Wastewater treatment plants (WWTPs) are important contributors to global greenhouse gas (GHG) emissions, partly due to their huge emission of nitrous oxide (N
2O), which has a global warming potential of 298 CO
2 equivalents. Anaerobic ammonium-oxidizing (anammox) bacteria provide a shortcut
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Wastewater treatment plants (WWTPs) are important contributors to global greenhouse gas (GHG) emissions, partly due to their huge emission of nitrous oxide (N
2O), which has a global warming potential of 298 CO
2 equivalents. Anaerobic ammonium-oxidizing (anammox) bacteria provide a shortcut in the nitrogen removal pathway by directly transforming ammonium and nitrite to nitrogen gas (N
2). 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 N
2O by anammox bacteria is lacking, the microorganisms coexisting in anammox-driven WWTPs could produce a considerable amount of N
2O and hence affect the sustainability of wastewater treatment. Thus, N
2O emission is still one of the downsides of anammox-driven wastewater treatment, and efforts are required to understand the mechanisms of N
2O emission from anammox-driven WWTPs using different nitrogen removal strategies and develop effective mitigation strategies. Here, three main N
2O production processes, namely, hydroxylamine oxidation, nitrifier denitrification, and heterotrophic denitrification, and the unique N
2O consumption process termed
nosZ-dominated N
2O degradation, occurring in anammox-driven wastewater treatment systems, are summarized and discussed. The key factors influencing N
2O emission and mitigation strategies are discussed in detail, and areas in which further research is urgently required are identified.
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