| Nitrous oxide(N2O) is an important greenhouse gas. It is about 300 times more effective than carbon dioxide(CO2), and a major sink for stratospheric ozone. GHG emissions could be categorise as nature emission source and anthropogenic emission source, and the anthropogenic emissions from water treatment is dominant for globle climate change. So limiting anthropogenic N2 O emission is such an urgent requirement now. N2 O emissions from the biological treatment of sewage, landfill leachates and industrial effluents have gained considerable interest among policy makers and scientists. Estimated global emission rates from these sources can contribute up to 10% of the anthropogenic N2 O emissions. The climate change would exceed the level that human and nature could endure if no proper mitigation measures to implement, and a high price would be paid in social, environmental and economic area.N2O emissions are associated with several processes in wastewater treatment plants. Its emission fluxes are variable and mostly depend on the operational arameters and environmental conditions. A lot of researches has been made to investigate the mechanism of N2 O emissions and operating factors that affect N2 O production in SBR treating wastewater. But little study has been done about the process of N2 O production during biological nitrogen removal in SBBR filed. In this study, by treating domestic wastewater with SBBR, the effect of membrane density, hydraulic load, aeration method, DO concetration and temperature on N2 O emissions were investigated. The optimal operating parameters that meet the water quality standard and mitigation of N2 O emission were found in this research, and some strategies to mitigate N2 O emissions from SBBR were proposed.① Membrane density is known to affect both N2 O emission and water qulity during water treatment. It is showed N2 O emissions increased with the membrane density, but the COD concentration was decreased from 52.12mg/L to 28.77mg/L, the removel rate was increased from 73.2% to 85.6%, and the TN concentration reached its nadir when the density is 50%. However, there was an increase in N2 O production when the membrane density was up to 70% and the efficiency of water treatment was inhibited as well. The N2 O yield and convertion rate are lowest, at 0.89 mg and1.55%, when the membrane density is 15%. As a result, the optimal membrane density in SBBR treating wastewater is 50%.② Hydraulic load has an effect on both N2 O emission and water qulity during water treatment, and a linear relation was shown among N2 O, HRT and drainage ratio. Under the given HRT, N2 O emissions increased with the hydraulic load(when the drainage rate varied from 1/4 to 2/3), the COD concentration was increased and the removal rate was decreased relately. Under the given drainage ratio λ, N2 O emissions showed a decrease when hydraulic load increased(when the HRT varied from 8h to 14h), both the COD and TN concentration were increased as the same time. The N2 Oyield varied from 0.69 mg to1.58 mg and the conversion rate varied from 2.28% to 3.26%. However, there is no law to follow when HRT or λis not given, it’s depend on specific operating factors and environmental conditions.③ Aereation process has a certain effect on N2 O emission, as to water qulity, it is unobvious under the given operating conditions. The COD concentration varied from 47.13mg/Lto 54.57mg/L, and the TN concentration was increased from 18.39mg/L to 21.39mg/L,the removel rate was decreased from 70.5% to 65.7% relately. It is showed N2 O emissions is higer in continuous aereation process than intermittent aeration process. Aereation has a great influence on the way of N2 O emissions. According to the study, N2 O emissions was mainly occurred during the aereation, for continuous aereation, it experienced an uptrend in the first then a downtrend. For intermittent aeration, little emission could be seen during intermittent time, which is mainly depend on the ratio of aereation and intermittent time. The N2 O yield and conversion rate reached its nadir, at 0.98 mg and 1.57%, when the ratio of aereation and intermittent time was 2h: 1h. As a result, the optimal ratio of aereation and intermittent time in SBBR treating wastewater is 2h: 1h.④ DO concentration is considered a very important parameter controlling N2 O emission during nitrification. The COD concentration decreased from 47.11mg/Lto 32.74mg/L, and the removel rate was increased from 77.3% to 84.2% relately. The TN concentration increased from 17.41mg/Lto 27.39mg/L, and the removel rate was decreased from 66.4% to 45.9% relately. It is showed when the DO concentration increased from 2mg/L to 5mg/L, the N2 O yield and conversion rate were decreased from 0.94 mg to 0.58 mg and 1.83% to 1.12%, respectively. Though it is beneficial to adopt a high DO concentration, overaeration would lead to unnecessary cost, so the optimal DO concentration should depend on the practical needs and conditions.⑤ Temperature has little effect on COD removal, the COD concentration varied from 32.94mg/L to 35.76mg/L. An uptrend can be seen in TN removal when the temperature increased from 20℃ to 30℃, the TN concentration was decreased from 17.31mg/L to 14.94mg/L,the removel rate was increased from 62.5% to 67.6% relately.. When the temperature was higher than 35℃, a lightly decrease was seen in TN removal, to 66.7%, at 15.39mg/L. The N2 O emission was increased with the temperature in the study, the yield and conversion rate reached its nadir at 0.41 mg and 0.99% when the temperature was 20℃.There are some strategies to mitigate N2 O emissions from SBBR: Avoiding overaeration; Avoiding low COD/N ratio, it could be solved by external carbon addition; Avoiding inhibitors of enzyme activity; The on-line monitoring system is needed to establish a realistic quantification of N2 O emission and microbial activity, which is benefical for AOB nitritation and avoiding rapidly changing process conditions; Other strategies, such as intermittent aeration process, innovative wastewater treatment process with low N2 O emissions, long SRT, and so on. |
PDF Full Text Request