Microbial Electrosynthesis(MES) Technology Was Used To Promote Methane Production From Organic Wastewater At Low Temperature

Energy shortage and environmental pollution have become more and more important factors restricting social and economic development in China.How to solve the problem of energy crisis and environmental pollution has become the focus of researchers.The microbial electrosynthesis（MES）technique has found a better solution to this problem.MES can not only treat organic wastewater to achieve clean treatment of wastewater but also recover the energy in it to produce clean energy such as methane,which is more efficient than ordinary anaerobic digestion（AD）.At present,most of the research on MES focuses on the medium temperature（25～37℃）and high temperature（55～65℃）,while the research on low temperature（<25℃）is still very few.However,for people in low-temperature areas and winter conditions,wastewater treatment and energy consumption have become two major problems that need to be solved urgently.The discharged low-temperature organic wastewater needs to be heated to medium temperature conditions for treatment,making AD uneconomical and reliable,and the cold creates a high demand for energy（biogas）.Therefore,it is very important to explore the use of MES to improve the performance of low-temperature anaerobic digestion.In this study,a single-chamber and a dual-chamber MES reactor were used to simulate organic wastewater with complex composition in dog food.The effects of electrification on low temperature（15℃）anaerobic digestion were investigated under an additional voltage of 1.0 V.The methanogenic performance,microbial community structure,and methanogenic pathway of dog food were analyzed combined with two different structures of single-chamber and dual-chambers.Finally,the methanogenic inhibitor 2-bromoethane sulfonate（BES）was used to explore the effect of methanogenic inhibitors on single and double-chamber MES and ordinary anaerobic digestion under low-temperature conditions.The conclusions are as follows:（1）Electricity could significantly improve the biogas production capacity of low-temperature anaerobic digestion.The biogas production capacity of dual-chamber MES was higher than the single-chamber MES.The coulombic efficiency of single-chamber and dual-chamber MES was far greater than 100%,and the energy recovered was much higher than the input energy.In terms of biogas components,the anode with a higher potential had a higher CO₂ concentration and a lower CH₄ concentration than the cathode,while the cathode had the opposite.From the perspective of the whole process,both the ammonia nitrogen concentration and p H value showed a downward trend,indicating that these two factors are not the cause of the difference in gas production performance.It was worth mentioning that although the p H of the MES anode drops the fastest,it was always in a suitable range.This indicates that low temperature is more resistant to changes in system p H than medium temperature and can better maintain a stable fermentation environment.（2）Propionic acid（～620 mg/L）was the acid with the highest concentration in low-temperature anaerobic digestion,followed by acetic acid（～220 mg/L）.With the addition of the methanogenic inhibitor sodium 2-bromoethane sulfonate,propionic acid concentration was significantly reduced in all reactions and remained at a low level（50～100 mg/L）,while acetic acid concentration was consumed in the normal anaerobic digestion group and increased in the 2-bromoethane sulfonate group.By comparing the energy recovery efficiency of single-chamber MES and dual-chamber MES,it could be found that the energy recovery efficiency of single-chamber MES（9.43%～15.91%）was lower than that of dual-chamber MES（21.42%～28.54%）.（3）From the analysis of bacterial community structure,the hydrolyzed bacteria Firmicutes and Bacteroidota were dominant in low-temperature anaerobic digestion.There was no significant difference in the species of flora,but there was some difference in the abundance among the groups.Methanocorpusculum and Methanosarcina were dominant methanogens,whose abundance was the highest in common anaerobic digestion,followed by single-chamber MES,and the lowest in dual-chamber MES.With the addition of methanogenic inhibitors,the species and abundance of microflora changed significantly,which indicated that2-bromoethane sulfonate could not only inhibit the activity of methanogenic archaea but also affect the metabolic activities of some bacteria.（4）The results of functional flora analysis showed that the abundance of electroactive microorganisms in the MES group was higher than that in the anaerobic digestion group,the abundance of single-chamber MES was higher than that of dual-chamber MES,and the abundance of carbon brush electrodes was higher than the fluid sludge.But for some other fermentation bacteria,the abundance of ordinary anaerobic digestion group was higher.In low-temperature anaerobic digestion,methanogens were mainly acetoclastic methanogens and hydrogenotrophic methanogens.The abundance of anaerobic digestion group was higher than that of MES group,and the abundance of dual-chamber MES was higher than that of single-chamber MES.The abundance of carbon brush electrodes was higher than that of fluid sludge.Metatranscriptomics analysis showed that low-temperature anaerobic digestion was dominated by the acetoclastic methanogenesis pathway.The metabolic capacity of the MES group was higher than that of ordinary anaerobic digestion,and electricity can promote methanogenesis.In this study,16S-r RNA amplification sequencing and metatranscriptomic sequencing were used to elucidate the differences of methanogenic performance,the composition of related functional microorganisms,and the expression of functional genes between single-chamber,dual-chamber MES,and common anaerobic digestion system at low temperature（15℃）.The feasibility of using MES technology to promote methanogenesis under low temperature were compared and analyzed,which laid a theoretical foundation for the research of promoting methanogenesis under low-temperature anaerobic digestion.