Nitric oxide and nitrous oxide emissions from a full-scale activated sludge anaerobic/anoxic/oxic process

• Online monitoring emissions of NO and N2O from a full-scale A2/O for the first time.
• NO and N2O emitted per N load of 0.002–0.021% and 0.095–3.44%.
• NO and N2O production and emission mainly occurred in oxic zones.
• Nitrite accumulation triggered NO and N2O production and emissions.
• N2O emission patterns closely resembled those of NO.

Both nitric oxide (NO) and nitrous oxide (N2O) have strong negative effects on the environment. Although N2O emissions in wastewater water treatment plants (WWTPs) have been widely studied, the concurrence of NO and N2O emissions has not been well characterized, and their emission status from WWTPs in China is still not clear. In this study, online NO and N2O analyzers were used to investigate spatial and temporal variations of NO and N2O emissions in a full-scale activated sludge anaerobic/anoxic/oxic (A2/O) process over one year. The amoA gene of ammonia oxidation bacteria and the nosZ gene of denitrifiers were analyzed to identify possible NO and N2O production pathways. The emission factors of NO and N2O in the studied A2/O process were 0.01 ± 0.006% (0.002–0.021%) and 1.29 ± 1.07% (0.095–3.44%) of the nitrogen load, respectively. Both NO and N2O were mainly emitted from the oxic zone, with emitting levels increasing greatly from the initial oxic zone to the zone end. The N2O emission pattern closely resembled that of NO emissions, but the emission amounts were 2–3 magnitudes higher than that of NO. Nitrite accumulation directly triggered NO and N2O production. This study provides novel insights into emission characteristics and the production pathways of both NO and N2O from a full-scale A2/O process, which is of great significance toward development of effective mitigation strategies for NO and N2O emissions from WWTPs.

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