Effects Of Straw Pretreatment And Additives On Biogas Production By Anaerobic Co-Digestion Of Corn Stover And Chicken Manure

Biogas production by biomass anaerobic digestion(AD)is a kind of clean and renewable energy technology,which is of great significance to the waste treatment and utilization of crop straw,livestock manure and other wastes.However,the AD process of a single substrate is unstable due to the imbalance of low nutrient components and low biodegradability.Corn stover(CS)has complex lignocellulosic structure,which is not easy to be degraded.Free ammonia and ammonium ions will be produced during AD process of chicken manure(CM)and other high-nitrogen substrate,and inhibit the activity of microorganisms.This is not conducive to AD to produce biogas.Anaerobic co-digestion(co-AD)is the simultaneous anaerobic fermentation of two or more different substrates with complementary characteristics to solve the limitation of single substrate degradation.Co-AD can not only improve the fermentation characteristics and improve the operational stability of AD system,but also alleviate the shortage of raw substrates and ensure the continuous supply of substrates.Therefore,it is of great significance to carry out the research of co-AD for promoting biogas production.In this study,experiments of batch and semi-continuous AD were conducted based on the co-AD of CS and CM.This research through the study of the pretreatment of CS,adjust the ratio of co-substrates,urea addition and biochar addition regulation method,to improve the nutritional balance of substrate,reduce the production of inhibitory substances,so as to improve the properties of AD.It can provide theory basis for producing biogas by co-AD technology and the resource utilization of agricultural wastes.The main contents and results of this study are as follows:(1)The co-AD of CS and CM was carried out in biochemical methane potential(BMP)device.Urea pretreatment and Na OH-H₂O₂ pretreatment of CS were used to study the synergistic effect and kinetics of co-AD to produce biogas at different VS ratios(1:0,2:1,1:1,1:2,1:3,0:1).Results showed that straw pretreatment can promote synergistic effect of co-AD.Synergistic effects were found in CS(Na OH-H₂O₂pretreated)/CM ratios of 2:1,1:2,1:3 and CS(urea pretreated)/CM ratios of 1:1,1:2.Synergistic effect was not found at CS(unpretreated)/CM ratios of 1:2 and 1:3.Pretreatment of CS can produce synergistic effect on co-AD and increase cumulative methane yield by 6.54%–24.65%.Na OH-H₂O₂ pretreatment effect on CS was better than urea pretreatment in terms of AD promotion.The highest cumulative methane yield of 332.7 m L/g VS was obtained from the CS(Na OH-H₂O₂ pretreated)/CM ratio of 1:3;and the highest cumulative methane yield of 319.7 m L/g VS was obtained from the CS(urea pretreated)/CM ratio of 1:2.Among the four kinetic models,modified Gompertz model was best fitted in describing the methane production during co-AD(R²=0.9845–0.9988).(2)The semi-continuous co-AD of CS and CM was carried out using continuous stirred tank reactor(CSTR).The addition of pretreated CS was used for semi-continuous co-AD based on CM mono-AD,so as to explore the mechanism of the VS ratio of CS/CM affecting the long-term operation of semi-continuous co-AD system and the recovery of biogas production.Results showed that CM semi-continuous mono-AD system could not keep stable operation.After running for a period of time,the biogas production reached a low level with the SMP of 0.06 m L/g VS,and the AD system showed strong inhibition with TAN concentration of 7000 mg/L and VFA concentration of 6000 mg/L.The addition of CS could promote the recovery of biogas production and maintain the stability of AD system.The ratio of CS(Na OH-H₂O₂pretreated)/CM was 1:3 has the highest SMP of 0.33 m L/g VS.The ratio of CS(urea pretreated)/CM was 1:2 has the highest SMP of 0.31 m L/g VS.However,the excessively high ratio of pretreated CS(when the ratio of pretreated CS to CM was 1:1)could affect the hydrolysis rate of AD process and reduced biogas production due to the high content of lignocellulosic materials.(3)The semi-continuous co-AD of CS and CM was carried out using CSTR.The effects of urea addition and urea pretreatment of CS on the characteristics of co-AD were studied,and the effects of urea addition and pretreatment in co-AD were compared.Results showed that urea addition in AD system can promote the operation stability of AD system and increase methane production.The combined effect of urea addition and urea pretreatment of CS had antagonistic effect on co-AD system and reduced methane production.Intermittent addition of urea can make up for the lack of urea pretreatment of CS,its advantage is more obvious under high OLR condition.At the OLR of 2.1 and6.3 g VS/L/d,the VMP of CS/CM-HRT-urea system was 1.94%and 14.17%higher than that of UPCS/CM system,while at the OLR of 4.2 g VS/L/d,the VMP of UPCS/CM system was 1.15%higher than that of CS/CM-HRT-urea system.(4)The performance of biochar mediated semi-continuous co-AD of CS and CM using CSTR was studied,and the effect of biochar addition with urea pretreatment was evaluated.Results showed that urea pretreated CS(UPCS)and biochar addition in AD system can improve co-AD for biogas production.The effect of urea pretreatment is similar to that of biochar addition,and their synergistic effect was apparent under medium and high OLR conditions.When the OLR was 4.2 and 6.3 g VS/L/d,the biochar mediated UPCS/CM co-AD operated stably with the VMP of 2.160 and 1.616L/L/d,and VMP of the biochar mediated UPCS/CM were 32.8%–89.6%and 27.8%–96.4%higher than other reactors,respectively.The results reveal that urea pretreatment and biochar addition promoted AD process through strengthening the buffer capacity system established by TAN and VFA and improving the degradation of lignocellulosic biomass.(5)The digestates of biochar mediated semi-continuous co-AD of CS and CM were sampled and the change of microbial community in the digestates was studied by16S r RNA gene sequencing technology.Results showed that both the biochar addition and the urea pretreatment of CS can promote the diversity of microorganism in the semi-continuous co-AD system of CS and CM.The dominant bacteria are Firmicutes and Bacteroidota,with the relative abundance of 39.52–85.04%and 8.60–48.93%,respectively.The dominant archaea are Euryarchaeota and Halobacterota,with the relative abundance of 30.97%–61.52%and 16.82%–62.69%,respectively.The dominant archaea on gunes level are Methanosarcina,Methanobacterium and Methanobrevibacter,with the relative abundance between 15.19%–61.41%,16.69%–40.11%and 8.00%–21.03%,respectively.Biochar addition reduced the VFA concentration by regulating the relative abundance of Proteobacteria,and enhanced the tolerance of Methanosarcina to promote the methane production of AD.When the OLR was 4.2g VS/L/d,biochar addition and urea pretreatment could adjust bacterial community structure to make it close to the structure under the low OLR condition by changing relative abundance of norank?f??Bacteroidales?UCG-001 and norank?f??norank?o??MBA03.This is advantageous to the AD for producing biogas.Different OLR conditions and different biochar addition and urea pretreatment conditions could change the microbial community structure.Biochar addition and urea pretreatment can improve the tolerance of bacteria and archaea for high OLR.This study applied various methods to promote the bacth and semi-continuous co-AD of CS and CM process.The results showed that straw pretreatment technology,increasing the ratio of CS in co-substrate,adding exogenous nitrogen source of urea and adding biochar can improve the AD characteristics in different aspect,adjust the microbial community structure,strengthen the stability of co-AD system,thus promote biogas production.