Study On The Emission Of N₂O In Anoxic-aerobic Biological Nitrogen Removal Process

N₂O is not only an important greenhouse gas,and also contribute to ozone destruction in the stratosphere,and it has become the major issue influencing the natural ecosystem and threatening the survival of mankind.Wastewater treatment process potentially contributes to N₂O emission,which can be produced in the process of nitrification and denitrification as well.Therefore,active study on production mechanism and control of N₂O in biological nitrogen removal process is of important practical significance.In this study,anoxic-aerobic SBR reactor was set up to investigate the emission of N₂O in nitrification and denitrification process.Colony library of functional gene in close relationship with N₂O emission was also constructed,which provided fundamental data for the regulation and control of N₂O release in the way of microbial population structure.The results showed that the activated sludge in anoxic-aerobic SBR reactor has a good settlement during stable operation with SV₃₀ 18-20%.The system was effective to remove pollutant and the average removal rates forf COD,ammonia nitrogen,total nitrogen and total phosphorus were 93.6%,96%,47.8%and 85.2%,respectively.In the stirring phase of a typical cycle,COD degradation mainly occurred in the first 60min and denitrification proceeded fast in the first 30min.At the 30th min, NO₃^--N concentration decreased to 0.44mg/L;and NO₂^--N concentration reached the maximum of 4.30mg/L,while almost could not be monitored at the end of the phase. NH₄⁺-N was mostly degraded in the aerobic nitrification and its concentration fell to 0 after 150 min.In the aerobic phrase,NO₂^--N reached the maximum concentration of 0.82mg/L at the 120th min and at the 310th min,the concentration of NO₃^--N was maximum when NO₂^--N was almost completely oxidized.The variations of DO,pH and ORP were basically consistent with the changes of pollutant concentrations with corresponding relations,which could be used as reference of stirring and aerobic reaction time in the forthcoming study.N₂O mainly emitted in the aerobic nitrification phrase and the emission in the anoxic denitrification phrase was far less than that from aeration.SPSS statistical analysis showed that the concentration of NO₂^--N and the emission of N₂O has a significantly positive correlation with the sig.of correlation coefficient 0.017.The emission rate was 0.01-0.20mg min^-1m^-3 in stirring phrase and 0.17-5.26mg min^-1m^-3 in aerobic phrase.Six DNA extraction methods were evaluated.The results showed that the concentration and purity of DNA extract from different methods were different. A260/A280 of the DNA extracted by Methodâ…¢was close to 1.80 with the highest purity.The cluster analysis of DGGE patterns for the six methods showed that the similarities between them are up to 74%.Primer concentration,Taq enzyme,Mg²⁺ concentration,DNA template dilution, annealing temperature and cycle number all have specific impact on PCR amplification.The optimized system and conditions were:primer concentration 0.3μM,Taq enzyme 0.5U,Mg²⁺ concentration 2mM,multiple of DNA dilution 5, annealing temperature 52℃and 34 cycles.Clone library analysis showed that 91.3%of amoA gene clones were classified asβ-proteobacteria and Nitrosomonas sp.was the dominant bacteria containing amoA functional gene which may play a leading role in the conversion of ammonia nitrogen into hydroxylamine.The diversity of nosZ gene was abundant which included Rhodobacter and Azospirillum ofα-Proteobacteria,Leptothrix and Azoarcus ofβ-Proteobacteria,and Pseudomonas ofγ-Proteobacteria.The largest group could be classified as Comamonadaceae ofβ-Proteobacteria.