Study On Enhancement In Anaerobic Co-digestion Of Cow Manure And Corn Straw By Thermal Pretreatment Combined With Zero-valent Iron Addition

With the development of rural economy and society,the discharge of agricultural wastes such as livestock manures and crop straws are increasing day by day,posing a serious threat to the ecological environment.How to treat them harmlessly and resourcefully has become an urgent problem to be solved.Anaerobic digestion technology can transform biomass in agricultural wastes into methane,and realize waste reduction and sustainable utilization of resources.However,there are some problems in anaerobic digestion of agricultural wastes,such as slow hydrolysis rate and low methane production efficiency,so it is necessary to explore technologies to strengthen anaerobic digestion of agricultural wastes.In this study,two typical agricultural wastes,cow manure and corn straw,were taken as the research objects,and the anaerobic co-digestion of cow manure and corn straw was strengthened by thermal pretreatment combined with zero-valent iron technology.Firstly,the optimal thermal pretreatment conditions were determined by analyzing the characteristics of thermal pretreatment products,and the effects of thermal pretreatment on liquid phase index,lignocellulose and apparent structure were explored.Secondly,the best material ratio was determined by analyzing the methane production and the changes of physical and chemical indexes of co-digestion of cow manure and corn straw under different ratios.Finally,based on the above experiments,zero valent iron was introduced to enhance the anaerobic co digestion of cow manure and corn straw,and the suitable type and dosage of zero-valent iron were determined to reveal its strengthening mechanism.The main conclusions are as follows:(1)Thermal pretreatment could promote the dissolution of soluble organic compounds and the degradation of lignocellulose.At the same temperature,the dissolution rate of soluble organic compounds and the degradation rate of lignocellulose increased gradually with the extension of time.At the same time,the dissolution rate of soluble organic compounds and the degradation rate of lignocellulose increased gradually with the rise of temperature from 90℃ to 100℃,but did not increase significantly with the further increase of temperature.Considering the dissolution efficiency and economic cost,the optimum conditions of thermal pretreatment were 100℃ and 60 min.At this time,the dissolution rates of SCOD,soluble polysaccharide and soluble protein were 88.8%,94.7% and 150.3%,and the degradation rates of cellulose and lignin were 8.2% and 3.3%.(2)Anaerobic co-digestion of cow manure and corn straw could improve buffering capacity of the system and the removal rate of dissolved organic matter and increase methane production.When the ratio of cow manure to corn straw was 6:4,the removal rate of dissolved organic matter and cumulative methane production reached the highest,with the removal rate of SCOD being 59.8%,the removal rate of VFAs being 63.8% and the cumulative methane production being 2255 m L.(3)ZVI could improve methane production on the basis of thermal pretreatment.When the dosage of micro-scale and nano-scale ZVI were 8 g/L and 4 g/L respectively,the cumulative methane production were the highest,which were 20.7% and 29.5%higher than that of thermal pretreatment group.The fitting results of Modified Gompertz model showed that proper ZVI could improve the maximum methane production potential and maximum methane production rate,and shorten the stagnation period..(4)ZVI could promote hydrolytic acidification,accelerate the conversion of propionic acid to acetic acid,and improve the removal efficiency of SCOD and VFAs.Nano-scale ZVI could promote the release of soluble organic compounds better than micro-scale ZVI,but excessive(8～12 g/L)nano-scale ZVI would have a negative impact on methane production.The removal rates of soluble organic compounds were the highest When adding 8 g/L micro-scale ZVI or 4 g/L nano-scale ZVI,and the removal rates of SCOD were 62.1% and 62.5%,the removal rates of VFAs were 77.2% and 78.5%.(5)ZVI could optimize the distribution of anaerobic microbial community.For the distribution of bacterial communities,the addition of appropriate ZVI increased the relative abundance of Petrimonas,Romboutsia and Saccharofermentans,thus promoting the hydrolysis and acidification of the system.For the distribution of archaea community,the addition of ZVI changed the dominant archaea from Methanobacterium to Methanothrix,which promoted the systematic acetic acid-type methane production.