| The 13th Five-Year Plan of the national rural biogas development had put forward new requirements for biogas industry in China,namely 172 full-scale biomethane projects and 3150 large scale biogas projects will be newly built,which makes the traditional raw materials could not satisfy the needs of industrialization for biogas that have been growing rapidly.It is extremely urgent to develop diversified utilization technology for substrate.The amounts of available crop straw and kitchen waste were approximately 180 and 90 million tons,respectively,which exhibited a huge production potential.The readily formation of scum,easily acidified system caused by largely fed feedstock,difficulties about charging and discharging make the practical biogas project about crop straw digestion difficult to operate,with low efficiency of straw utilization.To solve the problems,we established an integrated two-phase system,which combined an upper acidogenic phase and a lower methanogenic phase.By adopting an reconstructive continuous stirred tank reactor(CSTR)with screw-feeder and a trial-manufactured integrated two-phase system,we studied the efficient methane-producing mechanism about difficultly and easily biodegradable substrates digestion(corp straw and kitchen waste),and the effect of mixing intensities on the biogas-producing performance,we also studied the microbial communitits through high-throughput sequencing technology and real-time fluorescent quantitative polymerase chain reaction.(1)Co-digestion of high-solid rice straw and caw dung:The screw type CSTR,with 1.71 heigh-diameter ratio,was used in this study.The biogas yield increased first and then decreased when it reached the highest value as the organic loading rate(OLR)was raised from 8 to 24 gVS/(L·d).The highest volumetric biogas and methane production were achieved at 16 gVS/(L·d)(the hydraulic retention time,HRT,was 6.25 days),with values of 3.00 and 1.64 L/L·d.As the OLR increased,the HRT was shortened,resulted in a incomplete biogas potential release from substrate.The highest specific methane yield were achieved at 8 gVS/(L-d)with value of 157.03 mL/gVSadded.The low height-diameter ratio and the strong mixing intensity made it failed to establish an integrated two-phase system in the present CSTR.Firmicutes(the relative abundance was 36.18%)and Bacteroidetes(20.73%)were the predominant bacteria in the system,and the main methanogen was Methanosaeta,including relative abundance and quantity.(2)Co-digestion of high-solid kitchen waste and caw dung:The same CSTR was used in this study.The biogas yield increased as the OLR raised from 4 to 7 gVS/(L·d).The highest volumetric biogas and methane production were achieved at 7 gVS/(L·d)(HRT was 14.29 days),with values of 3.76 and 1.81 L/L-d.As the OLR increased,the HRT was shortened,resulted in a incomplete biogas potential release from substrate.The highest specific methane yield were achieved at 4 gVS/(L-d)with value of 293.44 mL/gVSadded.The fluidic substrate status,low height-diameter ratio and strong mixing intensity made it failed to establish an integrated two-phase system in CSTR.Bacteroidetes(43.58%)and Firmicutes(35.26%)were the predominant bacteria,and the main methanogen was Methanosarcina(61.43%in relative abundance)and Methanomicrobiales(51.00%in quantity).(3)Effect of mixing intensities on methane digestion performance fed with kitchen waste:The trail-manufactured integrated two-phase system,with the height-diameter ratio of 3.5,was adopted in this study.The OLR was set at uniform 4 gVS/(L·d)throughout the whole experiment process and the mixing intensities were choose as 20,45,and 70 rotation per minute(rpm).As the mixing intensity increased,the biogas yield from substrate showed an increase trend.The highest volume methane production and specific methane yield were both achieved at 70 rpm,with the values of 1.72 L/L·d and 430.67 mL/gVSadded.The fluidic status of kitchen waste made it failed to establish an integrated two-phase system,even the height-diameter ratio of reactor was as high as 3.5.With the low mixing intensity(20 rpm),the predominant bacteria was Firmicutes(48.66%and 46.12%in acidogenic and methanogenic phases,respectively),significantly higher than the second rich Bacteroidetes(24.18%and 20.94%in acidogenic and methanogenic phases,respectively).With the high mixing intensity(70 rpm),Bacteroidetes replaced Firmicutes and became the predominant bacteria with relative abundance of 28.44%and 29.96%in acidogenic and methanogenic phases,respectively.The relative abundance of Fimicutes in aforementioned phases were 24.18%and 20.94%.The enhanced mixing intensity increased syntrophic bacteria(Bacteroidetes,Synergistetes)richness that play critical roles in methanogensis.Methanosarcina was the predominant methanogen at low mixing intensity(67.15%and 64.74%in acidogenic and methanogenic phases,respectively),higher than Methanosaeta with relative abundance of 13.34%and 24.16%in acidogenic and methanogenic phases,respectively.However,the number of Methanosaeta was higher.Methanosaeta changed to be the predominant methanogen(63.81%and 74.13%in acidogenic and methanogenic phases),and it’s number significantly higher than Methanosarcina.The acetic acid concentration in digestive system was the main factor that affected whether Methanosarcina was the predominant methanogen or not,and the factor which influence Methanosaeta was its filamentous cells integrity.(4)Effect of mixing intensities on methane digestion performance fed with cornstalk:This study was also performed using the integrated two-phase system.The experiment was conducted at OLR 3 gTS/(L·d),feeding with two states of cornstalk:fresh cornstalk(FC)and silage cornstalk(SC).Two mixing intensities 20(R1)and 70(R2)rpm were choose.The high solid content of cornstalk with low density and the high height-diameter ratio made it successful to establish a clear two-phase system.The volumetric methane production of FC obtained from R1 and R2 was 0.80 and 0.78 L/L·d,the specific methane yields obtained from R1 and R2 were 290.87 and 281.54 mL/gVSadded.Correspondingly,the performance of SC obtained from R1 and R2 were 0.69 and 0.67 L/L-d,and 250.69 and 243.56 mL/gVSadded.The higher soluble substance content and lower lignocellulose content made FC achieved a better biogas biogas production efficiency,compared with SC.In addition,regardless of which kind of substrate was fed,R1 performance was higher than R2,which demonstrated that a low mixing intensity could improve the biogas yield release from cornstalk in this reached.Firmicutes was the main bacteria in acidogenic phase,the methanogenic phase’s predominant bacteria,however,were Bacteroidetes,Chloroflexi,and Synergistetes.No matter the acidogenic or methanogenic phases,the predominant methanogen was Methanosaeta.Cornstalk floated in the top of the reactor and completed acidogenesis,when the compactness was low,vigorous mixing could promote the acidogenesis;when the ompactness was high,however,strong mixing intensity could increase the compactness further,resulted in volatile fatty acids accumulation and difficult diffusion.A low mixing intensity could contribute to methanogens enrichment in methanogenic phase,in contrast,a high mixing intensity would enhance the methanogens loss along with slurry discharging. |
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