Research On Enhancement Of Anaerobic Digestion Of Straw And Sludge By Slurry Recirculation And Exogenous Enzyme Addition

The disposal of straw and sludge has become a major environmental problem.Anaerobic digestion is the most effective technology to treat solid waste.The low efficiency of anaerobic digestion technology is the main reason to limit the disposal of solid waste.Therefore,how to improve the efficiency of resource-based technology is imminent.In recent years,the domestic and foreign main concern on anaerobic digestion pretreatment technology and enzymatic hydrolysis technology.Nowadays,the scientific research of biomass energy and the recycling of solid waste put forward new requirements for energy efficiency.In order to construct a high-efficiency anaerobic digestion process,this study used single factor batch experiment method to study the enhancement effect of different pretreatment methods,different biogas sludge reflux,different amount of exogenous enzymes and different proportion of complex enzymes on the anaerobic digestion of straw sludge at medium temperature.To provide technical reference for the construction of an efficient anaerobic digestion process for solid waste mixtures.To study the effect of different pretreatment methods on mixed anaerobic digestion at medium temperature（35℃）.In the experiment,the mixture of straw sludge（w/w 1:2）was used as the substrate,and five different pretreatment methods were adopted,including straw thermo-alkali pretreatment,straw thermo-alkali ultrasonic pretreatment,non-pretreatment,straw sludge mixed thermo-alkali pretreatment and straw sludge mixed thermo-alkali ultrasonic pretreatment.The gas yield,methane content,ammonia nitrogen content,s COD and total volatile acid（VFAs）were analyzed under different pretreatments.The results showed that the gas production and methane production of straw pretreated by thermo-alkali ultrasound were the largest,1571 m L and 800.7 m L,respectively.The removal rates of TS and VS were 17.5%and 47.8%,respectively.The methane production per unit VS was381.29 m L.g^-1.（VS）,which was 81.8%higher than that of the untreated fermentation flask,and there was no VFAs accumulation.Thermo-alkali ultrasonic pretreatment of straw can be used as the best pretreatment method for anaerobic digestion of straw sludge.Under the condition of low load（6g VS/L/d）,the characteristics of heavy metals were studied in a CSTR reactor.The changes and existing forms of As,Cd,Cr,Cu,Ni,Pb,Zn and Hg were analyzed.The content of heavy metals in the anaerobic reactor effluent met the standard limits of pollutant control in GB 18918-2002 sludge for agricultural use.The requirement of alkaline soil application is that agricultural sludge can be used in theory.To study the effect of different biogas slurry reflux on mixed anaerobic digestion at medium temperature（35℃）.In the experiment,the mixture of straw sludge（w/w 1:2）was used as the substrate.Six different biogas sludge recirculation rates were used,including 0%,20%,30%,40%,50%and 60%.The changes of gas production,methane production,ammonia nitrogen content,s COD and total volatile acid（VFAs）were analyzed under different biogas sludge recirculation rates.The results showed that the gas production and methane production of 50%biogas slurry were the largest,1645 ml and797.5ml respectively.The removal rates of TS and VS were 22.5%and 48.1%,respectively.The methane production per unit VS was 613.45 m L/g.VS,which was 37.7%higher than that of fermentation flasks without biogas slurry,and VFAs accumulation was not observed.50%biogas slurry reflux can be used as the best reflux for mixed anaerobic digestion of straw sludge.The digestive juice of anaerobic digestion products can be partially recycled by biogas slurry reflux.To study the effects of different exogenous enzymes dosage and dosage ratio on the mixed anaerobic digestion process at medium temperature（35℃）.The experiment used straw sludge mixture（w/w 1:2）as substrate.Four different dosages of exogenous enzymes,including 0 mg/g（TS）,60 mg/g（TS）,80 mg/g（TS）and 100 mg/g（TS）,were used to set up seven groups of protease and cellulase.Then,under the optimal dosage,the two enzymes were added into five groups:1:1,1:2,2:1,1:3,3:1,respectively.The Changes of gas volume,methane content,ammonia nitrogen content,polysaccharide,protein,total volatile acid（VFAs）content and enzyme activity during fermentation was analyzed.The results showed that the highest methane yield of 373.05m L can be obtained when the weight ratio of neutral protease to cellulase is 1:2.The removal rates of TS and VS were 10.0%and 43.5%,and there was no accumulation of VFAs.The enzyme ratio（neutral protease:cellulase）of 1:2can be used as the optimal dosage of exogenous enzymes for anaerobic digestion of straw sludge.Molecular weight of digestive juice from multi-enzyme combined reaction was determined by HPLC.It was found that the molecular weight of digestive juice decreased with time.The main macromolecule substances in anaerobic digestion products were biopolymer and high molecular weight humic acid.The digestion yield of mixed anaerobic digestion of straw and sludge can be improved by ultrasonic pretreatment of straw,biogas slurry reflux and exogenous enzymes.The process is mature and reliable,and has certain engineering application significance.It can be used as a reference for the construction of high efficiency anaerobic digestion technology of straw and sludge mixture.