| As a precursor of several chemical products,caproate is a value-added product which can be used in the production of food additives,antibiotics and hexanol.Caproate is currently produced from chemical route where the severe environmental pollution and high energy consumption are observed.As an alternative way,caproate production from biological way has attracted lots of attention due to its advantages of green property and low energy consumption.In this study,wheat straw was used as the carrier to form the biofilm of Clostidium kluyveri,and the key parameters affecting caproate production were investigated,such as such as p H and ethanol/acetate ratio.With the optimum condition,the synthesis of caproate from food waste was explored in batch and fed-batch fermentation,and the mechanism of caproate fermentation was deeply explored.Finally,the open culture system with high productivity of caproate was established through combining the technologies of bioaugmentation of C.kluyveri,pretreatment on anaerobic sludge and biofilm.In the biofilm system,the effects of ethanol/acet ated ratio,total substrate concentration,initial p H,and type of electron acceptor on the production of caproate were investigated.The optimal fermentation conditions were ethanol/acetate ratio of8:3,total substrate concentration of 660 mmol/L(480 mmol/L ethanol and 180mmol/L acetate),and initial p H of 7.4.Further studies showed that the mixed electron acceptors of acetate and butyrate increased the production of caproate,and the optimal ratio of acetate to butyrate was 1:1.The highest caproate production of 146.6 mmol/L was obtained using the mixed electron acceptor.This result indicated that C.kluyveri was inclined to synthesize caproate using the mixed electron acceptors of acetate and butyrate.Analysis of scanning electron microscope on the wheat straw showed that the formed biofilm of C.kluyveri on the carrier was stable.The extracellular polymeric substances of extracellular polysaccharides and proteins enhanced the adhesion force between cells and the carrier,resulting in the improvement of biofilm stability.To achive efficient bioconversion of food waste to caproate,a three-stage integrated system was established,where the ethanol fermentation was conducted at the first stage,acetate fermentation was conducted at the second stage,and caproate fermentation was conducted at the final stage.Results showed that the highest concentration of ethanol and acetate produced from food waste were 1187.0 and 538.3mmol/L,respectively.Bath tests of caproate fermentation were conduted using ethanol and acetate fermentation effluents.Results showed that the biofilm of C.kluyveri formed on the wheat straw can be recycled.The biofilm maintained its stable structure and exhibited a high capacity in synthesing caproate after recovery of seven times.The mean caproate production was 127.6 mmol/L after eight batches of fermentation.To improve the production of caproate,caproate fermentation was further conducted in fed-batch fermentation,and the highest caproate production of159.6 mmol/L was obtained.The alleviation of inhibitory effect caused by substrates and undissociated caproic acid,and the high cell density explained the improvement of caproate production in fed-batch fermentation.The open culture system exhibited a higher stability in degrading biomass waste in comparison of pure culture system.The open culure system was therefore developt through bioaugmentation of C.kluyveri.Results showed that alkali-thermal pretreatment was the optimum method for pretreatment of anaerobic sludge.The highest caproate production was 94.8 mmol/L,which was 94.0% higher than that of the control.Further study indicated that addition of the carrier(wheat straw)into the open culture system improved the caproate production to 123.9 mmol/L.The lethal effect of alkali-thermal pretreatment to the mothanogen,the bioaugmentation effect of C.kluyveri and the biofilm formed were the main reason for the improvement of caproate production in the open culture system. |
