Methane Production From Hydrogen And Carbon Dioxide And Its Microbial Composition And Diversity In Anaerobic Digestion System

Global warming caused by greenhouse gas emissions is one of the most challenging environmental issues today.Carbon dioxide is the main greenhouse gas.Carbon capture,storage and utilization is an effective way to achieve carbon peak and carbon neutrality.The anaerobic digestion system can convert carbon dioxide to high value-added products such as methane or short-chain fatty acids through microbial metabolism,with the advantages of low operating costs,mild reaction conditions,small footprint and environmental friendliness.Therefore,this study adopted two-stage series-connected novel biomethanation-,trickle bed-,and single-stage novel biomethanation anaerobic digestion system to explore the process performance of hydrogen reduction on carbon dioxide to produce methane and biogas upgrading.Base on 16S r RNA gene high-throughput sequencing and bioinformatics analysis,the composition and diversity of microbial community was explored to reveal the changes of microbiota,and the main pathway of methanogenesis.The results were summarized as follows:(1)Study on hydrogen reducing carbon dioxide to produce methane and microbial composition and diversity in two-stage series anaerobic digestion systemUnder ambient temperature(6.4?26.4?),hydrogen and carbon dioxide(mixing ratio 4:1,v/v)were used as raw materials,the feeding gas rate increased from 0.40L/(L·d)to 7.86 L/(L·d).In the gas load increasing stage,the feeding gas rate that the two-stage reactor can bear was 7.33 L/(L·d),and the methane content and the carbon dioxide conversion rate were 90.82%and 93.28%,respectively.Results demonstrated that the methane production rate was 1.47 L/(L·d).In the ex-situ biogas upgrading with H₂ stage,the maximum feeding gas rate that the two-stage reactor could withstand was 8.76 L/(L·d),and the biogas upgrading from 60%to 91.46%CH₄.The carbon dioxide conversion rate was 93.60%,and the biogas upgrading rate was 3.37L/(L·d).In terms of microbial composition,the dominant functional microbiota were hydrogenotrophic methanogenesis(Methanobacterium and Methanobrevibacter).(2)Study on hydrogen reducing carbon dioxide to produce methane and microbial composition and diversity in trickle bed anaerobic digestion systemTwo sets of trickle-bed anaerobic digestion systems were used to study the methanogenesis pathway of hydrogen reduction on carbon dioxide,which are divided into ambient temperature(6.4?27.5?)and medium temperature(30?)conditions.The medium and ambient temperature trickle bed reactors use hydrogen and carbon dioxide mixing ratio of 4:1 as raw materials.The feeding gas rate gradually increases from 0.43 L/(L·d)to 8.04 L/(L·d)in the mesophilic temperature trickle bed anaerobic reactor.The optimum feeding gas rate was determined to be 7.39 L/(L·d),with the carbon dioxide conversion rate was 91.75%and the methane content was 90.97%.The achieved methane productivity was 1.48 L/(L·d).The feeding gas rate progressively increased from 0.40 L/(L·d)to 4.90 L/(L·d)in ambient temperature trickle bed anaerobic reactor,and the methane content gradually increased from15.5%to more than 90%.The optimum feeding gas rate was determined to be 4.65L/(L·d),with the carbon dioxide conversion rate was 94.33%and the methane content was 91.01%.The achieved methane productivity was 0.93 L/(L·d).In the ex-situ biogas upgrading stage,the maximum feeding gas rate that the mesophilic temperature trickle bed anaerobic reactor could withstand was 9.18 L/(L·d),and the corresponding methane content and carbon dioxide conversion rate were 93.31%and91.63%.The biogas upgrading rate was 3.53 L/(L·d).In terms of the microbial composition,the dominant functional microbiota of hydrogen reducing carbon dioxide to produce methane stage and ex-situ biogas upgrading were hydrogenotrophic methanogenesis in the mesophilic temperature trickle-bed anaerobic reactor.However,the dominant functional microbiota of hydrogen reducing carbon dioxide has changed from acetoclastic methanogenesis to both hydrogenotrophic and acetoclastic methanogenesis in the ambient temperature trickle-bed anaerobic reactor.(3)Study on microbial composition and diversity of in-situ biogas upgrading by exogenous hydrogenUnder mesophilic temperature conditions(30?),based on the pig manure biogas fermentation system,the experiment of upgrading biogas by in-situ reduction of carbon dioxide by exogenous hydrogen had been running for a total of 71 days.In the stable stage,the methane content in the biogas was 86.21%,the highest methane content was 88.96%,corresponding to the carbon dioxide content of 7%,the methane content of the control reactor without hydrogen was only about 65%,and the carbon dioxide content was as high as 32%.The cumulative methane production of the two reactors was 306.02 L and 277.48 L,respectively.In terms of microbial composition,the dominant functional microbiota was Methanosarcina.(4)Cmparative analysis of process performance and mian methanogenesis pathway among different anaerobic digestion systemsIn the stage of hydrogen reduction carbon dioxide,the optimal methane production rate of the two-stage series-connected novel biomethanation anaerobic reactor at ambient temperature was 1.47 L/(L·d),which was similar to the medium temperature trickle-bed reactor(1.48 L/(L·d))and significantly higher than 0.93L/(L·d)in the ambient temperature trickle bed reactor.In ex-situ biogas upgrading stage,the optimal biogas upgrading rate of the mesophilic temperature trickle bed was 3.53 L/(L·d),which was slightly higher than 3.37 L/(L·d)in the two-stage series-connected novel biological methanation anaerobic reactor at ambient temperature.The keystone microbiota for the hydrogen reduction on carbon dioxide stage and the ex-situ biogas upgrading stage were Methanobacterium,with relative abundances of 2.61%and 4.94%,respectively;the keystone microbiota for in-situ biogas upgrading stage were Methanosarcina,whose relative abundance was 5.92%.There are significant differences with the microbial community diversity among the three metabolic substrates,with the highest being ex-situ biogas upgrading,followed by hydrogen reduction on carbon dioxide and in-situ biogas upgrading.The biochemical reaction process of hydrogen reduction of carbon dioxide and ex-situ biogas purification was mainly based on the hydrogenotrophic methanogenesis pathway,while that of the in-situ biogas purification had both the hydrogenotrophic and acetoclastic methanogenesis.In conclusion,the process performance of the mesophilic temperature trickle bed reactor was better than the two-stage series-connected and the trickle bed anaerobic digestion system at ambient temperature.The biogas purification stage also showed that the process performance of the medium temperature trickle bed reactor was also better.The methane content of in-situ biogas purification can be stabilized above 86%.The most dominant archaeal taxa associated with methanogenesis keep constant under different reaction systems and process conditions.However,the sub-dominant taxa are different;Hydrogen reduction of carbon dioxide and ex-situ biogas purification are mainly based on hydrogenotrophic methanogenesis,and in-situ biogas purification has both hydrogenotrophic and acetoclastic methanogenesis.This study confirmed that the mixed microbiota producing methane through hydrogen reduction of carbon dioxide can realize the self-circulation of nutrients,without adding nutrients and trace elements for a long time,and the system was relatively stable.