Application of a mitigation method for nitrous oxide emission in a full-scale Carrousel reactor: Carbon footprint assessment.

Nitrous oxide (N2O) emissions from biological nitrogen removal processes in sewage treatment plants greatly contribute to their overall carbon footprint. The present study aimed to mitigate N2O emissions from an elliptical Carrousel bioreactor in a full-scale plant. The oxygen supply agitators equipped at the influent point and the opposite side of the reactor was operated alternately. The dissolved N2O (DN2O) concentrations were lowered when the agitator at the influent point was suspended while that on the opposite side was running. This scenario was associated with high levels of complementary DNA (RNA) from potential complete denitrifying bacteria, indicating increased N2O reduction activity utilizing influent organic matter. However, during periods of reduced influent organic load, dissolved oxygen (DO) levels temporarily increased; thereafter, DN2O increased, accompanied by a decrease in DO. This fluctuation was associated with the accumulation of nitrite and nitrate resulting from ammonia oxidation during the high-DO periods. Based on these findings, an N2O mitigation strategy was implemented: reducing the oxygen supply and increasing the running time of the opposite-side agitator during the low-organic-loading periods. This approach effectively decreased the DN2O levels, although a certain degree of instability remained during rainfall events. The median N2O emission factor decreased from 0.86 % to 0.28 %, reducing the annual carbon footprint of the plant by 14 %. This study provides valuable insights into N2O mitigation for full-scale plants and demonstrates the great impact of N2O reduction on their carbon footprint.