Simultaneous Hydrogen Production And Vivianite Generation From Waste Activated Sludge In Electro-Fermentation System Mediated By Iron Anode

In recent years,global energy utilization has been increased exponentially,resulting in a worsening global warming problem.Therefore,the global energy transition has been accelerated.As a renewable energy source,hydrogen（H₂）with high calorific value（120MJ/kg）,is regarded as an alternative energy carrier which can alleviate the world carbon neutral.As the carrier of resource and energy,recovery of valuable products such as volatile fatty acids（SCFAs）,methane and hydrogen from waste activated sludge（WAS）has been extensively studied.Electrofermentation（EF）as a new technology can regulate redox potential and electron transfer by electrodes,controlling microbial metabolism and strengthening anaerobic fermentation pathway.It can be used to recover hydrogen from WAS,providing a new idea for waste biomass recycling.Moreover,WAS is also rich in phosphate element（which can replace 25%of phosphate fertilizer consumption in China）.As a new phosphorus recovery product,recovering vivianite（Fe₃（PO₄）₂·8H₂O）is a promising method due to its simple formation condition and high economic value.Till now,the research on the vivianite recovery from WAS is still in the initial stage,which is mainly focused on anaerobic digestion system,while the relative research on electrochemical system has not been reported.In this study,the main objective is to improve the efficiency of hydrogen and vivianite production in EF system.This study determined the optimal conditions by exploring the performance of hydrogen production and synthesis of vivianite.In order to clarify the molecular ecological mechanism of interspecies interaction of the functional microorganisms in EF,a two-stage fermentation system with pre-fermentation-EF was constructed.This study may provide an innovative insight for maximal recovery of resource and energy from WAS.This study explored the effect of different types of iron sources（built-in iron electrode,built-in stainless steel mesh,ZVI,Fe₂O₃,Fe Cl₃,Fe OOH）on hydrogen production and vivianite crystal by using simulated fermentation liquid(PO₄^3-350 mg COD/L,SCFAs 3000mg COD/L).The results showed that the utilization effeciency of SCFAs reached 79.1%,especially acetic acid and propionic acid（the removal rate was 92.9%and 71.0%）.Furthermore,the current and hydrogen yield were further enhanced to 5.8 mA and 102.7±8.3mL within 3-4 d,respectively,which was increased by 1.1～19.3 folds and 2.0～8.9 folds compared with other iron sources.Meanwhile,the phosphate removal rate was up to 100%on the 2 d and 5 d in Fe electrode and stainless steel mesh electrode group,respectively,while that obtained in other groups was only 0～24.1%.Scanning electron microscopy（SEM）and X-ray photoelectron spectroscopy（XPS）analysis showed that the size of vivianite particles was larger than that of other groups.Microbial community structure analysis showed that Firmicutes wes the main functional bacteria in the embedded Fe electrode group,accounting for 85.6%,which increased by 11～642%compared with other groups.At the genus level,the microorganisms are mainly Acetoanaerobium,Acetobacterium and Azonexus in Fe electrode group,Acetoanaerobium and Acetobacterium accounted for 78.0%.Azonexus accounted for5.1%,which is mainly related to hydrogen production.In order to improve the efficiency of hydrogen production and vivianite synthesis in the electrofermentation system mediated by Fe electrode,the key parameters（electrolyte concentration,pH and voltage）were optimized.Results showed that the utilization efficiency of SCFAs decreased with the increase of electrolyte concentration.SCFAs concentration was 545.3±67.9 mg COD/L in the condition of 10 mM NaCl,which was 11.8～91.9%higher than other groups（30 mM,60 mM）,acetic acid（HAc）and propionic acid（HPr）were accounted for the lowest proportion（36.3%）compared with other groups,which are easily to be used by microorganisms.Accordingly,hydrogen production reached 387.8±14.2 mL,which was1.24-14.33 folds higher than other groups.As for pH optimization（6.0,7.0,8.0,9.0）,the results illustrated that the SCFAs utilization efficiency peaked at 77.6%at pH 8.0,and increased 13.1-83.9%compared with other groups.H₂ yield was 257.4±13.2 mL（5 d）,which was 1.1～2.3-fold of other groups.Meanwhile,the crystallinity of vivianite is higher than that obtained in other conditions.On this basis,the effects of different voltage conditions（0.4 V,0.6 V,0.8 V,1.0 V）has been investigated in this study.It was indicated that voltage increase（0.4 V～0.8 V）can effectively promote the utilization of SCFAs by microorganisms（utilization efficiency was 43.2～79.1%）,while that was only 67.1%in the condition of 1.0 V.Hydrogen and vivianite production obtained the peak value under the voltage of 0.8 V,and the SCFAs utilization rate reached 79.1%,which was 1.2～1.8-fold of other groups.hydrogen yield peaked at 113.0±1.6 mL.In addition,ORP in all groups decreased to-335～-426 mV,which provided a suitable environment for anaerobic microorganisms and met the generation condition of vivianite.At the same time,the electrochemical performance was effectively improved.The maximum current peaked at 9.2 m A under 0.8 V voltage,and obtained the maximal double-layer capacitance area.Finally,a two-stage fermentation system coupling with pre-fermentation and EF system was constructed to explore the feasibility and mechanism of simultaneous hydrogen production and synthesis of vivianite from WAS under the optimized operation parameters.WAS was pretreated by thermal-alkali（85℃,pH 12.0,1 h）and take as the substrate for anaerobic prefermentation（4 d）.Then,the fermentation liquid（SFL）and NaCl electrolyte solution were mixed and added into the EF system according to the volume ratio of 1:0（SFL）,1:1（SFL/NaCl=1:1）and 1:2（SFL/NaCl=1:2）.The results showed that the utilization rates of soluble carbohydrate and protein in SFL/NaCl 1:1 group were 59.9%and 65.1%,respectively,which were 1.1,2.1 folds and 1.8,1.1 folds higher than those in SFL and SFL/NaCl 1:2 group,respectively.The SCFAs utilization rate of SFL/NaCl 1:1 group was 86.8%,which was25.4-74.3%higher than that obtained in other two groups.Meanwhile,the SFL/NaCl 1:1group showed superior hydrogen production performance（65.4 mg/g VSS）,which was 1.7and 1.1 times higher than that of SFL and SFL/NaCl 1:2 group,respectively.The formation of vivianite was detected in all groups.Microbial community analysis illustrated that protein fermentation bacteria,polysaccharide fermentation bacteria and acetate producing bacteria were all enriched in SFL/NaCl 1:1 group.Petrimonas（an acetate producing bacteria）,accounting for 2.5%in SFL/NaCl 1:1 group,which was 1.4 and 4.7-fold of that enriched in SFL and SFL/NaCl 1:2 group.Meanwhile,Tissierella,related to the synthesis of vivianite,hydrogen-producing bacteria（i.e.,Pseudomonas and Comamonas）were all enriched.