Studies On Thermophilic Hydrogen Fermentation From Waste Biomass Coupled With Methane Production

Biofuels production from waste biomass provides a viable option to improve energy security,reduce environmental pollution and contribute to waste management.Waste biomass is the residue derived from the production,store and utilization of animals,plants and microbes.Hydrogen and methane production from waste biomass is a feasible technology.However,its large-scale production is limited due to low substrate degradation ratio and low efficiency and yield of products.The development of new strategies for hydrogen and methane fermentation improving energy recovery from waste biomass is one of the hot spots in the alternative energy production.In this study,two-stage hydrogen and methane production from waste biomass was explored.Anaerobic fermentation systems enhanced by C.thermocellum and biochar were established respectively,and the promotion mechanism of hydrogen production and substrate degradation was explored.Paper sludge(PS)from the secondary sedimentation tank of wastewater treatment in paper mill may contain cellulose-degradation and hydrogen-producing bacteria,so hydrogen fermentation from PS directly was investigated.PS(7.4%,w/v)can be fermented without any pretreatment and extra inoculation at 55°C under anaerobic condition and64.32 m M of hydrogen can be produced.The dynamic change of bacterial community structures during hydrogen fermentation at 55°C were investigated by analyzing 16S r DNA gene sequences.The results showed that microbial community was dominated by order Clostridiales and Thermoanaerobacterales.Genus Thermoanaerobacterium and Ruminiclostridium played a leading role in the fermentation process,which was responsible for the hydrolysis of PS and hydrogen production.The inoculation of Clostridium thermocellum improved the hydrogen production performance.The maximum hydrogen yield reached 0.165 L/g VS,which was one time higher than that of fermentation conducted by native microbes,and the degradation ratio of holocellulose was increased by 33%.Besides,the maximum hydrogen production rate increased by 6 times and the adaptation phase decreased from 7.59 h to 3.89 h after C.thermocellum inoculation.In order to improve the energy recovery from PS further,hydrogen-producing effluent was subjected to methane production,and the methane yield of 0.376 L/g COD was obtained.Energy yield of 4.5 MJ/kg PS was obtained in the two-stage hydrogen and methane production after C.thermocellum augmentation,50%higher than the single-stage anaerobic digestion(3.0 MJ/kg PS).The bacterial community of second-stage process was dominated by genus Propionispira,Mesotoga and Aminobecterium with stability,whereas the bacterial structure in single-stage process presented dynamic changes and genus Acetivibrio and Fibrobacter played an indispensable role.However,Genus Methanosaeta,Methanosarcina,Methanobacterium and Methanospirillum existed stably and predominantly in the archaeal community of single-stage and second-stage processes.Acetate was converted to methane by Methanosaeta and Methanosarcina.Methanobacterium and Methanospirillum worked synergistically with hydrogen-producing bacteria to produce methane,such as Acetivibrio,Mesotoga and Longilinea.Waste lignocellulose is thought to be good material for anaerobic fermentation due to the largely production and high holocellulose content.The efficient energy conversion is of great significance for waste biomass utilization and management.In the saccharification of hydrogen peroxide-acetic acid pretreated sugarcane bagasse(HPAC-SCB,2%,w/v)conducted by C.thermocellum,the reducing sugar content reached 14.58 g/L and the substrate degradation ratio was up to 90%.The sugar was used by C.thermocellum to produce hydrogen,resulting in a low yield of sugar.Theredore,HPAC-SCB was directly fermented to produce hydrogen.The hydrogen yield in the co-culture fermentation of T.thermosaccharolyticum and C.thermocellum from HPAC-SCB(2%,w/v)reached 226m L/g substrate.The long-term hydrogen and methane fermentation were successfully established with 1.59 L/(L·d)and 0.159 L/g substrate for average hydrogen productivity and yield,respectively and 0.341 L/g COD_added for Methane production.Average energy recovery of 8.79 MJ/kg SCB was obtained under the optimal conditions,54%higher than single-stage AD process.Biochar has the characteristics of highly aromatization,large specific surface area and p H buffer capacity.The effects of biochar addition on anaerobic fermentation was studied.The hydrogen production of cellobiose fermentated by C.thermocellum was increased by41%and 94%with sugarcane bagasse(SCB)biochar and spent mushroom compost(SMC7)biochar addition,respectively.SMC7 biochar addition improved the hydrogen yield and substrate degradation ratio of HPAC-SCB fermentated by C.thermocellum by 37%and10%,respectively and changed the metabolic flux of C.thermocellum.Therefore,the production of acetate,ethanol and hydrogen were enhanced.The mechanism of hydrogen production enhancement of C.thermocellum fementation with biochar addition was studied.It was found that the addition of biochar improved the hydrogen production performance of C.thermocellum by reducing ORP of fermentation broth,promoting the growth and improving the activity of hydrogenase and hydrolase of C.thermocellum.The effects of biochar addition on methane fermentation of hydrogen-producing efflunent were further studied.The results showed that the addition of biochar shortened the lag phase of methane fermentation and increased the maximum methane production rate.Compared with the hydrogen and methane production process and single-stage methane production process without biochar addition,the energy yield of hydrogen and methane production from HPAC-SCB with SMC7 addition was increased by 12%and 54%,respectively,and the anaerobic fermentation efficiency was improved significantly.