Enhanced Methane Production From Wastewater Through The Extrcellular Electron Transfer Mechanism

The methane recovery through conventional anaerobic digestion process depends on the final step of methanogenesis,which is dominated by the acetoclastic methanogens and hydrogenotrophic methanogens.The growth rate of hydrogenotrophic methanogens is significantly faster than the acetoclastic methanogens,resulting in a higher methane production rate.Naturally,the contribution of hydrogenotrophs to the methane production is only ~30%,while the acetotrophs accounts for nearly 70%.Therefore,the enhancement of hydrogenotrophs is crucial for promoting the performance of methane production in anaerobic digestion.In this study,the paired electrodes were introduced into the classical anaerobic bioreactor to inspire the microbial-driven extracellular electron transfer at anode and the carbon dioxide reduction at cathode.The pathway could stimulate the growth of hydrogenotrophic methanonges.The enhanced anaerobic bioreactor exhibited higher methane production rate(0.0564 m3 methane/m3 reactor/d)compared to the control reactor(0.029 m3 methane/m3 reactor/d).The degradation of volatile fatty acids(VFAs)was also enhanced at the same time.The microbial community was driven to enrich on the separately functional spots in terms of anode enriched with the exoelectrogens and cathode with the hydrogenotrophic methanogens.To verify the mechanism of electron transfer from electrodes to microbes,we specifically emphasized the study on cathode microenvironment and the specific microbial community.On the surface of cathode,p H increased to more than 10 in the close vicinity of the cathode due to the consumption of protons.Meanwhile,the mcr A sequencing showed that mostly methanogens belong to the species of Methanobacterium alcaliphilum,which was basophilic methanogens.Obviously,the p H determined the methanogenic community at the cathode.Hydrogen detected by the microsensor showed increasing concentration with moving closely to the cathode,indicating the hydrogen could be the electron carrier.A combination of microsensor and CV test showed the difference between abiotic and biotic cathode.The hydrogen concentration was clearly higher in abiotic model.According to the electron balance analysis,the total coulombs that was supplied from the current was nearly 2.23 C,while the hydrogen can store approximately 1.01 C,the rest should be directly transferred into microbes.The hydrogen takes 50% duty on transferring electrons.Overall,in the inter-face of cathode and microbes,electron can be transferred by hydrogen or enzyme(direct transfer pathway)to reduce carbon dioxide into methane.To overcome the limitation caused by the property of electrode materials and activity of attached biofilm on the electrodes in the enhanced anaerobic bioreactor,modification method was developed to decrease the catalytic resistance of electrode to enhance electron transfer.Moreover,we proposed a strategy on regulating the electron transfer through adding signal molecule(AHL)to induce the quorum sensing to achieve the directly enriched biofilm.After coating graphene on the surface of nickel foam to obtain nickel foam-graphene(NF-G),the EIS data showed the composite could effectively decrease the resistance of electron transfer during the electrochemical reaction.At 0.4 V conditions,the electron recovery efficiency was 27.17%.Additionally,the electron recovery efficiency of nickel foam was 14.91%,clearly lower than that of NF-G.The energy recovery efficiency improved from 52.91% to 92.46%.The improved results indicate that the modified NF is advantageous over the original material.Subsequently,the nickel foam phosphide(NF-P)comprised Ni5P4 and Ni P2,exhibited similar Tafel slope under the LSV test and EIS test,indicating that the catalytic resistance is not a rate limiting step.In practical bioreactor,cathodic electron recovery efficiency of NF-P was 81.83%,the value of NF was only 62.05%.The energy efficiency has been improved from 95.29%% to 125.69%%.Two signal molecules with different carbon chain were used to stimulate the bioreactor under high and low concentration conditions and the performances were investigated.The results reflected that 3OC6 can improve the electron recovery efficiency with high concentration(10 μM),the cathodic electron recovery was 88.92% that was higher than 80.21% of control reactor.The energy efficiency increased to 171.6% that was higher than that of control(160%).A further study was completed with the addition of 3OC6 under different external voltages to explore the mechanism of improvement.The final electron recovery efficiency increased by 5.57%,38.68% and 81.82% under 0.8 V,0.6 V,and 0.4 V respectively.Meanwhile,the electrochemical activity of bacteria increased,indicating that AHL can stimulate the growth of electrochemical activity of bacteria to reduce the resistance to achieve higher recovery efficiency.At 0.6 V condition,the energy efficiency reached 214.64%,which was higher than that of control(182.78%).Overall,the modified electrodes and the signal molecule exhibited sucessuful improvement in electron recovery efficiency,achieving higher energy recovery efficiency.