Effect Of Biogas Slurry Reflux On Carbon And Nitrogen Transformation And Microbial Community Structure In Anaerobic Fermentation Of Straw

Biogas production by anaerobic fermentation technology is one of the effective ways of straw resource treatment,and the large amount of by-products such as biogas slurry will inevitably cause secondary pollution.The most economical and effective way to solve this problem is to recycle the biogas slurry;the reflux ratio and organic load are the main factors affecting the biogas slurry recycling,and it is very important to study the characteristics of anaerobic fermentation.In this experiment,corn straw was used as fermentation material,and different reflux ratios（0%,50%,75%,100%）and organic loads（2g/（L.d）,3g/（L.d）,4g/（L.d））were set up to study the effects of different conditions on carbon and nitrogen morphology,enzyme activity and microbial flora structure in the system.The main conclusions are as follows:（1）Compared with four fermentation systems,the methane yield was 134.4,167.0,196.0 and 198.8m L/g,and the total reflux group was 47.9%higher than the non-reflux group.The redox potential in the reflux test group was lower than-300 m V,and the biogas slurry reflux improved the system stability.The redox potential was positively correlated with the biogas slurry reflux ratio.（2）The effect of different working conditions on the carbon morphology transformation was studied.It was found that the dissolved organic carbon content increased in each group at organic load of 4g/（L.d）,and gas production inhibition occurred in the non-reflux group.The removal rate of dissolved organic carbon could be improved by biogas slurry reflux,of which 75%was the best.There was no acid inhibition in the four fermentation systems,and there was a positive correlation between the reflux ratio of biogas slurry and the utilization rate of volatile fatty acids.Comparing the concentration of dissolved organic carbon（DOC）and volatile fatty acid（VFA）,it can be found that the increase of DOC concentration may be due to the accumulation of other refractory substances.（3）The influence of different working conditions on the transformation of nitrogen forms was studied.It was found that biogas slurry reflux could weaken the decreasing trend of Kjeldahl nitrogen content;ammonia nitrogen content decreased to 188.97-292.72 mg/L when organic load was 4 g/（L.d）,which did not inhibit and could meet the normal growth needs of microorganisms;biogas slurry reflux promoted the transformation of ammonia nitrogen to nitrate nitrogen and increased nitrate nitrogen content in the system.The concentration of nitrite nitrogen fluctuates in the range of 100 mg/L after running a water retention period,and the lower the slurry reflux ratio,the more obvious the decreasing trend of total nitrogen content in the system.（4）Both the reflux ratio and organic loading of biogas slurry could promote the activity of xylanase,cellulase,coenzyme F₄₂₀ and dehydrogenase,and the reflux ratio was positively correlated with the former three,but had no regular effect on the activity of dehydrogenase.（5）When studying the structure of bacterial community,it was found that the increase of reflux ratio and organic load reduced the diversity of bacterial community and increased the richness of bacterial community.At the phylum level,the dominant bacteria mainly include Bacteroidetes,Verrucomicrobia,Firmicutes,Chloroflexi and Proteobacteria.Relative abundance of Bacteroidetes was increased by biogas slurry reflux ratio and organic load,while the relative abundance of Firmicutes was opposite;higher biogas slurry reflux ratio was more conducive to slowing down the decrease of relative abundance of Proteobacteria.In 75%reflux test group,Chloroflexi was the first dominant bacterium when the organic load was 3 g/（L.d）;the organic load was positively correlated with the relative abundance of Verrucomicrobia.（6）The increase of biogas slurry reflux ratio decreases the diversity and richness of Archaea community,while the increase of organic load can enhance the diversity and richness of archaea,which is significantly lower than that of bacteria.The dominant archaea are Euryarchaeota and Crenarchaeota,among which Euryarchaeota occupies an absolute dominant position.The biogas slurry reflux changed the structure of microflora.With the accumulation of biogas slurry reflux time,the relative abundance of methanophilus（Methanospirillum,Methanobacterium,Methanosphaerula and Methanomassiliicoccus）increased,and the high reflux ratio slowed down the decline of dominant status of methanophilus acetophilus（Methanothrix）.Low.The increase of organic load changed the relative abundance of dominant bacteria and enhanced the dominant position of hydrogen-trophic methanogens in the whole fermentation system.（7）Proteobacteria、Bacteroidetes、Methanothrix and Methanosphaerula were positively correlated with cellulase and xylanase,while Synergistetes、Verrucomicrobia、Chloroflexi、Methanospirillum and Methanomassiliicoccus were positively correlated with dehydrogenase and coenzyme F₄₂₀.