| Methane production from lignocellulosic agricultural wastes is a huge potential technologyin the efficient use of these resources because it contributes to reducing carbon emission and providing clean energy.In recent years,high total solid(TS>10%)anaerobic digestion(AD)process has been given great attention since it has many advantages such as the minimized digester size,the less amount of water asrequired,no or less leachate output and the lower energy needed compared to the conventional wet(TS?10%)AD process.However,difficulties in operating high solid AD at industrial scale are related to some problems such as the long solid retention time and the unstable performance of biogas production.In addition,lack of an efficient reactor which has the function of continuous feeding feedstock and discharge digestate is another important problem.Furthermore,not much is known about the complex decomposition process of lignocelluloses and the structural changes of microbial communities during high solid AD.In order to develop a process with high and stable methane production of high solids content anaerobic degeation from lignocellulosic feedstockand and reveal the mechanism of efficient methane production,using cron stalk and dairy manure as the main material,some experiments in this paper were conducted as follows.Firstly,effects of substrate mix ratio,total solids content and substrate to inoculum ratio on methane production rate were discussed in batch anaerobic digestion experiments.Secondly,based on synthesizing the merits of continuous stirred-tank reactor and separated two-phase anaerobic digestion reactor,a novel anaerobic reactor with continuous operation to feeding feedstock and discharge digestate was designed,which was fit for high solids lignocellulosic wastes,then the reactor was started up quickly according to the optimal conditions from the previous batch experiments.Thirdly,the integrated two-division anearbic digestion system in the same reactor was bulit,then effects of total solids content,stirring intensity and organic loading rate on the formation of this systemand methane production were determined.The two divisions werethe acidification division of upside and the methanogenesis division of underpart in the reactor and they were separated.Finally,the dynamic changes of bacterial and archaeal communities'populations were analyzed using the next generation high-throughput DNA sequencing technique.The results were summarized as follows:1.Compared to the single feedstock of digestion,58.8%to 81.7%more methane productions from co-digestion were obtained at the optimal conditions.The highest methane production rate occurred at dairy manure to corn stalk ratio of 1:1 was 235.9 mL/g volatile solid(VS)and the shortest technical digestion time(T80)was 12d at the total solids(TS)of 15%and the feedstock inoculum(F/I)ratio of 2:1.The synergistic effects were proved that the buffering capacity of digestion system was improveddue to dairy manure with high alkalinity and the conversion rate from carbon dioxide to methane at the initial fermentation period was increased due to dairy manure with hydrogenotrophic methanogens.2.The methane productionperiod was shorter for the lower feeding concentrations(FC),while methane production rate decreased and methane production velocity increased firstly and then decreased with FC increasing at the constant inoculums in the batch experiments.The highest of methane production velocity of 0.34 L/L/d,methane production of 206 mL/g VS and T80 of 16d were obtained at the FC of 51 g VS/L.The fermentative bacteria and methanogens were inhibited by the accumulation of total VFAs and the low pH.3.Methane production at the initial period of digestion was improved and the biogas production period shortened with inoculum increasingin the batch experiments.The digestion system kept stable to produce methane at 5%to 25%TS when F/I ratio was 1.0.The highest methane production velocity of 0.38 L/L/d,methane production of 218 mL/g VS and T80 of 21d were obtained at F/I ratio of 2.0 and 20%TS.The failure of the digestion was caused by the accumulation of total VFAs at the initial period of AD.The excessive total solids content inhibited to the activities of bacteria with hydrolysis and acidogenesis.Total solids content had no effect on the dominant methanogen group during AD,which was the family Methanosaetaceae.4.The two-devision anaerobic digestion systemin the same continuous reactor were formed at the lower of stirring intensity,total solids content and organic loading rate(OLR).VS reduction and methane production of reactor with the lower mixing levels(2-6 h/d)was 20.9%to 28.6%and 17.2%to 37.5%were higher than those of reactor with the higher mixing levels(4-12 h/d)respectively,and the highest methane volumetric production rate(MVPR)of the former was 1.65L/L/d at the OLR of 5.50 to 13.44 g VS/L/d.The two-devision anaerobic digestion systems in reactors were stilled formed respectively at the TS of 15%and 20%.MVPR of the reactor with 15%TS increased and that of the reactor with 20%TS remained stable with OLR increasing at the OLR of 5.50 to 20.17g VS/L/d.The highest MVPR of 1.70 L/L/d was obtained from the former,which was 61.9%higher than that of the latter at the OLR of 20.17 g VS/L/d.5.The populations of bacteria and archaeal communities had obvious differences between acidification devision and methanogenesis devision.The dominant bacteria communities were Bacteroidetesand Firmicutes in the two devisons,and the represented bacteria communities were Proteobacteria and Spirochaetae in the acidification and methanogenesis devision,respectively.Acinetobacter belonging to Proteobacteria and Lactobacillales belonging to Firmicutes were found in acidification layer,which might play an important role in improving hydrolysis and acidogenesis rate of lignocellulosic biomass.Meanwhile,Ruminococcaceae and Syntrophomonas belonging to Firmicutes were found in methanogenesis layer,which might an important role in methane formation.6.The concentrations of main methanogens in methanogenesis devision were obviously higher than those inacidification devision,and the difference between the two devisions was narrower with stirred intensity,TS content and OLR increasing.The dominant methanogen group of the two devisons in the reactor with 10%TS was the aceticlastic family Methanosaetaceae at the OLR of 5.50 to 20.17gVS/L/d.The change of the domiant methanogen group was observed with a shift from the aceticlastic family Methanosaetaceae at the OLR of 5.50g VS/L/d to the hydrogenotrophic order Methanobacteriales at the OLR of 20.17 g VS/L/d in 15%TS and 20%TS reactors,meanwhile methane production decreased with OLR increasing. |
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