recirculation to enhanced aeration with fresh air addition resulted in the highest N2O formation (0.064 kg N m−3 d−1). The results indicate that adequate aeration control may be used to minimize N2O emissions from PNA reactors.