Study On Mechanism And Application Of Graphite In Electro-fermentation Promoting Methane Production Of Sludge

Anaerobic digestion(AD)has been one of the most common technologies for sludge treatment,the application of which can not only recaptures bio-energy(methane),but also offers some distinguished advantages such as the reduction of sludge volume,stability of the physicochemical characteristics of sludge,and improvement of sludge dewatering performance.Four key stages of anaerobic digestion involve hydrolysis,acidogenesis,acetogenesis and methanogenesis.Among them,the interaction between syntrophic bacteria and methanogenic archaea plays a key role on methanogenesis.Therefore,more efforts should be done to enhance their interaction to improve the anaerobic digestion performance.With consideration of the excellent conductivity of graphite and the high sensitivity of electrochemical technique,graphite powders was added into the electro-fermentation system(Graphite + Voltage),through the comprehensive analysis of methanogenic performance,electrochemical performance,microbial community structure,energy efficiency analysis and economic benefit analysis to investigate its potential for the improvement of anaerobic digestion performance,clarify the coupling effects between graphite,electricity and methane production,and understand the synergistic relationships and mechanism between electricity and anaerobic microbial community.Main contents and results are as follows:(1)Adding graphite powders significantly improved the anaerobic digestion performance of electro-fermentation system.Modified Gompertz fitted the experimental data well and revealed that the presence of graphite effectively improved the maximum methane production rate and shortened the lag time.The methane production potential in the Graphite + Voltage reactor reached 2.98 mmol with the potential of-0.6 V,which was 60.21%and 22.63%higher than that in Graphite reactor and Control reactor,respectively.Electrochemical analysis demonstrated that the addition of graphite in electro-fermentation system increased the charge storage capacity and decreased the electron transfer impedance and biofilm impedance.In-situ FTIR spectroscopy showed that the kinds and intensities of electro-active groups increased with graphite,which might be beneficial for the electron transfer process among microorganisms.(2)The performance of electro-fermentation system could be influenced by the potential applied.The results demonstrated that the optimum potential was-0.8 V during the range of-0.6 V and-0.9 V,under which the largest methane yield was obtained.Specifically,the maximum methane potential(P),maximum methane production rate(Rmax)and lag time(δ)with-0.8 V were 5.00 mmol,0.50 mmol/d and 3.74 days,respectively.It may be due to the high areal capacitance(8.26-8.43 mF·cm-2)and low electron transfer impedance and biofilm impedance(2.64-20.23 Ω),which indicated a fast electron transfer process and redox process(CO2 reduction).Confocal laser scanning microscope(CLSM)analysis showed that the increase of protential promoted the growth and distribution of biofilm on the cathode.However,when the potential was higher than-0.8V,higher ratio of active/dead cells were observed,which might be attributed to the unbalance between intracellular oxidation-reduction potential(ORP)and extracellular ORP.Meanwhile,it was found that the increase of potential could not change the species of microbial communities,but the relative abundances.The relative abundances of main microorganisms such as Hydrogenophaga,Methanobacterium reached highest with the potential of-0.8 V.In addition,the effectiveness of the Graphite + Voltage reactor with the aspects of energy and cost under-0.8 V was the highest,which was beneficial for its commercial application.In short,graphite powder can promote the process of electro-fermatation methanogenesis in sludge and improve its extracellular electron transfer performance.Through the analysis of energy efficiency and economic benefits,it has been confirmed that electro-fermatation methanogenesis has good economy.Our research not only expands the practical application of electro-fermentative methanogenesis in sludge treatment,but also provides a new perspective for understanding the extracellular electron transfer on sludge electro-fermatation.