Mechanism Of Electron Acceptor Promoting Acid Transformation During Anaerobic Fermentation

As there is still a certain amount of volatile organic acids in biogas slurry,it is not good for backflow or fertilizer utilization of biogas slurry.Generally,post-fermentation tank（tank）should be set up to reduce the acid content in biogas slurry.However,the residual volatile acid in biogas slurry is mainly propionic acid,which is difficult to transform,and its transformation process has the characteristics of high free energy and slow,which makes the post-fermentation cycle longer,and the equipment occupies a large area and investment.In addition,the electron acceptor substances in the anaerobic post-fermentation system are mainly single HCO₃^-,and the fatty acid oxidizing bacteria and carbonate reducing bacteria（hydromethanogens）are generally completed by indirect electron transport with low efficiency.Therefore,the lack of electron acceptor substances in biogas slurry may also be one of the main problems leading to propionic acid conversion difficulties.In this study,sodium propionate was used as the only substrate to simulate the anaerobic fermentation process of high acid content biogas slurry,the transformation characteristics of propionic acid with different electron acceptor substances(NO₃^-,SO₄^2-,Fe³⁺)were investigated.of propionic acid salt removal rate,methane-producing characteristics,water ion concentration index were measured,and by using 16S r DNA sequencing succession law of microbial community structure were analyzed,and the PICRUSt2 software is adopted to predict functional genes,explore using cheap electron acceptor material new way to strengthen the transformation of propionic acid in the biogas slurry,provide theoretical support for the production practice.The main results and conclusions are as follows:（1）The maximum removal rates of propionate in the experimental group were 94%,90%and91%,respectively,which were 36.23%,30.43%and 31.88%higher than those in the control group with the addition of electron acceptor NO₃^-,SO₄^2-and Fe³⁺.The maximum methane production rates were 295.49 m L/g COD,279.79 m L/g COD and 285.45 m L/g COD,respectively,which were29.9%,23.0%and 25.5%higher than those of the control group without electron acceptor.The system containing NO₃^-and Fe³⁺electron acceptor has less fluctuation in gas production and better resistance to acid shock.（2）The microbial diversity in activated sludge with electron acceptor NO₃^-,SO₄^2-and Fe³⁺was higher than that in the control group without electron acceptor,and the species diversity in the reactor containing electron acceptor NO₃^-was the highest.Thauera、Aquabacterium and Desulfomicrobium,Clostridium＿sensu＿stricto＿1 were used as dominant bacteria in the reactor of NO₃^-,SO₄^2-.Fe³⁺as independent electron acceptor in fermentation system.The results showed that adding different electron acceptors could significantly affect the microbial community structure of propionic acid anaerobic fermentation system.（3）Different electron receptors had different effects on microbial functional genes.The dominant functional genes related to REDOX reaction,such as K03711,K00384 and K03406,were enriched in the reactor with Fe³⁺and NO₃^-,indicating that electron receptors Fe³⁺and NO₃^-could actively participate in the electron transfer process of propionic acid transformation.In addition,from the enrichment degree of each key gene in the methane-producing metabolic pathway,it can be found that the methane-producing pathway of acetic acid metabolism is strengthened in the reactor with the addition of electron receptor Fe³⁺,and the methane-producing pathway of acetic acid metabolism and carbon dioxide are strengthened in the reactor with the addition of electron receptor NO₃^-.The enhancement of related functional genes was not obvious in the system with the addition of electron receptor SO₄^2-.