Study On Mechanism Of Biogas Slurry Reflux Combined With Biochar To Enhance Anaerobic Digestion Of Corn Straw And Pig Manure

The mature application of anaerobic digestion（AD）technology promotes the vigorous development of biogas projects in my country.A large amount of biogas slurry produced contains incompletely digested organic substances and is discharged without proper treatment,which is not only a waste of resources,but also brings environmental pollution risks.Therefore,this study analyzed the physicochemical properties of biogas slurry after mixed AD of corn straw（CS）and pig manure（PM）comprehensively.Proceed from the perspective of biogas slurry reflux ratio and reflux mode,use the sequencing batch AD experiment to refine the biogas slurry reflux ratio,and propose the two-phase synergistic reflux mode of biogas slurry.The semi-continuous AD experiment was used to explore the changes in the internal environment stability,gas production performance,etc.And by adding biochar,alleviates gas production inhibition and enhances to strengthen the system stability in the biogas slurry reflux.Based on microorganisms and methane-producing metabolic pathways,the mechanism of the effect of biogas slurry reflux combined with biochar on the AD system was deeply analyzed.Discuss and analyze the application potential of biogas slurry two-phase synergistic reflux mode to improve the stability and economy in the operation of biogas projects.Moreover,this technology not only provides theoretical basis and technical support for biogas slurry reduction and resource utilization,but also provides scientific support for the orderly advancement of the strategic goal of“carbon neutralization and carbon peaking”.The main research contents and conclusions are as follows:（1）Optimize biogas slurry reflux ratio（0%,25%,50%,75%and 100%）and reflux mode（hydrolysis phase reflux,digestion phase reflux,two-phase synergistic reflux）through batch AD experiments,study the effect of biogas slurry reflux on the internal environment and gas production characteristics of AD.The results of the batch biogas slurry reflux AD experiment showed that compared with the no reflux group（CK）,the hydrolysis phase reflux group and the two-phase synergistic reflux the hydrolysis degree of the treatment groups was enhanced,and the initial volatile fatty acid（VFAs）concentration increased by 20.35%-85.81%.Compared with the CK group,the biogas slurry reflux treatment can improve the conversion rate of VFAs,COD（Chemical oxygen demand）,total ammonia nitrogen（TAN）,and free ammonia（FAN）,thereby improve the substrate utilization efficiency of the digestion system.The cumulative methane production performance of system mixed of CS and PM were increased by biogas slurry reflux treatments.Among them,the maximum daily methane production of T20 group（75%,100%）was 65.82 m L/（g VS·d）,which was higher than CK increased by 88.07%,and the maximum cumulative methane production was 422.86m L/g VS,which was increased by 40.69%compared with CK.The modified Gompertz model（R²=0.9959-0.9989）could fit and predict the gas production performance of the mixed AD of CS and PM treated by biogas slurry reflux.The results showed that biogas slurry reflux significantly shortened the lag period of AD reaction（17.29%-48.32%）,and increased the peak gas production（7.54%-88.07%）.Moreover,the biogas slurry reflux treatment significantly enhanced the synergistic effect of the AD system（p<0.05）,and the biodegradation rate increased by 1.36%-40.95%compared with the CK group,and the T20 group had the highest biodegradation rate of 83.65%.Therefore,the reflux ratio of 75%and 100%in the two-phase synergistic reflux mode can maximize the comprehensive benefits of both biogas slurry reflux and cumulative methane production.（2）Based on the 75%and 100%two-phase synergistic reflux mode,the semi-continuous AD reactor was used to study the effect of biogas slurry circulation and reflux on the physical and chemical properties of the system and the stability of gas production.The results showed that the 2-5 hydraulic retention time（HRT）system was in the stage of stable and efficient gas production,and the volumetric methane production（VMP）was increased by 10.28%-43.51%higher than that in the non-reflux stage（1st HRT）.However,with the extension of the reflux period,the lifting effect gradually weakened.From the 6th HRT（126th day）,the gas production of the system was suppressed,and the average VMP was lower than that of the non-reflux period.By the 8th HRT（176-200 days）.The system average VFAs concentration was 15431.31 mg/L,TAN concentration was 10485.04 mg/L,FAN concentration was 381.60 mg/L,and COD concentration was 9702.12mg/L.At this stage,the environment in the system fluctuated sharply,and the high concentration of VFAs and TAN resulted in severe inhibition of gas production.（3）Based on the continuous operation of 8 HRTs（200 days）based on the circulation of biogas slurry in the semi-continuous AD reactor,2%,4%,6%,8%and 10%of biochar were added.In addition,the effect of biochar addition on the environmental changes and gas production characteristics of AD caused by the circulation of biogas slurry was studied.The results showed that the addition of biochar could strengthen the degradation and metabolic pathways of acetate and propionate,thereby reducing the concentrations of VFAs,TAN and COD concentrations.Biochar addition enhanced the buffer system formed by the combination of NH₄⁺and VFAs from C₂ to C₅,maintains the stability of p H,and effectively alleviated the inhibition of gas production in the system.When the biochar addition amount was 8%,the VMP increased from the 8th HRT（453.11 m L/（L·d））to 812.82 m L/（L·d）.It was accounted for 85.71%of the best stage of biogas slurry reflux improvement（948.35 m L/（L·d））.At this time,the VFAs concentration of the system was 5456.11mg/L,the TAN concentration was 5023.96 mg/L,and the COD removal rate increased from 46.42%to 66.27%.The addition of biochar can improve the recycling rate of substrates and enhance the gas production efficiency and stability of the biogas slurry return system.（4）The inoculum,normal operation period,gas production inhibition period（early and late period）and digestion broth of 8%biochar addition period were collected,and the 16S r RNA high-throughput sequencing technology was used to analyze the changes of microbial community composition and structure at each stage.The sequencing results showed that Firmicutes and Bacteroidota dominated the bacterial phyla,with relative abundances ranging from 43.52%to 83.31%and 6.75%to 40.27%,respectively.The gas production inhibition stages（S2,S3）were mainly caused by the decrease in the relative abundance of Bacteroidota.However,the addition of biochar could effectively increase the relative abundance of Bacteroidota,and at the same time increase the relative abundance of Chloroflexi,Spirochaetota and Synergistota bacteria.In Archaea phyla level,Halobacterota and Euryarchaeota were the dominant floras,and the gas production inhibition stage was mainly caused by the decrease of the relative abundance of Halobacterota.The addition of biochar could increase the relative abundance increased from 66.05%to 73.28%,returning to the dominant position,indicating that the addition of biochar could strengthen the tolerance of Methanosarcina.The analysis of bacterial hierarchical clustering and species differences showed that the two-phase synergistic return of biogas slurry and the addition of biochar caused changes in bacterial community structure.Mainly due to the significant increase in the relative abundance of Clostridium＿sensu＿stricto＿1 and Turicibacter,while the relative abundance of Ruminofilibacter,Bacteroidales＿UCG-001 and Sedimentibacter were significantly reduced.The results of archaeal hierarchical clustering and species difference analysis showed that the archaeal community structure was different,which was mainly caused by a significant increase in the relative abundance of Methanosarcina（p<0.01）and a significant decrease in the relative abundance of Methanobacterium（p<0.01）.The results showed that the addition of biochar could promote the evolution of the microbial community structure to a stable operation period.The results of metagenomic sequencing analysis showed that biogas slurry recirculation（S3＿190）directly reduced the relative abundance of methanotrophic metabolism of hydrogen-trophic（0.37%）,the relative abundance of acetate-trophic（0.74%）and the relative abundance of methanol/methylamine-trophic（0.22%/0.24%）methanotrophs.Therefore,the decreased relative abundance of the pathways indicated that the methane-producing metabolic capacity was inhibited at this stage.Compared with the gas production inhibition stage（S3＿190）,8%biochar addition（S4＿60）significantly increased the relative abundances of the above three types of methanogenesis metabolic pathways by 147.32%,44.20%and 139.57%/155.90%respectively.The comprehensive analysis showed that biochar can improve the regulation ability of the system,activate the metabolic activity of methanogens,and enhance the gas production characteristics of the AD system.