Nitrous oxide (N_2O) emissions from a pilot-scale oxidation ditch under different COD/N ratios, aeration rates and two shock-load conditions

摘要

Nitrous oxide (N_2O) generated from wastewater treatment plants (WWTPs) has drawn attention due to its high emission load and significant greenhouse effect. In the present study, N_2O 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 N_2O emission factor was 0.142 ± 0.013%, at COD/N of 5 and aeration rate of 1.8 m~3 h~(-1), 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 N_2O emissions and dissolved N_2O concentration along the channels showed a distinct spatial variation that N_2O emissions primarily occurred in the aeration zones due to the air stripping effect. However, both the aeration and anoxic zones contributed to N_2O generation due to autotrophic nitrification (AN), which was considered to be the main N_2O 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 N_2O emissions, especially at higher aeration rates, which could be explained by the improved N_2O generation by AN process during the shock-load period. This study offered new insights into the role of operational parameters to N_2O emission and the alternative approach for N_2O mitigation during both the steady-state operation and shock-load conditions in the oxidation ditch process.