| With the serious global warming, more and more concerns focus on the greenhouse gas. The fifth Intergovernmental Panel on Climate Change reported that the impact of nitrous oxide on atmosphere is more than 298 times greater than that of carbon dioxide. It has been the third greenhouse gas after the carbon dioxide and methane. The main source of nitrous oxide is agriculture. But, with the development of sewerage system and attention of nitrogen removal technology in wastewater treatment, the nitrous oxide emission from wastewater treatment is also increasing. In this study, nitrous oxide emission from wastewater treatment plant was investigated at Takasu wastewater treatment plant, Kochi, Japan during January 2014 to October 2014.In this wastewater treatment plant, the treatment processes were A/O, two-step feeding A/O and three-step feeding A/O, which shared same influent. We analyzed the monthly and diurnal variation of nitrous oxide emission from three different processes. By comparing the experiment results for these processes, it was expected to discover the process which had the lowest nitrous oxide emission. During the experiment, the three processes were in good treatment performance. The removal rate of COD and total nitrogen were 98.6±0.4% and 88.8±1.8%, respectively. Especially for the removal rate of ammonia, it was kept above 99%.Nitrous oxide emission from three-step feeding A/O process was the lowest among three different processes with 5.0g N/h. And the two-step feeding process emitted 7.8g N/h of nitrous oxide. Additionally, A/O process emitted 10.5g N/h which was the highest. These results showed that, different treatment process can result in different nitrous oxide emission under the condition of the same influent. The difference among those three processes was the feeding mode. Therefore, multiple feeding modes were advance to mitigate the nitrous oxide emission during wastewater treatment. Therefore, the ammonia oxidizing rate during nitrification and denitrification were possibly decreased with the change of C/N ratio with multiple feeding modes, and then nitrous oxide generation and emission decreased. Additionally, the influencing factors of nitrous oxide emission during nitrification and denitrification also were analyzed by this experienment. The results expressed that the main factors of nitrous oxide generation and emission were environmental temperature, ammonia oxidizing rate and dissolved oxygen. The nitrous oxide emission was increasing with the increase of temperature. And that its emission decreaesd when biological actively was stable after temperature achieving a certain level. The ammonia oxidizing rate has the positive relation with nitrous oxide emission. Therefore, multiple feeding modes in this wastewater treatment plant were the effective way to control the C/N ratio to mitigate nitrous oxide generation and emission. Meanwhile, dissolved oxygen was also a significant factor to nitrous generation and emission. Before this study, there were many lab-scale studies on nitrous oxide emission with the influence of dissolved oxygen. They indicated that different dissolved oxygen concentration resulted in significant variation of nitrous oxide emission. And the lowest nitrous oxide emission occurred when dissolved oxygen was 2mg/L in this investigation.From the experienment results, the different wastewater treatment processes will result in different amounts of N2O emissions. Influent mode, ammonia oxidation rate and dissolved oxygen concentration have obvious effect on N2O emission. Therefore, to reduce the impact of N2O on the greenhouse effect, wastewater treatment processes with low N2O emissions and control of operating parameters of the processes can be selected. |
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