Macroanalysis Of N₂O Emission And Microanalysis Of N₂O Production Inside Microbial Aggregates From Partial Nitritation And Anammox System

Nitrous oxide（N₂O） is a major greenhouse gas in the atmosphere, and its greenhouse effect per molecule is 265 times as great as that of carbon dioxide（CO2）. N₂O mainly produce from biological nitrogen removal（BNR）. Hence, the sources, mechanism, and minimization of N₂O emission from BNR process are of growing concern. Compare to conventional BNR, the operating conditions of a high level of nitrite accumulation and a low level of dissolved oxygen（DO） in innovative BNR have been found to trigger N₂O production. More N₂O emission in innovative BNR restricted the sustainable development of wastertreatment. In this paper, the mechanism of N₂O production and the characteristic of micro-environment inside sludge aggregates were investigated by combination of macro- and microanalysis. The main results were shown as below:1. The separating N₂O microelectrode which tip diameter around 50⁶0 μm was conctructed. The microelectrode was composed of tapered glass microelectrode and outer casing. After polarised process, microelectrod persent good performance under limit of detection. The 90% response times and life cycle of microelectrode were about 9 s and 20 d respectively.2. Inoculated with common activited sludge, a laboratory-scale sequencing batch reactor fed with synthetic high-ammonium wastewater was operated for autotrophic partial nitritation by adjusting influent NH4+ concentrations and air flow. During the stable operation phase, the partial nitritation system was operated at a nitrogen loading rate of 1.87 g?L-1?d-1. The concentrations of NH4+-N and NO2--N in the effluent were277.15±25 mg?L-1 and 293.79±29 mg?L-1 respectively, resulting in an effluent NO2--N/NH4+-N ratio of 1.06±0.08. The effluent was suitable for the subsequent Anammox process. The phylogenetic tree based on PCR-DGGE analysis inferred that autotrophic nitrifying bacteria（Nitrosomonas sp.） coexist with heterotrophic bacteria(Comamonas sp., Flavobacteria sp. and Acidovorax sp. in partial nitritation system.3. The macro N₂O emission characteristics and micro N₂O production inside flocs through an entire typical partial nitritation cycle was investigated. The results were shown follows: N₂O concentration in off-gas reached the peak（802.09 ppmv at 10 th min） and fall back quickly during the initial 50 min of aeration. During the aeration phase of 50³20 min, N₂O concentration in off-gas increased from 203.68 ppmv to 355.27 ppmv. The microprofiles concentration and net volumetric consumption/production rate inside flocs were determined using a microelectrode during the entire cycle（initial aeration phase, middle aeration phase, later aeration phase and settling phase） to investigated the nitrogen transformation and distribution characteristics. The mian results was:（1）air stripping of previously accumulated gas in the settling phase could lead to a higher N₂O emission in the initial aeration phase.（2）higher NO2- along partial nitritation could enhance the N₂O net volumetic production rate inside flocs（from 0.92 μmol?cm3?h-1 to 3.50 μmol?cm3?h-1）.4. Through by addition chemical inhibitor（ATU and DCD）, N₂O emission from different nitrogen substrate（NH4+, NO2-, NO3-, NH2 OH and combine nitrogen） and envirement（aerobic and anoxic condition） test showed that the N₂O mainly product from NH4+ oxidation and NO2- reduction process. NH4+ oxidation when presence of NO2- could improve N₂O emission obviously.5. Under oxygen-limited condition, as DO concentration increased from 0.85 mg?L-1 to 0.60 and 0.35 mg?L-1, N₂O-N/TNinf decreased from 0.57% to 1.81 and 2.35% respectively. Microelectrode analysis showed that the increasements of N₂O inside flocs increased from 29.23 to 66.82 and 78.80 mmol?L-1 respectively when DO decrease. Lower DO levels could stimulate N₂O production from denitrification pathway.6. Under oxygen-limited condition, p H effect on N₂O emission in partial nitritation was investigated. The result show that N₂O emission decreased from 71.85 mg to 37.72 and 44.21 mg respectively when p H increased from 7.5 to 8.0 and 8.5. Microelectrode analysis showed that N₂O mainly produced the surface of NH4+ oxidation zone under p H was 8.5 and 8.0. When p H was adjust to 7.0 and 7.5, N₂O production was mainly occured in the inner layer where DO was almost depleted, it revealed that N₂O production enhanced from denitrification pathway under lower p H conditions.7. The phylogenetic tree based on PCR-DGGE analysis inferred that Anammox bacteria（Candidatus Kuenenia sp.） and heterotrophic bacteria（Sphingobium sp., Sphingobacteriales sp. and Denitratisoma oestradiolicum sp.） coexist in Anammox system. In situ probing of Anammox activity within granules was exploring by microelectrode measurement. It revealed that Anammox and denitrification pathways coexisted in granules. Anammox activity zone was located in surface layer of 0¹500 μm and denitrification activity zone was located in core of 1500²500 μm respectively.8. The N₂O concentration in the off-gas varied from 11.38 ppmv to 29.84 ppmv from Anammox system. N₂O produced from Anammox granules was investigated by microelectrode measurement. The results reveal that N₂O mainly produced in the core of granules where denitrification occurred. This confirmed that N₂O primarily generated from denitrification pathway. The microprofile of N₂O concentrations within granules increased along with p H decreased from 8.5 to 8.0, 7.5 and 7.0 respectively. Higher p H condition could reduce N₂O production in Anammox granules.