| Straw is the largest and most difficult type of agricultural waste.It is an effective measure that to transform straw into biogas,biogas slurry,and biogas residue by anaerobic fermentation,which can get gas-liquid-solid three-state product from straw,initiativing sustainable development of agriculture.However,lignocellulosic components are difficult to decompose and transform,which become a bottleneck in biogas conversion.Therefore,it is of great significance to study the efficient pretreatment process to promote the decomposition of lignocellulose and improve the efficiency of biogas production.This study aimed at the biological pretreatment,partially combined with chemical pretreatment,studied the pretreatment effect of high-yield enzyme-producing fungi and their enzymes on anaerobic digestion.To explore the pretreatment mode for biogas conversion,optimized the pretreatment agent(microbial or enzyme),addition amount,addition time.Through the VFA,alkalinity,biogas production,methane content and microbial community to explore the role of pretreatment in the biogas conversion of straw.The results were summarized as follows:(1)Three strains of cellulase-producing fungi were screened and identified,which were Trichoderma harzianum,Trichoderma viride and Aspergillus sp.The higest enzymatic activity of the cellulase were obtained by the three strains when the solid-liquid ratio of medium was 1:2,temperatures were 25?,25 ? and 30 ?,respectively,pH was 3.5,and cultivatied for 3 days.(2)In order to explore the pretreatment effect of fungi on straw anaerobic fermentation,fungi were inoculated in different proportions of fermentation substrate,pre-cultured for 3 days,and the effects of fungal pretreatment on anaerobic fermentation were monitored.The greater the proportion of fungi inoculated on the fermentation substrate,the lower the total VFA concentration.The pre-inoculation of fermentation raw substrates was not conducive to the accumulation of VFA during the fermentation and the stability of the fermentation,and reduced the methane content.After 25 days fermentation,the cumulative methane production were:CK group>LT-50%treatment group>HE-50%treatment group>L-50%treatment group>HE-100%treatment group>L-100%treatment group>LT-100%treatment group,which were 377.58 mL/g TS,374.60 mL/g TS,338.72 mL/g TS,331.04 mL/g TS,296.05 mL/g TS,296.01 mL/g TS,294.06 mL/g TS,respectively.(3)According to the above studies,fungi is not conducive to methane production.In order to clarify the effect of fungi and enzymes in the pretreatment process,fungi and enzymes were used as treatment agents and combined with NaOH treatment to explore pretreatment ways to increase the methane production from maize straw.The C/N ratio of maize straw increased by 39.79%and the L/C ratio decreased by 23.08%after NaOH treatment,which increased the straw biodegradability.The reducing sugar concentration,methane content and methane production was reduced after pretreated by Trichoderma harzianum and Aspergillus sp.However,the reducing sugar concentration,methane content and methane production increased after enzymatic pretreatment,and enzymes and NaOH had a synergistic effect on the methane production.In the NaOH +Enzyme HE treatment group,the maximum methane yield was 300.85 mg/L TS,which was 20.24%higher than that in the CK group.The methane production in the Enzyme(L+HE)treatment group was 277.03 mg/L TS,which was 10.72%higher than that in the CK group.(4)It can be known from(3)that the pretreatment effect of the mixed enzyme was better than that of a single enzyme.In order to find the optimal mixed enzyme,three kinds of crude enzymes were combined in different ratios to explore the pretreatment effect of the mixed enzyme.It was found that enzymatic pretreatment increased the total VFA concentration at the beginning of fermentation,especially an increase of acetic acid and butyric acid.The start-up time of methane fermentation was accelerate and the initial methane content increased.The maximum cumulative methane production was obtained by the 0:2:3 treatment group,which was 465.16 mL/g TS,an increase of 47.38%compared with the CK group.In the single enzyme pretreatment,the cumulative methane production in the 5:0:0 treatment group was the highest,which was 376.52 mL/g TS,which was 19.29%higher than that in the CK group.High-throughput sequencing analysis showed that there was a large difference in bacteria between treatment groups.Bacteroidetes and Firmicutes were dominant bacteria for anaerobic fermentation.Methane production increased with the increase of Synergistetes abundance and decreased with the increase of Firmicutes abundance.Apart from the methanogenesis,such as Methanosaeta,Methanosarcina,Methanebacterium,Methanomassiliicoccus and Methanocorpusculum,the number of archaea associated with degrading cellulose,such as Bathyarchaeota and Deep sea Euryarchaeotic Group-DSEG,increased.It can be seen that the pretreatment has a great influence on the bacteria community structure during fermentation for 24 hours,thus affecting the production of methane.In addition to the methanogenesis,the archaea associated with degrading cellulose has played an important role.(5)0:2:3 mixed enzyme(the ratio of the crude enzyme solution from Trichoderma viride and Aspergillus sp.was 2:3)was selected to explor the optimal amount of enzyme and the addition time.With the increase of enzyme addition,the initial total VFA concentration of fermentation gradually increased.When the enzyme addition amount was 100 mL,the total VFA concentration reached 2.30 g/L,which was 80.09%higher than that of the CK group.Enzyme pretreatment improved the methane content of anaerobic fermentation,and the greater the enzyme concentration,the greater the increase.The maximum methane production was obtained by the Enzyme-100 treatment group(384.37 mL/g TS),16.34%increase compared to the CK group.When the enzyme added was 25 mL,50 mL,and 75 mL,the cumulative methane production was less than that of the CK group.This may be due to the low enzyme concentration and failed to achieve a good decomposition effect.The earlier the mixed enzyme is added to the fermentation system,the better the performance of anaerobic fermentation will be.The addition of mixed enzyme happened in pretreatment(-6)and at the beginning of anaerobic fermentation(0)can increase the methane content;adding mixed enzyme during the fermentation process(6d)has no beneficial effect on methane production.The effect of one addition is stronger than multiple additions of equal enzyme amounts.After 25 days fermentation,the maximum methane production was obtained by(0)treated group,which was 365.74 mL/g TS,24.61%higher than that of CK group.Followed by the(-6)treatment group(341.72 mL/g TS),16.43%higher than that of the CK group.In short,the enzyme pretreatment effect is better than the bacteria treatment,the mixed enzyme pretreatment effect is better than a single enzyme effect,and the chemical + enzyme combination pretreatment effect is better than the single mode treatment in the straw anaerobic fermentation.The sooner the mixed enzyme is added to the fermentation system,the more favorable it will be for anaerobic fermentation,and the one-time addition of equal amounts of enzyme will be better than multiple additions.Based on this,a new process for the biological pretreatment of agricultural cellulose waste was established in this study.By accelerating the hydrolysis efficiency of cellulose and increasing the gas production efficiency.As enzyme pretreatment is clean and environment-friendly,biogas slurry and biogas residue can be safely used in subsequent agricultural production,which is of great significance for establishing the self-circulation of the agricultural system. |
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