Regulation And Mechanism Of Medium Chain Fatty Acids Production From Waste Activated Sludge Hydrolysate By Microbial Electrosynthesis

The rapid growth of waste activated sludge production has posed serious challenges to China’s sewage treatment plants.Therefore,exploring and developing effective methods for the resource utilization of waste activated sludge is of great significance for the sustainable development of the sewage treatment industry.Recently,the novel resource utilization technology of converting waste activated sludge into medium chain fatty acids by chain elongation process has attracted more attention.However,this technology is still limited by the high dependence on the addition of external electron donors,low carbon conversion efficiency,and unclear microbial mechanisms.Based on this,this thesis proposes to utilize the green and sustainable electric energy to replace the external electron donors（like ethanol or lactate）,and realize the conversion of waste activated sludge hydrolysate to medium chain fatty acids by constructed microbial electrosynthesis system,thus breaking though the limitations of conventional chain elongating fermentation,developing novel regulating means to enhance the production performance of medium chain fatty acids,penetrating the carbon conversion,electron transfer and microbial mechanisms of electric filed stimulation,and promoting the development of medium chain fatty acids production from waste activated sludge.The main research contents and conclusions of this thesis are as follows:（1）The conversion of waste activated sludge hydrolysate to medium chain fatty acids without the addition of external electron donors was achieved in microbial electrosynthesis system,and the production performance was regulated by key parameters such as voltage and process p H.Results showed that the differences in cathodic fermentation environment（such as cathodic potential,p H）caused by open circuit and different voltages（0.6 V,0.9 V,and 1.2 V）,affected the production performance of caproate from acidogenic fermentation liquid of waste activated sludge.Due to the cathodic potential meeting the hydrogen evolution potential and the relatively stable p H（5.5～6.5）of the catholyte at 0.9 V condition,an effective electronic source and a mild fermentation environment was created for the chain elongator.As a result,the production concentration,the selectivity of caproate,and the carbon conversion rate respectively reached 3.01 g/L,52.2%,and 44.2%,which were significantly higher than the control group.Furthermore,under optimized voltage condition（0.9V）,the competition of the methane production pathway in the chain elongating process was suppressed by controlling the process p H（5.5,6.0,and 6.5）of the catholyte.It was found that the release of residual organic matter in the acidogenic fermentation liquid was also enhanced under p H6.0 condition,and the caproate concentration and the carbon conversion rate were further increased to 4.1 g/L and 51.1%,respectively.（2）Through the balance analysis of cathodic electron consumption and the electrons required for the production of medium chain fatty acids,it was determined that there were in situ electron donors present in the acidogenic fermentation liquid to drive the chain elongating reactions.The feasibility of amino acids and glucose as electron donors was verified by pure substrate experiments.Results demonstrated that there was a significant correlation（the R² of the fitting equation was greater than 0.7）between the changes in protein or total sugar and the production of caproate under different voltage and p H conditions,which indicated that residual protein,amino acids,and sugars might participate in the chain elongating reactions.Furtherly,under the open circuit and 0.9 V conditions,it was proved that the six amino acids combinations of alanine,cysteine,histidine,leucine,tryptophan,and tyrosine in the acidogenic fermentation liquid were acted as the in-situ electron donors to drive the chain elongating reactions,and electric filed could enhanced the conversion of amino acids into medium chain fatty acids,whereas the glucose could not directly drive the chain elongating reactions.（3）Considering that the main carbon sources of medium chain fatty acids production from acidogenic fermentation liquid included acetate,butyrate,CO₂,and residual organic matter,the carbon conversion efficiency and contribution of the above different carbon sources to medium chain fatty acids were determined by using acetate,butyrate,and CO₂ as the sole carbon source.Results indicated that the carbon conversion pathways of different substrates were different with each other,acetate was partly reduced to ethanol,and caproate was generated through ethanol mediated chain elongating reactions,while butyrate or CO₂was directly converted into caproate by hydrogen mediated chain elongating pathways.Moreover,the carbon conversion rates of different substrates were also different with each other,with the carbon conversion rate of acetate,butyrate,and CO₂converting into caproate being 3.1%,22%,and 16.6%,respectively.In addition,the residual organic matter such as proteins,amino acids,carbohydrates,etc.in the acidogenic fermentation liquid accounted for 74.8%carbon source for the caproate production.（4）In order to further improve the shortage of electron donor and the low carbon conversion rate,a strategy of nano zero valent iron as an additive was proposed to strengthen the electron transfer and substrate release performances.Results suggested that nano zero valent iron（NZVI）has the potential to promote the production performance of medium chain fatty acids from waste activated sludge by microbial electrosynthesis.Different concentrations of nano zero valent iron（2.5 g/L,5.0 g/L,and7.5 g/L）have been shown to be beneficial for providing more hydrogen as electron donors through anaerobic corrosion reaction,and the aged NZVI deposited on the cathode was conducive to the formation of biofilm and enhanced its redox performance.Eventually,the concentration and selectivity of caproate,and the carbon conversion rate were further respectively increased to 5.3 g/L,51.7%,and 61.2%under the optimized NZVI concentration（5 g/L）.Meanwhile,the acid consumption（HCl solution）for p H regulation was reduced by 37.5%due to the buffer performance of NZVI.（5）The changes in functional microbial community structure in biofilms and planktons were analyzed by microbial sequencing and functional prediction,as well as the response mechanisms to voltage,p H,and NZVI regulations.Results evidenced that voltage regulation was mainly conducive to the enrichment of chain elongator（Clostridium＿sensus＿stricto＿12）on the biofilm and the enhancement of acetyl-Co A biosynthesis activity（like acetyl-Co A hydrolase,acetyl-Co A carboxylase）.Moreover,the mechanisms of p H regulation on the enhancement of microbial community structure and function were mainly reflected in the high relative abundance of organic matter decomposer（like Pseudomonas）in the catholyte,and the more active activities of organic matter metabolism（amino acids transport and metabolism）,acetyl-Co A biosynthesis and utilization related enzymes（acetyl-Co A carboxylase）of biofilms and planktons.In addition,the cathode environment created by moderate nano zero valent iron was conducive to the enrichment of chain elongator（Clostridium＿sensus＿stricto＿12）of biofilm under long-term operating mode,and maintained the relative abundances of homo-acetogens（like Terrisporobacter,Romboutsia）in catholyte,while excessive NZVI would lead to the dominance of short chain fatty acids decomposers（like Paraclostridium,Desulfovibrio）,thus inhibiting the chain elongation.Therefore,this thesis has clearly confirmed the technical feasibility of medium chain fatty acids production from waste activated sludge by microbial electrosynthesis,and determined this technology is a process that utilizes the in-situ electron donor as the main electronic source and the cathode electron as the common electron donor to drive short chain fatty acids,dissolved CO₂,and residual organics in the acidogenic fermentation liquid into medium chain fatty acids.Meanwhile,this thesis improved the efficiencies of electron transfer and carbon conversion by voltage,p H,and NZVI regulations,so as to achieve efficient medium chain fatty acids production performance.Moreover,the distribution and response mechanisms of functional microbes under different regulations were clearly penetrated.Overall,the results of this thesis provide a reliable technical solution for alleviating the dependence on external electron donors during the conversion process of organic waste into medium chain fatty acids,and propose novel theoretic basis for understanding the electron transfer,carbon transformation,and microbial mechanisms in chain elongating process.