Nitrous oxide (NO) emissions from a pilot-scale oxidation ditch under different COD/N ratios, aeration rates and two shock-load conditions.

Nitrous oxide (NO) generated from wastewater treatment plants (WWTPs) has drawn attention due to its high emission load and significant greenhouse effect. In the present study, NO emissions from a pilot-scale Carrousel oxidation ditch under various chemical oxygen demand (COD) to nitrogen ratio (COD/N) and aeration rates were systematically investigated. The highest NO emission factor was 0.142 ± 0.013%, at COD/N of 5 and aeration rate of 1.8 m h, which was much lower than the majority of previous studies. The results could be attributed to the high internal recycle ratio of the oxidation ditch process which lightened the burden of influent load to the system. The profiles of NO emissions and dissolved NO concentration along the channels showed a distinct spatial variation that NO emissions primarily occurred in the aeration zones due to the air stripping effect. However, both the aeration and anoxic zones contributed to NO generation due to autotrophic nitrification (AN), which was considered to be the main NO generation process. In addition, two simulated shock-load conditions, ammonia overload shock and aeration failure shock, were carried out to explore the response of the biological nitrogen removal (BNR) system. The results indicated that both shock-loads lead to excessive NO emissions, especially at higher aeration rates, which could be explained by the improved NO generation by AN process during the shock-load period. This study offered new insights into the role of operational parameters to NO emission and the alternative approach for NO mitigation during both the steady-state operation and shock-load conditions in the oxidation ditch process.