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 (N(2)O), 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(2)O by anammox bacteria is lacking, the microorganisms coexisting in anammox-driven WWTPs could produce a considerable amount of N(2)O and hence affect the sustainability of wastewater treatment. Thus, N(2)O emission is still one of the downsides of anammox-driven wastewater treatment, and efforts are required to understand the mechanisms of N(2)O emission from anammox-driven WWTPs using different nitrogen removal strategies and develop effective mitigation strategies. Here, three main N(2)O production processes, namely, hydroxylamine oxidation, nitrifier denitrification, and heterotrophic denitrification, and the unique N(2)O consumption process termed nosZ-dominated N(2)O degradation, occurring in anammox-driven wastewater treatment systems, are summarized and discussed. The key factors influencing N(2)O emission and mitigation strategies are discussed in detail, and areas in which further research is urgently required are identified.