Selection of Cellulose-Degrading and Hydrogen-Producing Bacteria and Hydrogen Production Process from Cassava Residues

Author:Zhang Lu Yan

Supervisor:ren nan zuo ding jie


Degree Year:2019





Energy production from waste can not only solve the problem of wasting of resources and environmental pollution,but also provide a new approach for the development of renewable and clean energy.Cassava residues are the agricultural processing wastes after extraction of cassava starch extraction,with the characteristic of plentiful supplyment and low price.Fermentative hydrogen production from cassava residues can turn the waste into treasure and has a good application prospect.However,at present,the bacteria resources that can directly produce hydrogen by degrading lignocellulose and other complex substrates are relatively scarce.In addition,studies on hydrogen production from cassava residues and establishment of continuous flow fermentation process for hydrogen production from cassava residues are rarely reported.This study contained development of strain with the capacity of hydrogen production by cellulose,analysis of the hydrogen production performance and mechanism about degradation of cassava residues while production of hydrogen,and construction of hydrogen production process by fermentation from cassava residues.This study provided a new approach for the development and utilization of biomass waste resources and the energy conversion technology,and also provided theoretical basis and technical support for the industrialization process of biological hydrogen production technology.A cellulosic hydrogen-produing bacteria named Clostridium lentocellum Cel-10was isolated from giant panda feces.Under the conditions of CMC-Na 5.0 g/L,temperature 37°C,and initial pH 7.0,the maximum cumulative H2 yield were5.42±0.22 mmol H2/g CMC-Na.The hydrogen yield of the strain was in the forefront of mesophilic cellulosic hydrogen-producing bacterium.The research filled the gap in the field of fermentative hydrogen production by Clostridium lentocellum.The mutant strain CMU-7 with the good performance was obtained by nitrosoguanidine mutagenesis which was carried out to breeding efficient cellulosic hydrogen-producing bacteria.The activity of endo-glucanase,exo-glucanase andβ-1,4-glucosidase has been increased 123.08%,96.43%and 36.36%,respectively,according to the cellulase activity of the original strain Cel-10.Under the optimal conditions of hydrogen production from cellulose,the maximum cellulose degradation rate and cumulative H2 yield were 75.13±1.21%and 6.77±0.08 mmol H2/g CMC-Na,respectively,which were 47.86%and 24.91%higher than strain Cel-10.The hydrogen production performance was further improved,providing a new strain for the cellulosic hydrogen-producing strain resource.The capacity of strain CMU-7 to produce hydrogen from lignocellulosic materials was investigated.Under the condition of 5.0 g/L raw cassava residues,the maximum cassava residues degradation rate and cumulative H2 yield were 52.27±0.21%and4.512±0.12 mmol H2/g-cassava residues,respectively.The hydrogen production capacity was higher than that of some mesophilic and thermophilic bacteria reported,indicating that it has higher capacity of hydrogen production from lignocellulosic materials,which provided a possibility for the direct biotransformation of lignocellulosic raw materials to hydrogen production by fermentation.The changes of composition,morphological characteristics,physicochemical structure of cassava residues and cellulase before and after fermentation were compared.It was elucidated that the enzymes secreted by strain CMU-7 could hydrolyze cellulose and hemicellulose in cassava residues into small molecular substances and then used by the strain for growth and metabolism while had no obvious effect on lignin degradation.The degradation characteristics of cassava residues and hydrogen production mechanism of strain CMU-7 were revealed,which provided a basis for the strong degradation ability of cellulose and hemicellulose in lignocellulose by Clostridium lentocellum.After the pretreatment of cassava residues with NaOH solution to remove lignin effectively,under the conditions of 5.0 g/L cassava residues after pretretment,the maximum cassava residues degradation rate and cumulative H2 yield were 70.31±0.26%and 5.65±0.18 mmol/g-cassava residues,respectively,which were 34.51%and 25.22%higher than that of using unpretreated cassava residues.In addition,the time required for the end of fermentation was 8 h shorter than that for the hydrogen production from the fermentation of cassava residues,which effectively strengthened the hydrogen production process from the fermentation of cassava residues.The hydrogen production process using cassava residues as substrate was established.In addation,the control strategy of fed-batch fermentation was put forward.The best operating conditions were as follows:the stirring speed 20 rpm,running temperature of 37℃,the feed rate of 80%(cassava residue after the NaOH pretreatment,concentration of 5.0 g/L),HRT for 48 h(feed for 15 min,fermentation for 47.5 h,discharge for 15 min),the initial pH of 7.0.The process has stable operated for 14 cycles under the best operating conditions.The average degradation rate of cassava residues and cumulative H2 yield were 63.50%and 3.66 mmol H2/g-cassava residues,respectively.Magnified hydrogen production from cassava residues at the reactor level has been realized.