| The use of anaerobic bacteria to convert organic waste into caproate is a promising and sustainable technology for new energy and resource recovery.In this paper,the stable anaerobic acidizing phase was established by using the waste of fruits and vegetables as fermentation substrate.Using fermented liquid as the substrate of caproate-producing phase and adding exogenous ethanol as electron donor,functional mechanisms on caproate biosynthesis via microbial consortia are multidimensionally revealed by using real-time fluorescent quantitative PCR,microbial community structure and so on.The results can provide a theoretical basis of how to regulate and enhance the performance of caproate-producing phase.At the same time,the method of ion exchange extraction of caproate can relieve the toxicity of MCFAs in the carbon chain extension.D201 strong base anion exchange resin on the caproate-prodcuing reactor shape stability of caproate bacteria group,help to realize high production of caproate.The detailed information was as follows:1.The best anaerobic acidification phase can achieve with the conditions of the inoculation ratio of were 2:1,which can realize the efficient hydrolysis conversion of waste products.The reaction pH was set to 4,5,6 and 7,and the best condition of anaerobic acid phase was selected.The composition of liquid phase products was affected by pH,and it was ethanol-type fermentation at pH=4,and they were butyric acid-type fermentation at pH=5,6and 7.The hydrolysis conversion rate was lower under low pH,and the hydrolysis conversion rate was improved with the increase of pH.With the change of liquid phase composition,pH=6 was chosen as the best condition.The results showed that different inoculation ratios had little effect on the degree of hydrolysis.But with the decrease of the inoculation ratio,acidification rate reduce and concentration of butyric acid in the liquid phase components rise.Residual carbohydrates will be harmful to the subsequent caproic acid phase,so ratio of inoculum to substrate was 2:1 as the best condition.2.It is very important to control ethanol/acid ratio and pH to improve the fermentation efficiency of caproic acid.When the ethanol/acid ratio and pH control are 4:1 and 7.5,the yield of caproic acid can reach 14.9 g?L-1.The optimal dosage of ethanol is 18.2g?L-1?395.7mM?,when the ratio of ethanol to acid is 4:1 and the pH of 6.5.The highest production of caproate was 12.10g?L-1.When the ratio of ethanol to acid?4:1,the accumulation of caproate products occurred.The butyric acid in the liquid phase increased first and then decreased,and acetic acid could be fully utilized.When the ratio of ethanol to acid was 5:1,high concentration of ethanol produced significant inhibition of microorganisms,and caproate fermentation could not proceed.Therefore,the ratio of ethanol to acid=4:1 was selected as the optimal condition.Setting conditions of pHs at 5.5,7.5,8.5,9.5,that was found that the optimum fermentation performance under the conditions of pH 7.5,and butyrate accumulated under low pH condition,caproate was synthesized in a small amount;caproate synthesis was almost stagnant under high pH condition.The production of caproate at the optimum pH was up to 14.9 g?L-1.Caproate-producing bacteria?Clostridium kluyveri?index growth accompanied by the formation of butyric acid,caproate synthesis occurs mainly in growth later period,so the fermentation later period to ensure enough ethanol concentration is one of the main factors to increase caproate production.In addition,the appropriate increase of p H can help reduce the toxicity of organic acids and increase the biomass of caproate.However,alkaline environment can severely inhibit the growth and reproduction of caproic acid bacteria.By optimizing and regulating acidification and caproic acid respectively,the two-phase fermentation strategy is more conducive to improving the synthesis efficiency of caproic acid.3.Through the analysis of microbial community,it is shown that in the caproate-producing phase,the Firmicutes content is most abundant at the phylum level;the addition of proper amount of ethanol promotes the production of caproate,which may be due to the participation of ethanol as a carbon source,energy,and reducing equivalents.The four microorganisms,Sporanaerobacter spp.,Clostridium kluyveri DSM 555,Enterococcus asini ATCC 700915 and Eubacterium spp.,are closely related to the production of caproic acid and the utilization of ethanol.The concentration of ethanol has a significant influence on the abundance of caproate-producing bacteria,and the feedback inhibition of caproic acid on the microorganism in the system is also an important factor influencing the change of community structure.4.Ion exchange resin was used to reduce the potential toxicity of caproic acid accumulation in the fermentation process.The resin choice experiments showed that D201resin had a better adsorptive capability than others.Using D201 resin,the static and dynamic adsorption experiments were conducted.The OH-ions of D201 can exchange with ion in the acid solution in the conditions of 30?and pH of 7.5.D201 resin has a large adsorption capacity of caproate,and the adsorption capacity of other carboxylic acids and alcohols is small.When the temperature was 30?and concentration of eluent was 1mol·L-1,the resin elution efficiency is high.Under the optimum operation conditions,the repeated experiments data showed the good repeatability.This proved that the technological conditions for the extraction of caproate by D201 resin was effective and stable.5.The microorganisms in Chain elongation will be inhibited by high concentrations of ethanol and undissociated caproic acid,even if the reaction condition of pH,temperature,ethanol,acetate and butyrate are suitable.When the concentration of caproate reached11.17g·l-1,the carbon chain elongation process terminated.Using D201 resin in the reactor,it could effectively alleviate caproic acid in the process and make the reaction stability.The highest concentration of caproate in the process reached 17.31 g·L-1,which has the most productive rate was 2.02g·L-1d-11 and cumulative production was 29.09 g·L-1.Ensuring sufficient electron donor and carbon source in the reaction and continuous ion exchange can establish the stable process of chain elongation.Therefore,caproate can be synthesized efficiently by two-phase anaerobic fermentation. |
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