Efficiency And Mechanisms Of Methane Production In Bioelectrochemical Reactor Based On Optimization Of Built-in Electrodes

Based on the development demand of energy recovery,the mode of wastewater treatment has been transformed from the energy dissipation to the energy recovery and energy self-sufficiency.As a clean energy and efficient fuel,which can be sustainably recovered from waste organic matter and wastewater,methane has high industrial utilization value.However,the traditional anaerobic digestion for methane production often has the problems of slow reaction rate,long cycle,and difficult stable regulation.The introduction of microbial electrolysis makes the systems more stable and controllable,which has the superiority in organic conversion and methane recovery.Electrodes not only serve as the electron transport medium but also as the carrier of microbes.Previous studies have found that the increase of methane production was more dependent on the effect of biomass retention on electrodes,even rather than electrochemical interaction with the electrodes.However,the effect of electrode spatial distribution on the flow pattern and mass transfer in process,and the role of electrode spatial distribution on the growth and distribution of functional microbes,is not fully understood.At present,most researches focus on the laboratory stage,studying the improvement of reactor performance by single factor in the specific configuration.In practical application,it is of great significance to study the electrode arrangement and fluid state change in reactor.In this paper,some problems including the electrode material selection,optimization of electrode size and arrangement,regulation mechanisms by applied voltage,were studied.And fluid dynamics analysis method was used to obtain an optimization design strategy of reactor for practical application.An upflow bioelectrochemical methanogenic reactor was constructed.The efficiency of reactors with three kinds of metals（stainless steel mesh,nickel mesh and copper mesh）as biocathode was compared.Results showed that the COD removal rate of reactor with nickel mesh cathode was 1.12 times and 1.34 times of that of stainless steel mesh cathode and copper mesh cathode respectively,and the methane generation rate was increased by 45%and 148%,respectively.Therefore,nickel mesh was the the most suitable cathode material in this study.Furthermore,the effects of electrode position on organics removal rate,methanogenesis efficiency,electrochemical efficiency and microbial community structure were investigated.It was found that the reactor with the cathode placed below the anode was more conductive to the enrichment of electrochemical active bacteria and generating higher current.The reactor with the anode below was more conducive to attaching more methanogens and producting more methane.The configuration with electrodes placed at the bottom of the reactor and cathode placed above the anode generated maximum methane production efficiency.When the applied voltage was 0.8 V and HRT was 36 h,the maximum methane production rate,methane yield,and COD removal rate were 0.306 m³ CH₄/m³/day,0.222 m³ CH₄/kg COD,and92.1%,respectively.For making full use of reactor space,the cathodic space distribution of the reactor was optimized.The cathode space ratio（ratio of cathode surface area to reaction region volume）was taken as the index,three different cathodic configuration methods were compared（cathode space ratio:0.67 cm²/cm³,1.33 cm²/cm³ and 2cm²/cm³）.The influence of different cathode space ratio on the efficiency of organic matter conversion to methane（organic matter removal rate,methane generation）in the reactor was studied,and the hydrodynamic characteristics and flow state analysis of the reactor were studied.The results showed that by increasing the cathode space ratio to 1.33 cm²/cm³ and 2cm²/cm³,respectively,better flow patterns with the residence time of 1.336 times and1.363 times of theoretical hydraulic retention time could be obtained.The stacked structure of nickel meshes was beneficial to prolong the contact time of contaminant and improve the mass transfer.Considering the organic removal,methane recovery,electrons generation,and material consumption,the optimal cathode space ratio was 1.33 cm²/cm³.With this structure,COD removal efficiency reached 93.2±1.9%,methane production rate reached 0.332 m³ CH₄/m³/day under the HRT of 24 h.In addition,numerical simulation of the RTD experiment process was carried out.It was verified that the reactor solid model（especially the electrode solid model）and the calculation method based on some assumptions and simplification were feasible.Based on this electrode model,the optimized design of adding anode modules was obtained in this reactor with cathode space ratio of 1.33 cm²/cm³.The determined reactor configuration was the cross arrangement of anode-cathode-anode-cathode from bottom to top.For analyzing the mechanism of applied voltage promoting methanogenesis,the optimized cross configuration was adopted.The reactor efficiency,microbial community structure and shared OTU between biofilms and suspended microorganisms were compared under four kinds of reactors（dual-electrode module reactor,upper electrode module reactor,lower electrode module reactor and conductive carrier module reactor）.The results showed that the applied voltage can promote the methane production process of the reactor,and the maximum COD removal rate,methane production rate and methane yield were 96.7%,0.434±0.018 m³ CH₄/m³/day and 0.215±0.008 m³ CH₄/kg COD,respectively.The methanogenic capacity（methane production rate and methane yield）were 1.76 times and 1.69 times of the conductive carrier module reactor,respectively.The functional flora of acid-producing fermentation bacteria,electrochemically active bacteria and methanogens were enriched in both the conducting carrier module reactor and the electrode module reactor.Under the action of external voltage,the lower electrode module reactor has more advantages than the upper electrode module reactor in the utilization of organic matter to produce methane,and the electrochemical effect of the upper electrode module was higher than that of the lower electrode module.The microbial diversity on the surface of the electrode module was smaller than the surface of the conductive carrier,the shared network nodes between the electrode and the suspension solution were reduced.More Methanobacterium was enriched on the cathode,and the upper anode accumulated more Desulfovibrio.Compared with the conductive carrier,the electrode increased the pathway of the methane production by cathodic hydrogen evolution reaction.However,the Coulombic efficiency and Electrochemical contribution efficiency are low,the direct effect of electrochemistry is small.The effect of external voltage is more inclined to enrich more hydrogenophilic methanogens,mainly Methanocorpusculum,in the system.In this study,the effect and internal relation of electrode distribution on reactor flow pattern and electrode biofilm were studied.An upflow bioelectrochemical methanogenic reactor was optimized and designed,and a solid model was established and verified to simulate the internal flow field in this reactor.The optimal design of electrode position and configuration based on fluid dynamics will provide theoretical guidance for practical application.