Directional Degradation Of Lactic Acid In Soured Food Waste To Promote Methane Production With Enhanced Electron Transport

Methane production from anaerobic digestion of food waste is an efficient and clean way to utilize biomass energy.However,the technical bottlenecks such as weak electron transfer ability of anaerobic digestion of lactic acid,an intermediate product of lactic acid degradation,and low methane production efficiency due to unclear multiple competing reaction pathways of lactic acid degradation have not been solved.In this paper,we used microbial electrochemistry to enhance electron transfer in lactic acid anaerobic digestion and reveal the competitive pathways of electron transfer in lactic acid methanogenesis;we used nano zero-valent iron to promote efficient propionic acid degradation and regulate the structure of bacterial flora to enhance electron transfer for methanogenesis;we changed lactic acid degradation from inefficient propionic acidbased fermentation to efficient butyric acid-based fermentation by acidification pretreatment,and promoted hydrocarbon conversion to improve methane production efficiency.Microbial electrochemistry was used to enhance the electron transfer in the methanogenesis of lactic acid anaerobic digestion,three-dimensional fluorescence spectroscopy revealed the competing pathways of electron transfer in the methanogenesis of lactic acid,and high-throughput sequencing profiled the regulatory mechanism of electrochemistry on microbial flora.The methane production of lactic acid anaerobic digestion increased and then decreased when the constant voltage was increased,and the methane production increased by 27.4% at the optimal working condition at a constant voltage of 0.8 V compared with that without the applied constant voltage.The electrochemical effect reduced the content of humic acid with electron transfer function in the microbial extracellular polymer,which changed the electron transfer pathway between microbial cells from using extracellular polymer to more efficient direct electrode transfer.At a constant voltage of 0.8 V,the relative abundance of electron-producing bacteria Firmicutes in the microbial flora increased from 51.5%to 59.7%,and the microorganisms were able to form stable biofilms on the surface of the electrode sheet,and the electron transfer capacity of the anaerobic digestion system was enhanced.Nano zero-valent iron were used to promote the efficient degradation of propionic acid and optimize the regulation of microbial colony structure,which enhanced the electron transfer between microbial cells to promote methanogenic reactions.The relative abundance of Candidatus Cloacamonas in the microflora increased from 2.2%to 3.8% when nano zero-valent iron were added at 2 g/L,which promoted more degradation of propionic acid to acetic acid;meanwhile,the relative abundance of Methanomassiliicoccus and Methanosarcina increased from 14.6% and 2.1% to 24.2%and 3.9%,respectively,which promoted methanogenesis.The apparent electron transfer rate constant in the fermentation system was increased from 0.029 s-1 to 0.034s-1,and the electrochemical properties were improved.The amount of acetic acid in the lactic acid degradation products increased from 32.9% to 55.1% of the total soluble metabolites,resulting in a 37.1% increase in methane production obtained from lactic acid fermentation.The deep acidification pretreatment changed the lactic acid degradation mode from the inefficient propionic acid fermentation to the more efficient butyric acid fermentation,which promoted hydrocarbon conversion to improve methane production efficiency.The dynamic monitoring of reductive coenzyme I during lactic acid degradation using acidifying bacteria revealed that deep acidification pretreatment consumes NAD+ and converts it to NADH,which is consistent with the reaction type of butyric acid type fermentation,while the NADH/NAD+ ratio in the unprepared condition is almost unchanged,which is typical of the propionic acid type fermentation.The amount of propionic acid in lactic acid degradation products as a percentage of total soluble metabolites was significantly reduced from 69.6% to 19.5% after deep acidification pretreatment,and the methane production from lactic acid anaerobic digestion increased by 56.8% to 615.1 m L CH4/g TVS compared with that without pretreatment,and the peak rate of methane production increased by 2.7 times.