The emission of nitrous oxide (N2O) from full-scale anoxic/anaerobic/oxic (A2O) and sequencing batch reactor (SBR) processes was measured to evaluate N2O emission from urban wastewater treatment plants (WWTPs). The results showed that N2O flux in the A2O WWTP followed an order of A2O-oxic zone > aerated grit tank >> A2O-anaerobic zone > A2O-anoxic zone > final clarifier > primary clarifier, while in the SBR WWTP the order was SBR tank >> swirl grit tank > wastewater distribution tank and within the SBR tank in an order of SBR-feeding period > SBR-aeration period > SBR-settling period > SBR-decanting period. N2O emission from the A2O WWTP was approximately 486.61 kg d−1, 96.9% of which was from the A2O-oxic zone. In the SBR WWTP, the emission of N2O was 339.24 kg d−1 with 99.9% of the total emission coming from the periods of feeding and aeration. There was 6.52% of nitrogen-load in the influent being transformed to the emitted N2O in the SBR WWTP; the percentage was 3.35 times higher than that in the A2O WWTP.

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