Construction And Performance Improvement Of Microbial Electrosynthesis For CO₂ Reduction To CH₄ Integrating Acidic Organic Wastewater Treatment

The rapid development of the global industrialization has caused the energy crisis as well as the adverse environment pollution and greenhouse effect.Bioenergy is considered as the alternative energy sources because of the renewability,environmental friendliness,simple operation,and process safety.At present,methane production via anaerobic digestion is a well-established bioconversion technology,in which various types of biomass and organic wastes can be converted to biogas by anaerobic microorganisms at relatively low temperature and ambient pressure.Notably,hydrogen fermentation prior to methane fermentation can realize the co-production of hydrogen and methane,which is commonly used to enhance the total energy conversion efficiency.However,the excessive accumulation of volatile fatty acids（VFAs）during hydrogen fermentation can cause a significant drop of pH,thereby inhibiting the bioacitivity of microorganism.Additionally,it also has negative effects on the subsequent methane fermentation,resuling in low methane yield and production rate.Such an adverse issue could not be easily avoided using continuoue mode,perodically replacing the effluent,and diluting the effluent.In this case,we proposed to use microbial electrolysis cells（MEC）,which could produce hydrogen on the assistance of biocatalysts,to increase the pH of effluents of dark fermentation.Then the effluents were used for methane production using the system of intagreting methane fermentation and microbial electrosynthesis.This study discussed the results of hydrogen and methane production,chemical oxygen demand（COD）removal efficiency,electrochemical performance of electrode,the characterization of microbial film.Then the practical experiment was studied by two diffirent effluents of dark fermentation,one was the solution of mixed glucose and glycine by dark fermentation,the other was hydrothermol liquid of algae by dark fermentation.Finally,improving methane production of microbial electrosynthesis system（MES）was studied by chitosan modified cathode.The methane production rate,Faraday efficiency,electrochemical performance of cathode,the characterization of microbial film 0was also investigated.The major achievements were listed as follows:（1）The effluents of dark fermentation,which use mixed glucose and glycine as substrate,was firstly treated using MEC,and then the treated effluents were degraded by the integrated system containing anaerobic digestion and MES.The results showed that the pH of effluents increased from 4.5 to 8.7 after the treatment of MEC,accompanied with the production of 3.65 mmol H₂ in the cathode chamber of MEC.After 21 days’degradation of the pH-adjuested effluents using MES,the CH₄ production,COD removal efficiency,the overall energy recovery of the whole system was 18.8 mmol,89%,and3.78,respectively.In contrast,no CH₄ was produced in the control system which use the un-treated effluents,and no COD was degraded in the system.（2）The effluents of dark fermentation using hydrothermol liquid of algae was treated by MEC,and then the treated effluents were degradated by the integrated system containing anaerobic digestion and microbial electrosynthesis.The results showed that the pH of effluents increased from 4.7 to 9.5 after the pretreatment with MEC,accompanied with 8.22 mmol H₂ production in the cathode chamber of MEC.After 20days’operation with MES,the methane production,COD removal efficiency,the overall energy recovery of the whole system with the pH-adjusted effluents was 55.8 mmol,88%,3.67.In contrast,no CH₄ was produced in the control system which use the un-treated effluents,and no COD was degraded in the system.（3）The cathode of MES was modified with chitosan,which contain functional groups with positive charge.After 48 days of cultivation at a poised potential of-0.7 V vs.Ag/AgCl,the startup time of the biocathode modified with chitosan was largely reduced from 1 to 10 days.In addition,the current density of the biocathode modified with chitosan was around 0.16 A/m²,largely higher than that（0.038 A/m²）of the biocathode without modified.Accordingly,the CH₄ production rate of the biocathode was 46mmol/（m²·d）,more than 150-fold of the biocathdoe without modified.The reason was probably due to the increased dry weight（5.4 mg/cm²）of the biocathode modified with chitosan in contrast to that（0.22 mg/cm²）of the biocathode without modified.