| Nitrous oxide (N2O) is one of three major greenhouse gases in the world. With the intensification of human's activities, the concentration of N2O is rising in the atmosphere, and it is easy to cause global surface warming. Wastewater treatment plants (WWTPs) have been recognized as one of the important N2O emission sources. Due to the differences of process performance and operational parameters in different WWTPs, it is difficult to study and control N2O variations in full-scale WWTPs. This study was conducted in a full-scale WWTP in Japan treating the same water quality of urban wastewater with three different anoxic/oxic activated sludge processes. The spatial variation and daily variations of N2O emissions were investigated, and the effects of different environmental factors on N2O emissions were analyzed. In addition, the relationship between nitrification rate and gaseous N2O rate were discussed. The aim of this study is to clarify the variations of N2O in different time periods of the anoxic/oxic activated sludge process, and to determine the main affecting factors on N2O emission, which provide the basic data and theory for N2O emission reduction strategy.First of all, N2O emissions from three different anoxic/oxic activated sludge processes were compared. Grab samping conducted at 12:00 in the morning, and the concentrations of D-N2O and G-N2O were determined at all of the reaction tanks in the processes:Process 1(Anoxic-1/Oxic-1/Anoxic-2/Oxic-2), process 6(Anoxic/Oxic-1/Oxic-2/Oxic-3) and process 7 (Anoxic-1/Oxic-1/Anoxic-2/Oxi c-2/Anoxic-3/Oxic-3). Process 6 was determined as the main contributor of N2O emission in the WWTP, in which D-N2O and G-N2O were detected at much higher concentrations. At the same time, N2O emissions from each reaction tanks of the process were compared, and highest D-N2O and G-N2O concertrations of 41.1?g N/L and 36.2?g N/L, respectively were observed at Oxic-3. This results demonstrate a large spatial variation of N2O emission in the WWTP, and imply the higher potential of N2O production via nitrification in oxic stage of this anoxic/oxic process.Secondly, continuous on-line measurement was used for monitoring D-N2O emissions at Oxic-3 in process 6, the time of continuous on-line measurement were lday,3days and 4days with N2O wastewater sensor (made in Denmark). N2O concentrations appeared the large fluctuaiton at different times of the day. In general, the peak of D-N2O concentrations appeared in the afternoon between 14:00 and 17:00, very low D-N2O concentrations were detected at midnight and early in the morning, it kept stable until next morning, and then increased. The results demonstrated the daily variations of N2O emissions in process 6.Diurnal 8h investigations and intensive 24h investigations were conducted in different seasons to clarify the daily variation of N2O and affecting factors. The N2O wastewater system was used for on-line monitoring of D-N2O in Oxic-2 and Oxic-3 of process 6 at 15 minutes intervals. D-N2O concentration was maintained stably at lower levels in Oxic-2, while sharp peak was observed in Oxic-3. The highest D-N2O peak of 68.4?g N/L appeared in June, and the lowest peak of 15?g N/L was observed in September. The highest daily mean D-N2O concentration was 31.5?g N/L in October. Factors affecting N2O emission were DO, temperature, and influent BOD loading, respectively. As a result of aeration management in the WWTP, DO concentration became high before 11:00-12:00 in the morning, therelater it was stable at low concentration. High D-N2O concentrations were observed in Oxic-3 when DO became lower than 0.4 mg/L; Water temperature also affected N2O emissions, judging from the survey results of different seasons. When temperature became lower than 22.1 ? in winter, nitrification reaction was incomplete at the end of Oxic-2, leading to accumulation of N2O in Oxic-3. When the average DO was 1.12mg/L, negative correlation between the influent BOD loading and daily variations of D-N2O was observed, the lower influent BOD loading, the higher D-N2O production.Finally, it was studied to evaluate the N2O emission rate, quantitative analysis for material quality of input and output, and then study the relationship between nitrification rate and N2O emission. In oxic stage, with the lowest nitrification rate, D-N2O concentration was the highest, N2O will produced through hydroxylamine oxidation in oxic process when nitrification was incompleted; low NH4+-N concentration in nitrification will enhance denitrification in nitrification stage, resulting in more N2O accumulation. G-N2O emission rate increased with the increase of nitrification rate in October, when nitrification rate higher than 0.41mg N/L/h; On the contrary, in December, G-N2O emission rate decreased with the increase of nitrification rate, when the nitrification rate higher than 1.07 mg N/L/h. |
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