Minimizing N2O emissions and carbon footprint on a full-scale activated sludge sequencing batch reactor

• N2O emissions from a full-scale sequencing batch reactor were studied.
• N2O accounted for 60% of the carbon footprint of the treatment plant.
• N2O minimization was achieved implementing intermittent aeration at the bioreactor.
• Performance or electricity consumption were unaffected by N2O minimization strategy.

A continuous, on-line quantification of the nitrous oxide (N2O) emissions from a full-scale sequencing batch reactor (SBR) placed in a municipal wastewater treatment plant (WWTP) was performed in this study. In general, N2O emissions from the biological wastewater treatment system were 97.1 ± 6.9 g N2O–N/Kg NH4+−NNH4+−N consumed or 6.8% of the influent NH4+−NNH4+−N load. In the WWTP of this study, N2O emissions accounted for over 60% of the total carbon footprint of the facility, on average. Different cycle configurations were implemented in the SBR aiming at reaching acceptable effluent values. Each cycle configuration consisted of sequences of aerated and non-aerated phases of different time length being controlled by the ammonium set-point fixed. Cycles with long aerated phases showed the largest N2O emissions, with the consequent increase in carbon footprint. Cycle configurations with intermittent aeration (aerated phases up to 20–30 min followed by short anoxic phases) were proven to effectively reduce N2O emissions, without compromising nitrification performance or increasing electricity consumption. This is the first study in which a successful operational strategy for N2O mitigation is identified at full-scale.

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