Biogas Production Characteristics And Microbial Community Structure Of Rapeseed Cake Anaerobic Fermentation

Rapeseed cake is a by-product obtained from rapeseed oil by pressing,is a resource-based agricultural solid waste.China has a huge output of rapeseed cake,if it is not fully utilized,it will not only cause a waste of resources,but also pollute the environment.At present,among the methods of resource utilization of agricultural waste,biogas production by anaerobic fermentation is a clean and efficient way,which can realize the resource utilization of waste and reduce environmental pollution.In this study,rapeseed cake was used as raw material to carry out the potential of anaerobic fermentation to produce methane at medium temperature(35±0.5℃);Taking the idle agricultural biogas digester in Qinghai Province as the fermentation device,the pilot test at low temperature(15.2～17.8℃)was carried out.The characteristics of biogas production by mixed anaerobic fermentation of rapeseed cake and sheep manure at different ratios and the characteristics of microbial community structure were analyzed.The main results are as follows:1.In order to explore the combined effect of solid content and inoculation ratio on the characteristics of methane production in rapeseed cake anaerobic digestion,this study carries out anaerobic fermentation experiments of rapeseed cake with different solid content(2%,4%,6%,8%)and inoculation ratio(1.5,2.0,3.0)at medium temperature using automatic methane potential test system.Cumulative methane yield and production dynamics characteristics,as well as the various indicators of fermentation broth were analyzed.The results show that solid content and inoculation ratio both have an impact on the fermentation start-up time and cumulative methane production,and the inoculation ratio has a greater impact on the gas production characteristics of anaerobic digestion than solid content.Within the range of experimental parameters,the larger the inoculation ratio,the faster the fermentation starts.When the inoculation ratio is constant,the cumulative methane production increases first and then decreases with the increase of the solid content.Under the condition of high solid content,the cumulative methane production increases with the increase of the inoculation ratio.When the inoculation ratio is 3.0 and the solid content is 6%,the maximum cumulative methane production is obtained,which is507.31 m L/g(measured as VS).The kinetic model shows that all treatments can well reflect the methane production law of anaerobic fermentation of rapeseed cake.When the solid content is constant,the larger the inoculation ratio,the shorter the residence time of gas production.Compared with other fermentation raw materials,rapeseed cake has obvious potential as a raw material for anaerobic digestion.2.Based on the agricultural biogasdigester as the fermentation device,to study the characteristics of mixed anaerobic fermented 40d gas of rapeseed cake and sheep manure in 7 different proportions(calculated by dry material).According to the result,it indicated that the entire fermentation process of each mixed treatment is under good operation,which suggested that the mixed fermentation could improve the anaerobicdigestion and operation effect,of which,the accumulated biogas yield had high potential when the dry material proportion of rape seed cake and sheep manure was 2:1,up to 122.92m~3/t.The modified Gompertz model indicated that each treatment could fit the process of producing biogas well,of which,the value of maximum biogas yield(P_m)and maximum biogas production rate(R_m)increased with the increase in the addition volume of rape seed cake,while(λ)decreased with the increase in the addition volume of rape seed cake.Meanwhile,the pure rapeseed cake fermentation had the phenomenon of low p H and late VFAs accumulation.When mixed with sheep manure,the indexes of fermentation tend to be normal,indicating that mixed fermentation can balance nutrients and improve system stability.3.The analysis of microbial community changes during the fermentation of rapeseed cake and sheep manure showed that the bacteria in 29 samples belonged to 7phyla and 17 genera;archaea belong to 5 orders and 10 genera,and different raw material ratios resulted in significantly different on microbial community compositions.Firmicutes,Proteobacteria,Bacteroidetes,and Actinobacteria were the dominant bacterial phyla among the 29 digester samples(total relative abundances>79.23%),followed by Synergistetes(average relative abundances:7.3%).Lactobacillus was the most abundant genus in the biogas digesters with high rapeseed cake contents(14.68%),while Peptoniphilus exhibited higher abundances(12.69%)in the mixed fermentation digester treatments.In addition,unclassified Synergistaceae abundances(6.64%)were positively associated with biogas production variation among treatments.Bacteroides(5.74%)and Pseudomonas(5.24%)both accounted for larger proportions of communities in the digesters that used more sheep manure.Methanomicrobiales(66.55%)was the most dominant archaeal group among digesters,with Methanogenium(41.82%)and Methanoculleus(16.55%)representing the main gas-producing archaeal genera;they were more abundant in biogas digesters with higher sheep manure contents and higher rapeseed cake contents,respectively.No matter which ratio,the relative abundance of Methanoculleus was the highest on the 4th day of fermentation.VFAs and p H were the main factors associated with differences in microbial communities among the 29 samples.Specifically,VFAsconcentrationswas most important,which positively correlated with Lactobacillus,Methanoculleusand Methanothrixabundances,while p H was positively correlated with Bacteroides,Pseudomonasand Methanobacterium abundances.