Study On The Characteristics Of Wood-based Biochar Addition On Energy Production Enhancement During Anaerobic Digestion Of Organic Wastes

Food waste and sewage sludge derived from municipal wastewater treatment plants are two main components of municipal organic wastes.By anaerobic digestion?AD?treatment,two goals could be achieved.On the one hand,the wastes could be reduced and harmless.On the other hand,the enriched organics of wastes could be converted to cleaner energy,which largely achieves energy saving and GHG emission reduction.However,several crucial issues exist during conventional AD treatment,such as the low organics conversion rate,and low energy production rate.The reason is that during AD of organic wastes,the rate of acidogenesis process is generally higher than that of methanogenesis process,which lead to the excessive accumulation of volatile fatty acids?VFA?.Consequently,the whole efficiency of AD process is largely limited.With the aim of prmoting AD process efficiency,in this study,the waste sawdust was used as the feedstock for wood-based biochar preparation.This biochar was added to AD system,to explore the efficiency promotion of AD with the concept of“treating waste with another waste”.The main results of this study were listed as below:?1?The effect of pyrolytic temperature on wood-based biochar physio-chemical characteristics was studied.By comparing the differences of wood-based biochar prepared under different pyrolytic temperature,it was found that the biochar yield decreased as pyrolytic temperature rising,whereas the ash content,pH,specific surface area,electrical conductivity of biochar showed increase trends as pyrolytic temperature rising.Moreover,the organic structure of wood-based biochar went thourgh a coversion from aliphatic carbon to aromatic carbon,and eventually to graphite-like layer structure as the pyrolytic temperature rising,which resulted in the variation of organic functional groups on wood-based biochar surface.Scanning electron microscope observation found developed porous structure on the surface of wood-based biochar.Consequently,the particular characteristics of wood-based biochar,such as alkalinity,certain electrical conductivity,high specific surface area,and enriched organic functional groups in surface made it have a great potential as an additive to promote the methanogenesis process during AD.?2?The promotion efficiency of energy production during AD was studied with wood-based biochar as an additive.By comparing the methanogenesis performance of AD system with biochar prepared under different pyrolytic temperature addition,it was found that biochar prepared at 500?showed the optimal performance.The promotion efficiency of anaerobic methane production with different biochar dosage addition were compared.The results indicated that compared with the control group without biochar addition,adding wood-based biochar with dosages of 2-15g/L achieved methanogenesis lag time shortening and maximum methane production rate promoting significantly.Meanwhile,as the wood-based biochar dosage rising,the ratio of acetate to the total accumulated volatile fatty acids?VFA?increased,whereas that of butyrate?mainly accumulated VFA type?decreased,which confimed that wood-based biochar addition likely promoted butyrate syntrophic oxidation process.Among these conditions,the dosage of 15g/L optimally mitigated VFA accumulation during lag time of methanogenesis,which was confirmed as the optimal biochar dosages.Thereafter,15g/L biochar were added into AD system to investigate the effects of biochar addition on methane production under different organic loading rate?OLR?conditions.The results demonstrated that differing from the methane production inhibition in control group of high OLR condition due to the severe VFA accumulation,the groups with wood-based biochar addition triggered this process after a lag time duration,which identified the potential of wood-based biochar as additive for methane production promotion under high OLR condition.?3?The characteristics of wood-based biochar addition promoting AD process were investigated.By analyzing the pH variation of AD systems under different OLR conditions,it was found that wood-based biochar addition was beneficial for mitigating pH decreasing.With VFA as substrate for AD,it was further confirmed that the alkaline organic functional groups?-COO^-,-O^-?and particular ash content of wood-based biochar?HCO₃^-?could substitute conventional buffering chemicals to buffer hydrogen ion and mitigate pH decrease,which is beneficial to maintain the metabolic activity of methanogenic archaea.The effect of wood-based biochar addition on VFA syntrophic oxidation was investigated under two conditions,which were high hydrogen partial pressure and no electron acceptors existing conditions.The results showed that wood-based biochar addition promoted methane production under these two conditions,which indicated that wood-based biochar not only stimulated interspecies electron transfer process without using hydrogen as mediators,but also acted as electron acceptors to assisted VFA oxidation via direct interspecies electron transfer?DIET?.Moreover,microbial community analysis revealed that wood-based biochar addition enriched particular microbes,which had extracellular electron transfer capacity to participate DIET.Consequently,integrating the key physiochemical properties of wood-based biochar,it was proposed the mechanism that biochar played as redox-active mediators to promoted DIET during AD process.?4?The performance of regulating technology of AD process with wood-based biochar assistance was studied.Long-term AD system operation results demonstrated that the addition of wood-based biochar greatly mitigated VFA accumulation via syntrophic oxidation enhancement under mesophilic condition,which achieved the OLR promotion from 6.8g VS/L/d to 16.2g VS/L/d.Integrating the electrical conductivity of AD sludge and microbial community analysis,it was found that wood-based biochar regulation largely promoted the electron transfer capacity of AD sludge,and enriched syntrophic bacteria and methanogenic archaea,such as Petrimonas,Syntrophomonas,and Methanosarcina with DIET capacity under high OLR condition.Under thermophilic condition,the performance of wood-based biochar addition on accumulated propionate mitigation was insignificant,whereas butyrate and valerate accumulation was largely mitigated,which could be attributed to the enrichment of typical electro-active microorganism,such as Tepidimicrobium and Methanothermobacter,by which doubled the OLR of thermophilic system.Furthermore,for an acidified AD system,biochar addition could accelerate the methanogenic recovery via rapid accumulated VFA consumption.Consequently,the regulating technology of AD process with wood-based biochar assistance for high-efficient energy production was proposedAccording to the results of this study,adding wood-based biochar with reasonable dosage to AD system is beneficial for enhancing the buffering capacity of system,mitigating the excessive accumulation of VFA,promoted the conversion rate of organics,strengthening the syntrophic oxidation of VFA via accelerating potential DIET,enriching functional microorganism,by which achieved high-efficient methanogenesis process during AD under high OLR condition.The results of this research provide theoratical and technical supports for the high-efficient resource utilization of organic wastes.