Effect of COD/N ratio on N2O production during nitrogen removal by aerobic granular sludge

N 2 O-production was investigated during the nitrogen removal using aerobic granular sludge (AGS) technology. A pilot sequencing batch reactor with AGS achieved an effluent in accordance with national discharge limits, although presented nitrite accumulation rate of 95.79% with no simultaneous nitrification-denitrification. N 2 O-production was 2.06 mg L −1 during anoxic phase, with N 2 O-emission during air pulses and aeration phase of 1.6% of the nitrogen loading rate. Batch tests with AGS from the pilot reactor verified that at the greatest COD/N ratio (1.55), the N 2 O-production (1.08 mgN 2 O-N L −1 ) and consumption (up to 0.05 mgN 2 O-N L −1 ), resulted in the lowest remaining dissolved N 2 O (0.03 mgN 2 O-N L −1 ), stripping the minimum N 2 O gas (0.018 mgN 2 O-N L −1 ). Conversely, the carbon supply shortage, under low C/N ratios, increased N 2 O-emission (0.040 mgN 2 O-N L −1 ), ascribed to incomplete denitrification. High abundance of ammonia-oxidizing and low of nitrite-oxidizing bacteria were found, corroborating the partial nitrification occurrence. Denitrifying heterotrophic community, represented mainly by Pseudoxanthomonas , was predominant in the AGS. Overall, the AGS had shown stable partial nitrification ability representing capital and operating costs. The SBR operation flexibility could be advantageous to control N 2 O emission, which extension of anoxic phase would benefit complete denitrification in cases of low C/N influents.