Perfromance And Mechanism Of Electrochemical Pretreatment Enhancing Anearobic Co-fermentation Of Waste Activated Sludge And Food Waste Into Volatile Fatty Acids

With the increase of the annual output of waste activated sludge（WAS）and food waste（FW）in China,it is vital to develop rational and efficient disposal strategies for treating WAS and FW.Anaerobic co-fermentation（Co-AF）is considered as one preferred method for WAS and FW treatment for several favorable attributes including obvious technological advantages remarkable stabilization effect for organic solid waste,and efficient recovery of high valueadded products like volatile fatty acids（VFAs）.Nevertheless,Co-AF process still meets some limitations,such as low hydrolysis and biodegradation efficiency,lack of essential data,and unclear metabolism pathway.Recently,electrochemical pretreatment（EPT）,benifitting the improvement of anaerobic fermentation（AF）efficiency,has attracted many scholars’attention.In this study,EPT was employed to improve the yield of VFAs.The feasibility of EPT enhancing the performance of WAS-FW anaerobic co-fermation was analyzed and the key influence factors（pretreatment voltage,material ratio and electrode material）were investigated.The application potential of this process and VFAs was also evaluated.Then,the related mechanisms of EPT enhancing Co-AF of WAS-FW into VFAs were discussed.Firstly,the feasibility of EPT enhancing Co-AF of WAS-FW into VFAs was investigated.The mixed substrate（with 50%FW）was pretreated for 60 min at a voltage of 5.0 V,and then subjected to the 15-day Co-AF.The results showed that chloride ions in FW was oxidized into reactive chlorine species（RCS）that significantly promoted the disintegration of WASFW and improved the biodegradability of dissolved organic matter（DOM）.90%of RCS were consumed by the mixed substrate,but some residual RCS entered the subsequent AF system.The residual RCS was the key role to change the microbial community structure.The different tolerance to residual RCS for microorganisms in AF system promoted the screening and enrichment of acidogenic bacteria（Prevotella₇,Lactobacillus,and Veillonella）and inhibited methanogens,which resulted in acidogenic bacteria gradually become the dominant bacteria.These results benefited the production of VFAs.Consequently,the VFAs yield of EPT enhancing Co-AF of WAS-FW（549.1±1.5 mg COD/g VS）was 40.8%higher than that of the control group,demonstrating the feasibility of EPT enhancing of the VFAs yield.Secondly,the effects of different pretreatment voltage（0 V,5 V,10 V,and 15 V）and material ratio（0%,25%,50%,75%,and 100%FW）on the EPT process and Co-AF process were explored.The increase in voltage and the proportion of FW promoted the disintegration of mix substrate during 60 min EPT.During the subsequent 16-day Co-AF,the moderate residual RCS（4.4--14.5 mg Cl2/L）showed an inhibitory effect on the activities of acidogenic bacteria,hydrolysis enzyme,and acidification enzyme in the early stage of CoAF,but it exhibited a promoting effect in the middle and late stages.Particularly,the relative abundance of Prevotella₇ increased from less than 2%to 70%,and the relative activity of hydrolysis and acidification enzyme increased from 85.6%-89.2%to 135%-176.6%.However,more than 14.5 mg Cl2/L of residual RCS retarded all fermentation bacteria and key enzyme activity.The optimal condition was determinied to be that pretreatment voltage and FW ratio being 5 V and 75%,respectively.The VFAs yield of Co-AF under the optimal condition was 476.9±6.3 mg COD/g VS,which was 78.6%higher than that of the control group.Thirdly,under the optimal condition of pretreatment voltage（5 V）and FW ratio（75%）,the performance of the different metal oxide-coated anodes（Ti/PbO2,Ti/IrO2-Ta2O5,Ti/SnO2-RuO2,and Ti/IrO2-RuO2）on EPT enhancing of the VFAs yield was investigated.These results showed that the metal oxide-coated anodes produced RCS and singlet oxygen（1O2）during EPT.The disintegration efficiency of the mixed substrate was positively correlated with the yield of these two oxidants.RCS mainly promoted the release of the total amount of DOM,while 102 had a stronger effect on improving the biodegradability of the substrate.During the subsequent 16-day fermentation,the appropriate residual RCS screened and enriched acidogenic bacteria during the decay period（2-4 days）.For examples,Prevotella₇ became the dominant fermentative bacteria,and the relative abundance increased from less than 1%in the early stage of Co-AF to 53%-78%in the middle and late stages.These acidogenic bacteria utilized the biodegradable organic matter that improved by 1O2 to enhance the yield of VFAs.The VFAs yield of Co-AF with electrode material of Ti/IrO2-Ta2O5 was 545.5±36.0 mg COD/g VS,which was highest among these metal oxidecoated anodes and 101.8%higher than that of the control group.Fourthly,under the optimal condition of pretreatment voltage（5 V）and FW ratio（75%）,the application potential of the process of EPT enhancing Co-AF of WAS-FW and its products was evaluated.The VFAs from Co-AF of WAS-FW could be used as the carbon source in a denitrifying phosphorus removal system,in which the phosphorus removal efficiency and denitrification rate was increased by 11.6%and 16.7,respectively.The relative amounts of Candidatus_Competibacter（glycogens accumulating organisms）decreased by 86.6%,while the relative amounts of Tetrasphaera and Dechloromonas（phosphorus accumulating organisms）increased by 24.9%and 135.1%,respectively.Halogenated byproducts would be generated during the EPT process,whose concentration was determined to be 40.1-525.1 μg/L.While,the amount of the halogenated by-products gradually decreased with a reduction of 81.6%-99.9%under the subsequent AF.Finally,the economic analysis indicated that the profit of the process of EPT enhancing Co-AF of WAS-FW under the optimal EPT condition was 4.61 times than that of the control process.Therefore,the process possesses good practicability,safety and economy.Overall,EPT can effectively enhance the yield of VFAs of Co-AF of WAS-FW,which provides a new strategy for the recycling of organic solid waste such as WAS and FW.