Effects of feeding and aeration strategies on N₂O production and emission by an aerobic granular sludge system for municipal wastewater treatment

The effects of a single feeding cycle followed by a continuous aeration phase (AND_C) and a step-feeding cycle followed by intermittent aerobic/idle phases (AND_I) on the production and emission of nitrous oxide (N₂O) from aerobic granular sludge (AGS) from real domestic sewage were studied. Higher N₂O emissions were observed in the AND_I treatment, and 9.2 ± 4.1% of the influent TN was emitted as N₂O, while in the AND_C treatment, 4.6 ± 2.5% of the influent TN was emitted as N₂O. Both strategies were similar for carbon and total phosphorus removal; but AND_I was advantageous for ammonium nitrogen and total nitrogen removal. Regarding the microbial populations associated with N₂O production, genera such as Thauera, a heterotrophic denitrifier, were found to have a relative abundance of 2.1% in AND_C and 3.8% in AND_I. Defluviccocus and Tetrasphaera, organisms capable of denitrification and phosphorus removal, especially the latter, were present in AND_C. Under AND_I conditions, these organisms may have been replaced by fast-growing organisms, such as Thauera. Principal component analysis (PCA) showed that incomplete denitrification was the dominant effect in the AND_C strategy. This may be related to the nitrate and phosphate concentrations and effluent characteristics (low C:N ratio). In the AND_I strategy, incomplete denitrification and low polyhydroxyalkanoate (PHA) consumption were the main effects. This is indicated by the high nitrite and phosphate concentrations. Therefore, according to the PCA results, the combination of the AND_C and AND_I strategies can play a crucial operational role in the dynamics of N₂O production and emission, especially considering that real domestic wastewater was used in the present research.