Funding: AGAUR and ACC1Ó (Secretaria d'Universitats i Recerca del Departament d'Economia i Coneixement de la Generalitat de Catalunya), 2010VALOR0096. The authors thank Eduard Moliné (DAM) for helpful discussions during the course of the research. M. Pijuan also acknowledges the Ramon y Cajal research fellowship (RYC-2009-04959) provided by the Spanish Government.

Nitrous oxide (N₂O) and methane emissions were monitored in a continuous granular airlift nitritation reactor from ammonium-rich wastewater (reject wastewater). N₂O emissions were found to be dependent on dissolved oxygen (DO) concentration in the range of 1-4.5 mg O₂/L, increasing within this range when reducing the DO values. At higher DO concentrations, N₂O emissions remained constant at 2.2% of the N oxidized to nitrite, suggesting two different mechanisms behind N₂O production, one dependent and one independent of DO concentration. Changes on ammonium, nitrite, free ammonia and free nitrous acid concentrations did not have an effect on N₂O emissions within the concentration range tested. When operating the reactor in a sequencing batch mode under high DO concentration (>5 mg o₂/L), N₂O emissions increased one order of magnitude reaching values of 19.3+/-7.5% of the N oxidized. Moreover, CH₄ emissions detected were due to the stripping of the soluble CH4 that remained dissolved in the reject wastewater after anaerobic digestion. Finally, an economical and carbon footprint assessment of a theoretical scaled up of the pilot plant was conducted.