Medium-chain Fatty Acids Production From Excess Sludge Based On Anaerobic Fermentation

Anaerobic fermentation could recover energy or energy precursor while treating organic waste by anaerobic bacteria,which lead to a sustainable energy supply and an alleviated environmental pollution caused by improper disposal of organic waste.The target products of energy recovery from anaerobic fermentation mainly include methane,hydrogen,volatile fatty acids,and medium-chain fatty acids.Medium-chain fatty acids have long hydrophobic carbon chains and are easily separated from water.They are precursors of some high value-added products and can be used in the manufacture of biofuels,lubricants,rubber and dyes.Existing tests show that ethanol,lactate,acetate and butyrate can be the substrate for the chain elongation fermentation.In this study,ethanol and lactate served as electron donor,while acetate and butyrate served as electron acceptor for chain elongation fermentation to investigate the effect of different substrate combinations on fermentation results.The combination of lactate as sole electron donor and acetate as sole electron acceptor is the most suitable combination when there is only one kind of electron acceptor for chain elongation fermentation of anaerobic mixed culture.This combination was added to the mixed culture for anaerobic fermentation and obtained the highest caproate concentration of 482.94mMC(9.35g/L)among the four groups of experiments.In the ethanol-acetate combination test,after caproate concentration reached 132.72mMC(2.57 g/L),the microorganisms were inhibited and the caproate concentration was no longer increased.When microorganisms use lactate as an electron donor and acetate and butyrate as electron acceptors for carbon chain elongation fermentation,the higher concentration of acetate is beneficial to the production of high concentration of n-caproate,and the higher concentration of n-butyrate may be able to shorten the process of producing butyrate and increase the proportion of n-caproate in the product.Therefore,the combination of lactate,acetate and butyrate may be more advantageous for the production of high concentration of n-caproate.In order to explore the feasibility of producing medium-chain fatty acids from excess sludge,different concentration of excess sludge was first fermented to volatile fatty acids,and then further converted to medium-chain fatty acids.The low-concentration excess sludge(Including 300ml excess sludge and 200ml supernatant)was anaerobically fermented at 30°C,pH 5 and 10 for 7 days,117.86mMC(3.54g/L)of acetate and 25.56mMC(0.56 g/L)of butyrate was obtained from the supernatant in medium bottle at pH 10.Subsequently,the high-concentration excess sludge was fermented under the same conditions(temperature 30?.,pH?10)for 7 days.The highest acetate and butyrate concentrations reached 224.1 mMC(6.72 g/L)and 27.2mM(0.60 g/L),respectively.Then the supernatant was mixed with anaerobic mixed culture and then lactic acid was added for carbon chain elongation fermentation.The final yield of caproate was 2.01 g/L,indicating that it is feasible to produce medium-chain fatty acids using excess sludge by anaerobic fermentation.