Study On The Influence Mechanism Of Ferroferric Oxide And Ethanol On Methanogenesis From Propionate

Anaerobic digestion has many advantages,such as low energy consumption,wide application,and high energy recovery.As the core technology of environmental protection and resource recovery,anaerobic digestion process is in line with the current sustainable development direction advocated in China.The key to improve the efficiency of anaerobic digestion is to reduce the accumulation of volatile fatty acids（VFAs）during anaerobic digestion.Propionate in VFAs is the most difficult to remove due to its high Gibbs free energy during oxidation,and its complete removal requires efficient interspecies electron transfer between fermenting bacteria and methanogens.Direct interspecies electron transfer（DIET）has more advantages in thermodynamics and kinetics than traditional interspecies hydrogen transfer（IHT）.Stimulating DIET is one of the methods to promote the degradation of propionate and improve the ability of methanogenesis.The effect of ferroferric oxide（Fe₃O₄）on the methanogenesis of propionate was investigated by adding conductive material Fe₃O₄ during the long-term domestication of the anaerobic sequencing batch reactor（ASBR）.At the same time,the effect of Fe₃O₄on the co-metabolism of ethanol and propionate and the role of Fe₃O₄ and ethanol in this process were also investigated by adding ethanol.Studies showed that Fe₃O₄ was not conducive to the degradation of propionate and the production of methane（CH₄）during the long-term domestication,but Fe₃O₄ was beneficial to the co-metabolism of ethanol and propionate and CH₄ production in the presence of ethanol.The batch experiment results showed that the maximum CH₄ production rate of ASBR_P+Fe（with Fe₃O₄）was 70%lower than that of ASBR_P（without Fe₃O₄）fed with propionate,and the maximum CH₄ production rate of ASBR_EP+Fe（with Fe₃O₄）was 81.4%higher than that of ASBR_EP（without Fe₃O₄）fed with propionate and ethanol（COD ratio of 2:1）.The contents of humic acid,riboflavin and NADH related to electron transfer in SMP and EPS secreted by ASBR_P+Fe+Fe at the end of the cycle were lower than those of ASBR_P.The sludge conductivity and ETS activity of ASBR_P+Fe were 3.15 and 3.54times of ASBR_P,respectively.Microbial community structure and metagenomic analysis results indicated that both Syntrophobacter and Smithella were dominant propionate oxidizing bacteria in ASBR_P and ASBR_P+Fe.However,the relative abundances of propionate-oxidation-related genes contained in Smithella in ASBR_P were high,while the relative abundances of genes contained in Syntrophobacter in ASBR_P+Fe were high.The dominant acetate oxidizing bacteria in ASBR_P were Corynebacterium and Mesotoga,while that in ASBR_P+Fe was Mesotoga.The relative abundances of Mesotoga and its genes related to acetate oxidation in ASBR_P+Fe were higher than those in ASBR_P.The dominant methanogens in ASBR_P were Methanosaeta and Methanobacterium,while that in ASBR_P+Fe was Methanosaeta.The relative abundances of Methanosaeta and its genes related to methanogenesis were similar in ASBR_P and ASBR_P+Fe,and the relative abundance of gene contained in Methanobacterium in ASBR_P+Fe was lower than that in ASBR_P.The contents of humic acid,riboflavin and NADH related to electron transfer in SMP and EPS secreted by ASBR_EP+Fe at the end of the cycle were lower than those of ASBR_EP.The sludge conductivity and ETS activity of ASBR_EP+Fe were 2.66 and 2.73times of ASBR_EP,respectively.The dominant ethanol oxidizing bacteria in ASBR_EP and ASBR_EP+Fe were both Geobacter,while the dominant propionate oxidizing bacteria in ASBR_EP and ASBR_EP+Fe were both Syntrophobacter and Smithella.The relative abundances of these bacteria and their genes in ASBR_EP+Fe were higher than ASBR_EP.The dominant methanogens in ASBR_EP and ASBR_EP+Fe were both Methanosarcina.The relative abundance of Methanobacterium in ASBR_EP and Methanosaeta in ASBR_EP+FeP+Fe were also high.The methanogenesis genes in ASBR_EP and ASBR_EP+Fe were mainly detected in Methanosaeta,and the gene abundance of ASBR_EP+Fe was higher than that of ASBR_EP.In the presence of ethanol,the DIET was enhanced with the addition of Fe₃O₄.The energy saved by DIET than IHT could be used by other microorganisms for faster metabolic activities.Secondly,DIET could consume propionate oxidizing products and make propionate metabolism more thermodynamically favorable.Fe₃O₄ coupled with ethanol could better promote methanogenesis from propionate.