| With the accelerating of global warming, the reduction of greenhouse emission has aroused international attention. Nitrous oxide (N2O) is an important greenhouse gas. Its greenhouse effect is 310 times higher than CO2. It can also destroy the ozone layer and promote the formation of acid rain. Because its concentration in the atmosphere has been increasing rapidly in recent years, N2O has been listed as one of the major problems which can endanger the natural ecosystems and human living foundation. It had been reported that biological nitrogen removal is an important anthropic source of N2O. Therefore, it is very important to study the production mechanism and reduction control of N2O during biological nitrogen removal process.Our study focused on the anoxic-oxic SBR biological nitrogen removal system. Nitrogen transformation under different technological conditions, i.e. anoxic/oxic time phase fraction, temperature, aeration rate, carbon source and carbon nitrogen ratio, were systemically studied. The sources of N2O emission under different technological conditions were also elucidated through chemical inhibition method. By the use of molecular bio-techniques, the changes of microbial community structure related to N2O emission under different technological conditions were successfully tracked. The contributions of different microorganisms on N2O emission were figured out through the comparative analysis of changes between microbial community structure and system feature. Furthermore, feasible N2O reduction strategies were proposed through two angles:optimization of operation conditions and regulation of microbial community. The reduction effects of the proposed strategies were evaluated through experiments. The main research conclusions are as follows:(1) All of the technological conditions researched, i.e. anoxic/oxic time phase fraction, temperature, aeration rate, carbon source and carbon nitrogen ratio, have an effect on the N2O emission of the anoxic-oxic SBR biological nitrogen removal system. Considerating the pollutant removal and N2O emission performance at the same time, the optimum operation conditions for the anoxic-oxic system were PF=0.5 (that is 2h of anoxic and 4h of oxic), T=25?, aeration rate at 2.7 Lair/(Lreactor·h). In addition, use the mixed carbon source and improve the carbon nitrogen ratio as much as possible.(2) No significant N2O emission was observed during the anoxic phase, while most of the N2O emission occurred during the oxic phase. The chemical inhibition method was used to identify the source of N2O emission from anoxic-oxic SBR biological nitrogen removal system. Results showed that:under the optimum operation conditions found in the preliminary research, during the anoxic phase, nitrate ammonification was the major source of N2O emission while denitrification performed as a sink of N2O; during the oxic phase, nitrifier denitrification was the major source of N2O emission while coupled-denitrification has a "negative absorption" effect on N2O emission. However, some factors may affect the source of N2O emission. Aeration rate has an effect on N2O during the oxic phase; Carbon source has an effect on N2O during the anoxic phase; Temperature has no significant effect on the production mechanism of N2O in the anoxic-oxic SBR biological nitrogen removal system.(3) The changes of microbial community structure related to N2O emission under different technological conditions were analyzed by the use of molecular bio-techniques, such as PCR-DGGE, cloning and sequencing, targeting functional genes amoA and nosZ. Results showed that aeration rate and carbon nitrogen ratio has an effect on the community structure of ammonia oxidizing bacteria, with the accumulation of Nitrosomonas-like bacteria under mild aeration rate or carbon nitrogen ratio higher than 6.8. Carbon source has an important impact on community structure of denitrifying bacteria. The decrease in diversity and abundance of denitrifying bacteria were observed when sodium acetate was used as the carbon source. Through the comparative analysis of changes of microbial community structure and N2O emission feature, it was shown that the major source of N2O emission during the oxic phase was nitrifier denitrification of Nitrosomonas spp.(4) Feasible N2O reduction strategies were proposed through two angles: optimization of operation conditions and regulation of microbial community. Results showed that:addition of denitrifying activated sludge during the initial stage of oxic phase, external carbon addition and step-feed technique can reduce the N2O-N conversation rate in the anoxic-oxic SBR biological nitrogen removal system, and the extent of reduction was 39.8%,16.2% and 46.2%, respectively. Meanwhile, these reduction strategies can also improve the removal performance of total nitrogen. |
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