| The use of solid waste to generate biomass energy is increasingly becoming a research hotspot.The hydrolysis and acidification of the sewage plant residual sludge together with the chain elongation can produce medium-chain fatty acids,which is an important precursor of biodiesel.While,there is no research of this mixed fermentation to realize the value of excess sludge resource utilization and sustainable energy production.This research project takes the residual sludge of urban sewage treatment plant as the research object,and carries out anaerobic chain elongation biological mixed fermentation on sludge hydrolysis and acidification liquid.On the basis of studying the composition of high-solid sludge hydrolysis and acidification liquid,the accumulated and chain elongation bacteria enriched sludge was used as inoculum to optimize the acid/alcohol ratio of chain elongation.The characteristic of chain elongation with single acid and mixed acids as electron acceptor together with the effect of nitrogen and phosphorus concentration in hydrolysis and acidification liquid on chain elongation process were investigated by batch simulation experiments.Finally,the practical residual sludge hydrolysis acidification liquid together with ethanol was used as the influent of chain elongation mixed fermentation experiment,with the purpose of proving the feasibility of using the excess sludge of lunicipal sewage treatment plant to produce medium chain fatty acid.The results show that the total acid yield of TS=8%high-solids sludge hydrolysis acidification reached 5 g L-1 on the 10th day with a acetate,propionate and n-butyrate proportion of 5:2:3.The ammonia nitrogen concentration is 3434.02 mg L-1,and the phosphate concentration is 596.21 mg L-1.And the optimized acid/alcohol ratio is 1:3.In this study,for the first time a mixed acids with a proportion of acetate,propionate and n-butyrate of 5:2:3 was used as an electron acceptor to carry out chain elongation experiments with ethanol.The results showed that the chain elongation of the mixed acids performed the best effect.The concentration of target product n-caproate high up to 56.69 mmol/L,and the corresponding total chain elongation efficiency and n-caproate production efficiency were 82.55%and 63.32%,respectively,which were both higher than that in chain elonation of single acid.Through the analysis of chain elongation results,we innovatively proposed a potential microbial reaction pathway and proved its thermodynamic feasibility.And we also designed an analysis mode to illustrate the the synergistic mechanisnl of acetate,propionate and n-butyrate in mixed acids in the view of the microbial metabolic mechanism ot chain elongation and competition process.For chain elongation process,the optimum NH4+and PO43-concentrations are 900 mg/L and 1500 mg/L,respectively.However,the concentration of nitrogen and phosphorus has little effect on chain elongation,therefore the chain elongation of the actual sludge hydrolysis acidification liquid does not require adjustment of nitrogen and phosphorus.The usage of actual sludge hydrolysis acidification liquid as a matrix can achieve a certain degree of chain elongation to produce n-caproate,confirming the feasibility of applying chain elongation to the carboxylic acid platform.However,in order to ensure a high chain elongayion and n-caproate production efficiency,it is still necessary to add an appropriate amount of inorganic carbon source and carry out both experimental research and industrial production of actual sludge hydrolysis acidification liquid chain elongation fermentation in a continuous flow reactor.The research of this subject enriched and perfected the process conditions and mechanism of chain elongation,and successfully proved the feasibility of combining sludge hydrolysis acidification fermentation with chain elongation technology,which then provides a new possibility direction for sludge resource utilization as well as an important theoretical basis and application platform for the subsequent mixed acid proportion optimization and directed acid production fermentation technology. |
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