| Nitrous oxide (N2O) is the third most efficient greenhouse gas after carbon dioxide (CO2) and methane (CH4). Two-thirds of the overall N2O is emitted by microbial processes. The biological nitrogen removal (BNR) process in wastewater treatment occupies an extremely important position among the microbial processes of N2O emission. In the BNR process, microorganisms are utilized to convert inorganic nitrogen compounds into dinitrogen gas through different biochemical reactions. N2O can be an intermediate or end product in the metabolism of both nitrification and denitrification processes. Over the past decade, controlling the emission of N2O has become an important part of the biological wastewater treatment process.Our study study attempts to elucidate the dynamics of N2O emission and emission characteristics in a partial nitrification system producing an influent suitable for the anammox process. The study was conducted in long term operation. By using the molecular bio-techniques, the changes of microbial community structure and the evolution of the bacteria community related to N2O emission were successfully tracked. N2O emission under different technological conditions, i.e. temperature, aeration rate, ammonia consentration, and salinity, were systemically studied. Furthermore, feasible N2O reduction strategy was proposed through reducing the feeding rate. The main research conclusions are as follows:(1) Partial nitrification (PN) was achieved respectively in Sequence Batch Reactor (SBR) and Sequence Batch Biofilm Reactor (SBBR). High ammonia nitrogen (500mg L-1) wastewater was used as feed wastewater. Partial nitrification was achieved after57and31days in SBR and SBBR, respectively. Both PN-SBR and PN-SBBR could provide suitable water for anammox in long term. Compared with SBR, the start up of partial nitrification in SBBR was shorter and less energy was consumed with intermittent aeration.(2) The N2O emission was measured in PN-SBR and PN-SBBR. The emission characteristics and dynamics were illustrated. The N2O emission which was about (1.50±0.22)%of nitrogen load in PN-SBBR was higher than that of about (0.80±0.19)%in PN-SBR. The dynamic behavior of the quantity of N2O emission in the partial nitrification process in SBR can be fit to a logarithmic function (y=5.211n(x)-11.31, R2=0.98). The dynamic behavior of N2O emission in the partial nitrification process can be fit to a line (y=0.0375x±0.48, R2=0.99).(3) The microbial mechanism of N2O production in partial nitrification was analysised. The dominant microorganisms in PN-SBR and PN-SBBR are Nitrosomonas sp, and Nitrosospira sp., respectively. The content of EPS and SMP were in a decreasing trend with the operation of partial nitrification. The amount of protein and humic acid reduced obviously. With the achievement of partial nitrification, the amount of carbohydrates increased significantly.(4) The main factors affecting N2O emission during the process of partial nitrification were determined. Measures reducing N2O emisiion in partial nitrification process has carried in the discussion. Temperature had important effect on partial nitrification, the performance of partial nitrification was enhanced and the emission of N2O rose with the temperature going up in the range of15-35℃.The moderate aeration rate contributed to the reduction of the emission of N2O. The increase amount of ammonia concentration and salinity resulted in the growth of the emission of N2O. To provide a stable running environment in partial nitrification process, feasible N2O reduction strategies were proposed in operational process. These measures include controlling suitable hydraulic retention time to reach effective oxygen supply without anaerobic phase, reducing the inflow rate, using moderate aeration rate and so on. |
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