Mechanism Research And Performance Optimization Of Electrocatalytic Methanogenic System (EMG)

Biogas production from anaerobic digestion is an important part of bio-energy.The calorific value of biogas generated in the traditional anaerobic digestion process is low.Electrocatalytic reduction of carbon dioxide in biogas to methane can further improve the calorific value of biogas and reduce carbon dioxide emissions.In this paper,the mechanism and key operation factors of methane production enhanced by EMG were studied by a self-designed,developed and operated single chamber microbial electro catalytic methanogenesis（EMG）reactor.This study designed and developed a single chamber EMG reactor,and developed a supporting online monitoring system to achieve online monitoring of temperature,pH,voltage,current,and gas production during the operation of the reactor.Based on Lab VIEW,an control software was developed to achieve real-time operation parameter monitoring and data storage.At the same time,the ESP32 module and Raspberry Pi were used to achieve remote data communication based on the MQTT protocol.The operation status of the reactor can be viewed remotely in real-time from mobile devices,providing technical support for the future application of EMG reactors in practical scenarios.The effects of different conditions on the start-up and microbial acclimation process of EMG reactors were studied.（1）The start-up stage of the EMG reactor was inoculated with different sludge to have anaerobic microbials obtain electro-activity.The results showed that the start-up acclimation time of the reactor using anaerobic sludge was 10 days,which was reduced by 12 days compared to inoculating excess sludge.And achieved a maximum methane concentration of 77.4%and a methane yield of 0.68 m³·m^-3·d^-1.（2）Comparing the performance of different cathode materials,the results showed that carbon fiber brushes exhibited higher cathodic activity during EMG start-up due to their higher specific surface area and better microbial adhesion.The highest current density could reach 6.7 A·m^-3,and they exhibited better methane production performance.The highest methane concentration was 78.8%,and the average methane yield was 0.71 m³·m^-3·d^-1.（3）The effects of four different start-up voltages on the start-up and operation processes of EMG reactors were investigated.The results showed that the EMG reactor started with a 1.0 V voltage exhibited the best methane production performance,with a methane content of 75.8%～88.4%and a maximum daily methane production rate of 1.67 m³·m^-3·d^-1.The performance of EMG reactor and traditional anaerobic digestion reactor was compared.Compared with the traditional anaerobic digestion reactor,the EMG reactor has expanded the electron transfer path for anaerobic microorganisms to metabolize organic pollutants due to the presence of electrocatalysis,especially the direct electron transfer path with cathode output electrons.Helps electrically active methanogens obtain H₂ from the substrate solution for reducing CO₂.The experiment shows that the methane production performance of EMG reactor is always better than that of traditional anaerobic digestion reactor in different feeding stages.The average batch capacity methane yield of EMG reactor can reach 1.16 m³·m^-3·d^-1,which is 50.6%higher than that of traditional anaerobic digestion reactor.The maximum methane concentration can reach 90.3%,which is 24%higher than the traditional anaerobic digestion reactor.In addition,under the action of electrocatalysis,propionic acid was smoothly oxidized on the anode,and the propionic acid content in EMG decreased from 1100.9 mg/L to 16.4 mg/L within 13 days.EMG reactor can overcome the inhibition of high concentration propionic acid on anaerobic digestion process and improve the stability of anaerobic digestion process.The microbial high-throughput sequencing analysis of the two reactors showed that the archaea community in the EMG reactor was dominated by Metanoculleus,while the traditional anaerobic digestion reactor was dominated by Metanothrixz.At the same time,the proportion of methane producing bacteria that can generate methane by reducing CO₂in the EMG reactor is over 80%.In terms of bacterial community,the bacterial community in the EMG reactor is dominated by Firmicutes,which is related to the generation of current in the bio electrochemical system.Therefore,the enrichment of Firmicutes is conducive to the stable operation of the EMG reactor.Research on the stable operation of EMG reactors has shown that:（1）The stability of reactor temperature is a key factor affecting EMG performance.（2）The application of different substrates can lead to fluctuations in the pH value and methane production performance of EMG reactors.Compared to glucose,the methane production efficiency of the EMG reactor is better when sodium acetate is the sole substrate,with a maximum methane concentration of 91.4%and a batch average pool volume methane yield of 1.16m³·m^-3·d^-1.The application of different substrates in the EMG reactor can be used as a means to regulate the concentration of H⁺in the matrix liquid,so as to adjust the relative advantages of acetophilic methanogenesis and hydrophilic methanogenesis and optimize the methanogenic performance.（3）For the operation of EMG reactors,pH value can be both a regulatory measure and a characterization of changes in reactor performance.Whether using any of the three substrates,glucose,sodium acetate,or CO₂,the methane concentration in the produced gas and the pH value of the matrix liquid have the same trend of change.The pH value for stable operation of the EMG reactor should be controlled between 7.6 and 9.2.