Biogas Upgrading Associated With In-situ Hydrogen Production By Bioelectrochemical System (BES)

The anaerobic digestion（AD）of blue algae suffers from the inhibition of high concentrations of ammonia nitrogen and the low purity of methane in the biogas produced.Bioelectrochemical systems（BES）,which have received much attention in recent years,not only have the ability to remove and recycle ammonia nitrogen,but can also be combined with anaerobic digestion to enhance methane production.At the same time,the hydrogen produced in the BES cathode can also be applied to the purification of biogas.In this study,a coupled three-stage chamber BES-AD reactor was designed and constructed by embedding the anaerobic digestion system in a bioelectrochemical system.Based on this reactor,the process of hydrogen production using ammonia nitrogen released during the anaerobic digestion of Taihu blue algae as the nitrogen source and the purification of biogas using the hydrogen produced at the cathode were investigated.The main findings of the study are as follows:（1）Hydrogen-producing microorganisms and anodic electroproducing microorganisms were acclimated in fermentation flask and the anode chamber of BES,respectively.The results showed that the anodic potential stabilized at-0.50 V after 12 days of domestication.After two weeks of domestication,the H₂ content in the gas produced by hydrogen-producing microorganisms was stable at about 55%and no CH₄ was detected.（2）Different nitrogen sources were added to the fermentation flask and the cathode chamber of BES for hydrogen production.The results showed that the hydrogen production in the fermentation flasks was comparable between 1000 mg/L peptone（399.13 m L）and 1000 mg NH₄⁺-N/L ammonium bicarbonate solution（448.45 m L）,indicating that ammonia nitrogen could replace organic nitrogen as the nitrogen source for the hydrogen producing microorganisms.When 0.4 V was applied to the BES,the biohydrogen production in the cathode was 2.34 and 2.90 times than the group of 0 V and 0.8 V,which indicates that the nitrogen source is insufficient at too low a voltage and the activity of the hydrogen-producing microorganisms is inhibited at too high a voltage.（3）When 1000 mg NH₄⁺-N/L ammonium bicarbonate solution was added to the middle chamber of the BES-AD reactor,the methane yield was less different from the control group,while the hydrogen yield reached 524.16 m L,which was better than the other groups.Comparing the methane and hydrogen production,it can be found that the BES-AD reactor still has the ability to remove and recycle ammonia nitrogen under the operating conditions of high concentration of ammonia nitrogen.（4）In the purification experiments,the injection of 0.25 bar of H₂ on reaction day 0 was able to promote the hydrogenotrophic methanogens pathway and the aceticlastic methanogens pathway to improve the production and content of CH₄.Compared with the control group,the administration of 0.25 bar H₂ on reaction day 0 increased the methane yield（286.62 m L）and methane purity（75.73%）by 22.98%and 14.04%,respectively,while the addition of 0.75 bar H₂ at the beginning of the process inhibited the degradation of VFAs,resulting in lower methane yield and purity than the control group.（5）High-throughput sequencing results showed that Romboutsia（42.98%）,unclassified＿c＿Bacterodia（40.85%）,norank＿f＿Rikenellaceae（37.24%）,norank＿f＿norank＿o＿Methanomicrobiales（32.58%）,and norank＿f＿Synergistaceae（29.72%）associated with the methanogenic process in the R₁ group were all higher in relative abundance than the control group;PICRUSt results showed that the appropriate amount of hydrogen inlet could not only affect the biomethanation performance of the reactors in each group,but also enhance methane production by promoting the extracellular electron transfer process.