Nitrous Oxide Emission from Full-Scale Anammox-Driven Wastewater Treatment Systems

Wastewater treatment plants (WWTPs) are important contributors to global greenhouse gas (GHG) emissions, partly due to their huge emission of nitrous oxide (N₂O), which has a global warming potential of 298 CO₂ equivalents. Anaerobic ammonium-oxidizing (anammox) bacteria provide a shortcut in the nitrogen removal pathway by directly transforming ammonium and nitrite to nitrogen gas (N₂). Due to its energy efficiency, the anammox-driven treatment has been applied worldwide for the removal of inorganic nitrogen from ammonium-rich wastewater. Although direct evidence of the metabolic production of N₂O by anammox bacteria is lacking, the microorganisms coexisting in anammox-driven WWTPs could produce a considerable amount of N₂O and hence affect the sustainability of wastewater treatment. Thus, N₂O emission is still one of the downsides of anammox-driven wastewater treatment, and efforts are required to understand the mechanisms of N₂O emission from anammox-driven WWTPs using different nitrogen removal strategies and develop effective mitigation strategies. Here, three main N₂O production processes, namely, hydroxylamine oxidation, nitrifier denitrification, and heterotrophic denitrification, and the unique N₂O consumption process termed nosZ-dominated N₂O degradation, occurring in anammox-driven wastewater treatment systems, are summarized and discussed. The key factors influencing N₂O emission and mitigation strategies are discussed in detail, and areas in which further research is urgently required are identified.