| Since the Industrial Revolution,the human economy had the rapid development.Meanwhile,due to the large use of fossil fuels,CO2,CH4 and other greenhouse gases(GHG)had been excessively emitted,which had finally led to the global warming.The development of biogas digesters can not only accelerate the resource utilization of livestock and poultry breeding wastes,protect the rural living environment,but also reduce the GHG emissions which is in line with the international trend of carbon emission reduction.When accounting the carbon emission reduction of biogas digesters,the establishment and application of methodologies are involved.Among the many indicators involved,methane conversion factor(MCF)is the most critical,and its establishment process is extremely complicated.At present,the most biogas projects use the default or empirical MCF values,which are highly arbitrary and valuable.This study combines the international background to respond to climate change,and meets the needs of domestic ecological civilization construction and independent carbon emission reduction.Based on clarifying the establishment principles of MCF,this study made some researches and calculates the regional MCF for a typical biogas fermentation ecosystem,and finally fitted a new calculation model of MCF for a typical biogas fermentation ecosystem,which will provide a reference for the establishment of the related database of default MCF in Southwest China.This study mainly used the traditional technique and the static chamber technique to investigate the methane emission flux in a typical biogas fermentation ecosystem,and then used the 16 S r DNA sequencing technique and the stable isotope technique to investigate the relationships among the distribution of microbial communities,methane conversion pathways,and methane emission flux.Finally,this study calculated the MCF for a typical biogas fermentation ecosystem according to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories,and established the suitable new calculation model of MCF for a typical biogas fermentation ecosystem.The main research conclusions are as following:1.This study performed a characteristic analysis on the methane emission fluxes from the inlet,outlet,and in total of a 100 m3 hydraulic biogas digester.The study found the diurnal patterns of CH4 emission fluxes from the inlet and outlet were inconspicuous,CH4 emission fluxes from the inlet,outlet,and in total showed evident monthly pattern,which was mainly attributed to the fermentation temperature and feeding quantity.For the 100 m3 biogas digester fed with pig manure,the measured CH4 emission fluxes from the inlet,outlet,and in total were respectively 1.199 g/(m2·h),2.432 g/(m2·h),and 53.845 g/(m3·d).Importantly,the 100 m3 biogas digester had obvious CH4 leakage,with the leakage ratio of 15.06 %.2.Through the changes of the microbial community in the system,we found that:(1)Climate changes caused changes in the fermentation temperature in the system,and the changes of fermentation temperature affected the composition of microbial community from the biogas fermentation ecosystem.The smaller the change of the fermentation temperature,the smaller the change of the diversity of microbial community.(2)Clostridiales,Methanosarcinales,Bacteroidales,and Anaerolineales were the four dominant bacteria groups under the stable operation of biogas fermentation,with their relative abundances of 33.80 %,27.70 %,18.60 %,and 5.38 %,respectively;the subdominant floras were Synergistales,Lactobacillales,Methanomicrobiales,and Selenomonadales,with their relative abundances of 4.63 %,3.28 %,2.18 %,and 1.11 %.(3)Methanogens were mainly dominated by Methanothrix,Methanospirillum and Methanosarcina,with their relative abundances of 42.00 %,1.38 %,and 0.88 %.The experimental results showed that during the stable operation of biogas fermentation,acetic acid-type methanogens were dominant.3.During the stable operation of a typical biogas fermentation ecosystem,the isotopic ratio of CH4 was maintained between-36 ‰ and 48 ‰,and the isotopic ratio of CO2 was maintained between-14 ‰ and 30 ‰;the contribution rate of CH4 production in the CO2 reduction pathway were all less than 30%,and the overall value of ?mc was small.The experimental results show that during the stable operation of a typical biogas fermentation ecosystem,the methanogenic metabolic pathway was mainly dominated by the CH3 COOH cracking pathway,and the methanogenic metabolic pathway changed little during this process.4.When the Chao 1 index,Simpson index,and Shannon index were used as indicators to evaluate the changes of the microbial community,the changes of the microbial community during the experiment did not cause a significant change in the methane conversion pathway,and the effects of them on the daily CH4 production and CH4 emission flux were not significant.The reason may be that the research area had a small change in regional pressure and temperature.Meanwhile,the biogas fermentation ecosystem used semicontinuous fermentation,which had been in a stable gas production stage for a long time.As a result,even if there is a small change of the microbial community in the system,it is always in a dynamic stable equilibrium.The RDA analysis between environmental factors and Top20 bacteria showed that the fermentation temperature,feeding quantity,daily methane production and methane emission flux had a very significant positive correlation with Methanothrix,which indicated that the increase of temperature and feeding quantity would promote the growth and reproduction of Methanothrix,and then made it an absolute dominant flora in the system.Therefore,the CH3 COOH cracking was the dominant methanogen pathway during the experiment period,and the corresponding methane emission flux also changed.5.Based on the characteristics of a typical biogas fermentation ecosystem,the MCF values of the 100 m3 biogas digester was calculated by simplifying the calculation model,with 14.10%.Meanwhile,this study fitted a new MCF calculation model of a typical biogas fermentation ecosystem,and obtained MCF =(-0.349Xfeeding-0.262 Xtemperature + 1.622Xflux-15.029)* 100 %(P <0.01,R2 = 0.9552)... |
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