Effect Of Kiwifruit Branch Biochar Addition On Methane Production Characteristics Of Mixed Materials Under Different Anaerobic Digestion Conditions

Climate warming is a global problem faced by mankind.Under the impetus of the national "double carbon" goal,it is imperative to recycle waste into materials and clean energy.With the development of society,people’s demand for fruits is gradually diversified,such as kiwifruit,and the kiwifruit branches that come with it become a difficult problem to deal with.It is an effective method to prepare biochar from waste fruit branches.Biochar has been widely used in single anaerobic digestion because of its micropores on the surface,which can enrich microorganisms and promote interspecific electron transfer among microorganisms.Based on the outstanding effect of biochar in anaerobic digestion,the research on biochar in anaerobic co-digestion is less,the research on the influence mechanism of anaerobic co-digestion is not deep enough,and the relationship between biochar and anaerobic co-digestion technology is not clear.Therefore,it is necessary to explore the influence of kiwifruit branch biochar addition on methane production characteristics of mixed materials under different anaerobic digestion conditions.In this study,food waste,wheat straw and chicken manure were used as digestion substrates for anaerobic co-digestion.Kiwifruit branches were used as raw materials to prepare biochar,and it was modified by NaOH.By controlling the amount of biochar,digestion temperature,total solid concentration(TS)and C/N ratio,the effects of biochar and anaerobic co-digestion process on methane production were explored.By measuring the daily biogas production,methane content,pH,volatile fatty acids(VFA),ammonia nitrogen(AN),lower fatty acids,key enzyme activities(acetate kinase,coenzyme F420)and microbial community structure of the digestion system,the effects of different biochar additions and digestion conditions on methane production in anaerobic co-digestion were explored.Meanwhile,the process conditions of anaerobic co-digestion were optimized by response surface methodology,and the economic benefits were analyzed to provide technical reference for the application of biochar in anaerobic co-digestion.The main conclusions of this study are as follows:(1)The effects of different biochar addition and temperature on anaerobic co-digestion are as follows: biochar addition and temperature have significant effects on methane production in anaerobic co-digestion.In medium-temperature(35 ℃)and high-temperature(55 ℃)digestion systems,the addition of biochar promoted the utilization of VFA in different degrees,shortened the lag period and increased the cumulative methane yield(3% ～ 81.7%).Among them,the cumulative methane yield of the treatment group with 2.1 g biochar(3 g/L)at 55 ℃ was 10573 mL;When the addition of biochar is 2.1 g,the AN mitigation effect of digestion system is better,and the anaerobic co-digestion effect under various temperature conditions is improved.However,when the biochar addition amount is 4.9 g(7 g/L),the digestion effect of the high-temperature digestion system is poor because the higher AN inhibits the activity of microorganisms,and the cumulative methane production is only 4351 mL.Under the same addition of biochar,the effect of temperature on digestion system is significant,and the cumulative biogas production at low temperature(25 ℃)is much lower than that at medium-temperature and high-temperature.(2)The effects of different biochar addition and TS on anaerobic co-digestion are as follows: the cumulative methane production increases with the increase of biochar addition and TS.Under the same TS digestion conditions,biochar at the initial stage of digestion increased the degradation rate of VFA in the digestion system and promoted the stability of pH,and the cumulative methane production increased with the increase of addition amount.Under the same biochar addition,the cumulative methane production also increased with the increase of TS.The higher the TS,the higher the concentration of VFA.At the same time,the biochar promoted microorganisms to use VFA to produce methane,so the digestion effect showed different.When the TS was 9.5% and the biochar addition amount was 4.9 g,the cumulative methane production reached 10854 mL,which increased by 73.1%.(3)The effects of different biochar addition and different C/N ratio on anaerobic codigestion are as follows: the addition of biochar will increase the biogas production of digestion system,and the higher the addition of biochar,the shorter the delay period and the higher the utilization rate of VFA;The adsorption effect of biochar on AN is positively correlated with the C/N ratio of digestion system.Biochar can increase methane production by reducing the stress of AN and VFA,shortening the lag period and promoting the utilization of acetic acid and propionic acid.Biochar increased the activity of acetate kinase during digestion,but did not increase the activity of coenzyme F420.By enriching the abundance of Firmicutes,Synergistetes,Proteobacteria,Methanosarcina and Methanobacterium,biochar improves the ability of microorganisms to adapt to complex environment,thus promoting substrate hydrolysis,acid production and electron transfer between different bacteria and methanogens.In addition,biochar promotes the metabolic pathway of acetic acid dominated by Methanosarcina and the metabolic pathway of hydrogen nutrition dominated by Methanobacterium through direct interspecific electron transfer(DIET),and the acetic acid pathway contributes the most to methane production by anaerobic co-digestion.Among the three anaerobic co-digestion systems with C/N ratio under the action of 2.1 g biochar,the Methanosarcina abundance in the anaerobic co-digestion system with C/N ratio of 20 is the highest with 71.4% and its methane production is the highest(9805 mL).However,in the anaerobic co-digestion system with C/N ratio of 25,not only the abundance of Methanosarcina is low,but also the Synergistetes are lower than the other two groups,and the digestion effect is also poor,the cumulative methane production is only 5918 mL.(4)The response surface method was used to optimize the process parameters of methane production by anaerobic co-digestion of food waste,wheat straw and chicken manure,and the established quadratic regression mathematical model was highly significant(p<0.0001).The predicted value of the experiment had a linear correlation with the actual value,and the accuracy of the model was high.The results show that the technological parameters affecting the cumulative methane production by anaerobic co-digestion are from strong to weak:temperature > biochar addition > TS > C/N ratio.According to the model prediction,the optimal process of methane production by anaerobic co-digestion of food waste,wheat straw and chicken manure is as follows: temperature is 48.1℃,biochar addition is 4.9 g,TS is 9.5%,C/N ratio is 20,and the expected maximum cumulative methane production is 11094 mL.The process parameters optimized by response surface methodology are all within a reasonable range,which is suitable for fermentation engineering.Through economic benefit analysis,it is found that the optimal digestion conditions can increase the annual income by 896.9yuan/ton,and the economic benefit is also increased by 37.3%.