Study On The Disinhibition Effect Of Iron Modified Biochar On Enhanced Anaerobic Fermentation In Reflux System

Anaerobic fermentation not only produces clean energy,but also produces a large amount of biogas slurry.The large-scale and long-term operation of biogas project makes the supply of biogas slurry exceed the demand,but also restricts the rational utilization of biogas slurry due to transportation,seasonal farming and other problems.Biogas slurry reflux not only solves the problem of biogas slurry drainage,but also improves the fermentation performance of the system to a certain extent.However,the long-term biogas slurry reflux and high load operation are prone to the accumulation of volatile organic acids,a mmonia nitrogen,humic acid and other inhibitory intermediates,which affect the biogas production.How to mitigate the inhibition effect,maintain the stable operation of the system and improve the methane conversion efficiency is an urgent problem to be solved at present.In this study,biogas residue carbonization and anaerobic digestion were combined,and biogas residue rich in lignocellulose was used to modify its surface properties by iron salt,and the modification method was optimized to develop iron-modified biochar for straw anaerobic fermentation system to improve the synergistic effect of iron oxide and biochar.The regulation effect of iron-modified biochar on the operation of high-load anaerobic fermentation under reflux system was explored,and the influence mechanism of iron-modified biochar on methane production by anaerobic fermentation was analyzed based on the changes of fermentation characteristics and microbial community structure.The main conclusions are as follows:Using corn straw biogas residue as matrix,iron modified biochar was prepared by ferric nitrate impregnation pyrolysis at different temperatures（500℃,600℃,700℃）and impregnation concentrations（0.2mol/L,0.6mol/L,1.0mol/L）.The results showed that the modified biochar had higher conductivity（155.9μS/cm～276μS/cm）and p H value was slightly alkaline（7.31～7.95）.Because nitrate provided additional oxygen,the number of oxygen-containing functional groups（-OH）was enhanced and a new Fe-O bond was formed,and iron ions were successfully loaded onto the biochar.After modification,iron exists in bivalent and trivalent forms on biochar.Through XRD analysis,the two newly added obvious diffraction peaks are the diffraction peaks of Fe₃O₄,indicating that Fe₃O₄ was successfully synthesized on biochar.The surface of the original biochar is smooth,while the surface of the modified biochar is rough and attached with a large number of Fe₃O₄ particles.The modified biochar has a larger specific surface area（140.405m²/g～227.164m²/g）and a larger micropore volume（0.047m³/g～0.07m³/g）.It can provide more attachment sites for microorganisms.In order to investigate the effect of biochar and iron-modified biochar on anaerobic fermentation,the results showed that the addition of biochar improved the biogas production performance of anaerobic fermentation based on corn straw as substrate,and maintained the stability of anaerobic fermentation system.The original biochar prepared at different temperatures increased the cumulative biogas production by 11.0%～33.8%,and the iron-modified biochar showed a greater advantage in promoting biogas production,increasing the cumulative biogas production by 16.1%～64.1%.Among them,the iron-modified biochar prepared at 600℃and 0.6mol/L has the best promoting effect on straw anaerobic fermentation biogas production.The comparison of the effects of biochar modification on microbial community structure of anaerobic fermentation showed that the addition of iron-modified biochar increased the abundance of microorganisms in the system.The optimal biochar was added to semi-continuous biogas slurry reflux anaerobic fermentation system to explore the regulation effect of iron-modified biochar on corn straw anaerobic fermentation obstacles under reflux system.The results showed that the addition of iron-modified biochar alleviated the acidification phenomenon in the biogas reflux system under high load operation.The average daily methane production in the added group was11.46L/d,which was 162.24%higher than that in the non-added group.The methane content was 11.11%higher,and the hydrogen sulfide content decreased from 138ppm to 40ppm.Compared with the untreated group,it decreased by 70.33%;The total volatile acid content was decreased.The content of humic acid in the liquid phase of iron-modified biochar decreased by 10.31%,and the inhibition of humic acid on anaerobic fermentation was reduced.Microbial results showed that the addition of iron-modified biochar changed the microbial community structure,increased the relative abundance of Firmicutes and actinobacteria,and increased the relative abundance of hydrogen and acetic acid producing HN-HF0106 bacteria（9.51%）.The relative abundance of the dominant hydrogen-producing bacteria DMER64 was decreased（8.54%）,and the relative abundance of the three sulfate-reducing bacteria Syntrophobacter,Sporotomaculum and Pelotomaculum was decreased.The relative abundance of Methanosarcina,the main electron accepter,was increased（51.20%）by iron-modified biochar,and the direct interspecific electron transfer was promoted in the addition group.When the organic load in the anaerobic fermentation system was increased and acid inhibition occurred,the interspecific hydrogen transfer was inhibited.Iron-modified biochar can promote direct interspecific electron transfer to become the main mode of intertrophic metabolism,and maintain the basic stability of methanogenesis.It also promoted the production of acetic acid methanotrophic form.