Anoxic phases are the main N2O contributor in partial nitritation reactors treating high nitrogen loads with alternate aeration

• 3.6% of the influent TN removed as N2O when treating mature leachate in a PN-SBR.
• Anoxic and aerobic phase contributed to 60% and 40% of N2O production, respectively.
• NO2− denitrification rate always higher than N2O denitrification rate.
• Anoxic phase should be avoided in PN reactors treating high N industrial wastewater.

Partial nitritation (PN) reactors treating complex industrial wastewater can be operated by alternating anoxic-aerobic phases to promote heterotrophic denitrification via NO2−. However, denitrification under stringent conditions can lead to high N2O production. In this study, the suitability of including anoxic phases in a PN-SBR treating real industrial wastewater was assessed in terms of process performance and N2O production. The PN-SBR was operated successfully and, when the HCO3−:NH4+ molar ratio was adjusted, produced a suitable effluent for a subsequent anammox reactor. 10–20% of the total influent nitrogen was removed. N2O production accounted for 3.6% of the NLR and took place mainly during the anoxic phases (60%). Specific denitrification batch tests demonstrated that, despite the availability of biodegradable COD, NO2− denitrification advanced at a faster rate than N2O denitrification, causing high N2O accumulation. Thus, the inclusion of anoxic phases should be avoided in PN reactors treating industrial wastewaters with high nitrogen loads.

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