Production Of Medium Chain Fatty Acids From Agricultural Cornstalk By Anaerobic Fermentation

As a renewable material,agricultural biomass is increasingly playing an important role in the replacement of energy and chemical materials from fossil resource.However,traditional resource recovery techniques,such as incineration and anaerobic digestion,encounters the problems of low added value and poor economy of ultimate products.Recently,microbial chain elongation as a substitute for traditional anaerobic digestion has received extensive attention,which produces medium chain fatty acids?MCFAs?with a high value through inhibiting the conversion of organic acids into methane and regulating the reduction potential.The products of MCFAs have a high energy density,which could be used to prepare liquid fuels and functional chemicals.Compared with the traditional biogas fuel,MCFAs possess more environmental and economic advantages.In this study,a typical agricultural biomass-corn stalk was used as the substrate to produce high-value-added MCFAs via anaerobic chain elongation.The preliminary experiments were conducted to investigate the different operating factors,such as the types of substrate and electron donors,pH effect.And then second series of experiments were carried out to examine the pH effects on preliminary acidogenesis of cornstalk and secondary chain elongation with electron donor addition.Finally,it was evaluated to use aqueous products of thermochemical treatment as methanogenesis inhibitor instead of BES for MCFAs production from stalk.The detail results and conclusions are listed as follows:?1?Firstly,a set of experiments were conducted to investigate the effects of substrate type,electron donor type and concentration,inoculation sludge source,pH value on the mixed culture chain elongation process.The results showed that that a maximum 3.5 g/L of caproate was obtained with cornstalk as a substrate after 8-day fermentation,which was similar to the value of 3.6 g/L from the 14-day glucose fermentation.Ethanol as electron donor was proved to be more effective than propanol,but the MCFAs productions were deteriorated when the dosage was too high or low.The anaerobic sludge from anaerobic reactor for methane production could be used as an inoculum for microbial chain elongation,which contained abundant microorganisms for MCFAs productions.?2?Another set of experiments were carried out to systematically examine the effects of pH on acidogenesis of cornstalk were studied,and the corresponding microbial community dynamics at different pHs were analyzed.The results indicated that neutral pH was favorable for the chain elongation process.Under neutral condition,the yields of short chain fatty acids and MCFAs were 258 mg/g VS _fed and 93 mg/g VS _fed,respectively,which were significantly higher than 176 mg/g VS _fed?short chain fatty acids?and 45 mg/g VS _fed?MCFAs?under acidic condition,respectively.Under the neutral condition,the Firmicutes were the dominating phylum accounting for 49.9%after enrichment.?3?The effects of pH on ethanol consumption and caproate production during the chain elongation in the second feeding process were investigated.The results showed that no obvious increase of the caproate concentration was observed after ethanol addition at pH 5.6.This was mainly attributed to that the high level of toxic free organic acids including caproic acid under acidic conditions on chain elongation microorganisms.In contrast,when the pH was adjusted to 7.0,a nearly saturated concentration of caproate was obtained.In addition,the conversion efficiency of ethanol to caproate at the second stage was as high as 83.6%,which was 5.3 times of that in first feeding process.?4?Finally,the aqueous products of thermochemical conversion were explored as methanogenesis inhibitor instead of BES.The results indicated that an appropriate concentration of aqueous products significantly promoted the caproate production,and the optimum dose was found to be 7 g/kg,by which methane production was inhibited and caproate production reached 3.2 g/L.In summary,the above results proved that the typical agricultural waste biomass-cornstalk could be used as a substrate to produce MCFAs in the mixed culture chain elongation process.The neutral pH condition was favorable for the chain elongation process to produce MCFAs.In addition,the aqueous products of thermochemical conversion was proved to be superior methanogenesis inhibitor for anaerobic chain elongation reaction to BES with the with the advantages of low cost and environmentally friendly These findings could provide useful information for the development of mixed culture chain elongation technology for MCFAs production,and also greatly expand agricultural stalk waste treatment and resource utilization.