Study On Medium Chain Carboxylic Acid Production From Chain Elongation Technology Induced By Ethanol And Lactate

Facing the severe resource shortage and environmental pollution,researchers pa y more attention to the production of renewable energy and biochemicals from waste biomass.Recently,an emerging chain elongation?CE?technology for medium chain carboxylic acid?MCCA?production from waste biomass has caught the attention of researchers.MCCA is a multi-functional chemical with high energy density and strong hydrophobicity,and its large-scale production and utilization are expected to reduce environmental pollution and the dependence of chemical industry on non-renewable fossil fuel.However,the current research on CE is still in the early stage,and many factors that limit the MCCA production have yet to be resolved.In order to overcome the defects of ethanol and lactate as single electron donors,this study proposed combining ethanol and lactate as co-electron donors of CE process,and revealed the pathway and mechanism of enhanced MCCA production from co-electron donors of ethanol and lactate.In addition,this study applied the conclusion for MCCA production from real waste biomass.Firstly,this study investigated the effect of molar ratio of single electron donor?ethanol or lactate?to electron acceptor?acetate?and substrate concentration on MCCA production performance.The results showed that when the electron donor?ethanol or lactate?/electron acceptor?acetate??1 go against MCCA formation,and the main product is butyrate.Under electron donor/electron acceptor>1,caproate is the main product.The higher the ratio of electron donor/electron acceptor,the more favorable it is for caprylate production.The optimum ratioes of ethanol and lactate to acetate for MCCA production are 3/1 and 2/1,respectively.The optimal substrate concentration for MCCA production from ethanol and acetate,and lactate and acetate are 300-500 and 400-600 mM C,respectively.Ethanol and lactate as co-electron donor obtained higher substrate utilization ratio,MCCA production and selectivity than ethanol or lactate as a single electron donor.With ethanol and lactate as co-electron donors,the optimal ethanol/lactate molar ratio is 2/1.To reveal the pathways and mechanisms of enhanced MCCA production from co-electron donors of ethanol and lactate,a large number of sequencing experiments were carried out.According to the experimental results,the pathways to enhance MCCA production with ethanol and lactate as co-electron donors are:The CO₂produced by lactate provides a carbon source for the ethanol-using CE bacteria to synthesize protein,and also reacts with H₂ produced by ethanol metabolism,which lowers the H₂ bias,thereby accelerating the oxidation of ethanol?the first step of CE process?.The H₂ produced by ethanol reduces the propionate,produced by the competitive reaction of lactate-depleting acrylate pathway,to propanol,and the propanol as an electron donor of CE process reacts with acetate and propionate to produce heptylate and caproate,respectively.In addition,the H₂ reacts with CO₂ to form acetate by homoacetogenesis and ethanol by acetate reduction,and the acetate and ethanol produced in situ would be converted to caproate and caprylate by CE process.It is noted that the reutilization of lactate-carbon flow of CO₂ and propionate is the main reason for the enhanced MCCA production from co-existed ethanol and lactate.Phylum Firminutes,class Negativicutes,and genus Veillonella are the key microorganisms that cause the different MCCA production from co-electron donors of ethanol and lactate.The above experimental conclusions show a guiding significance for substrate selection and collocation of CE process:the substance containing ethanol,lactate,and SCCA is the best substrate for MCCA production,and the substance containing only lactate or ethanol or SCCA can be combined for MCCA production.Based on this,the study selected the real waste biomass of China strong-flavor-type liquor wastewater?CSLW?,which contains ethanol,lactate,and SCCA,as substrate for MCCA production.Firstly,sequencing experiments were carried to confirm that the MCCA production from CSLW without the addition of exogenous electron donor s and electron acceptors is feasible.Then,an expanded granular sludge bed?EGSB?reactor was operated to investigate the performance of long-term continuous MCCA production from CSLW,and the optimal EGSB operating parameters for MCCA production are obtained,which are temperature of 40 ^oC,pH of 5.4,ULV of 3.5 m/h,HRT of 8 h,and OLR of 40 g COD/?L·d?).Under the optimal operating parameters,MCCA production of 10.27 g COD/L,production rate of 31.12 g COD/?L·d?and substrate conversion rate of 76.8%were obtained.The long-term and stable MCCA production performance is related to the special structure of granular sludge,which can weaken the damage from toxic substrate and undissociated MCCA to CE bacteria to some extent.The granular sludge after long-term CE process is mainly composed of spore-forming rod and fusiform spore-forming rod shaped bacteria,and its dominant species are phylum Firminutes,class Clostridia and genus Clostridium IV.