Nitrous oxide emissions from secondary activated sludge in nitrifying conditions of urban wastewater treatment plants: Effect of oxygenation level

In order to better understand the mechanisms of N2O emissions from nitrifying activated sludge of urban WWTPs, sludge from the Valenton plant (Paris conurbation) are subjected to lab-scale batch experiments under various conditions of oxygenation. The results show that the highest N2O emissions (7.1 μgN-N2O gSS−1 h−1 in average) occur at a dissolved oxygen (DO) concentration of around 1 mgO2 L−1. These high emissions at low oxygenation (from 0.1 to 2 mgO2 L−1) are due to two processes: autotrophic nitrifier denitrification and heterotrophic denitrification. Nitrifier denitrification always dominates, representing from 58% to 83% of the N2O production. This N2O production originating from nitrifying activated sludge becomes 8 times higher when nitrite is added at a DO of 1 mgO2 L−1; a decrease is observed both at higher and lower oxygenation. Heterotrophic denitrification represents less than 50% of the N2O production, decreasing from 42% to 17% when oxygenation increases from 0.1 to 2 mgO2 L−1. We show that ammonium oxidizing bacteria (AOB) can shift to nitrifier denitrification when oxygen is depleted in the environments including in the WWTPs, nitrite then plays the role of oxygen as the final electron acceptor. As opposed to what happens in nitrification, the end products of nitrifier denitrification are gaseous forms of nitrogen, where N2O is not negligible compared to N2. Overall, N2O emissions represent 0.1–0.4% of oxidized NH4+, depending on the oxygenation level. N2O emissions would range from 0.11 to 0.42 T N-N2O day−1 for a tertiary treatment of the Paris wastewater effluents, consisting exclusively of activated sludge nitrification.