Promoting Anaerobic Digestion Of Sewage Sludge Through Regulating The Electrochemical Properties Of Biochar: Investigation On Mechanism

In recent years,many studies have shown that adding biochar to anaerobic digestion system can significantly increase the methane yield.Regrettably,the mechanism about how the electrochemical characteristics of biochars（electron donating and accepting capacity,electrical conductivity）affect the sludge anaerobic digestion process remains to be explored.Based on the above background,by regulating electrochemical properties of biochar,the effects of biochar with different electrochemical characteristics on the methane production of anaerobic digestion of sludge were analyzed,and the composition of microbial communities was analyzed.The main conclusions are as follows:（1）Regulation and analysis of the electrochemical properties of biochars:Biochars（labeled BC300,BC500,and BC700,respectively）were prepared with different pyrolysis temperatures（300°C,500°C,and 700°C,respectively）,using corn stalk as biomass raw material.And the electron exchange capacities（EEC,the sum of the electron donating ability（EDC）and the electron obtaining ability（EAC）,which reflects the level of redox activity of biochars）of BC300 and BC700 were both higher than that of BC500,whose EEC was 1.007 mmol·g^-1,0.968 mmol·g^-1 and 0.626mmol·g^-1,respectively.Most of the redox functional groups on the surface of BC300 are in the reduced state（EDC>EAC）,while that on the surface of BC700 is mostly in the oxidized state,and the oxidized and reduced states of functional groups on the BC500 surface are more evenly distributed.The EC of biochars increases with increasing pyrolysis temperature.Five approaches were selected to modify the functional groups on the surface of BC300,which were chemical modification based covalent bonds（including p-benzoquinone and hydroquinone treatment）,physical AQDS adsorption,oxidation with H₂O₂ and oxidation with HNO₃,and the biochars were labeled as BC300-对苯醌,BC300-对苯二酚,BC300-AQDS,BC300-H₂O₂and BC300-HNO₃.The results showed that the oxidation with HNO₃ had the strongest ability to improve EEC of BC300.The EEC of BC300-HNO₃is 276.85%higher than that of BC300.The physical adsorption of AQDS had the second strongest enhancement on EEC of BC300 and the EEC of BC300-HNO₃ is 90.96%higher than that of BC300,the EEC of BC300-AQDS increased by 90.96%.The chemical modification had a small increase in EEC of BC300.Among the five modified biochars,except that the EC of BC300-AQDS increased compared with BC300,the EC of the other four biochars nearly remain same.（2）The effect of biochars with different electrochemical properties on the anaerobic digestion of sludge:BC300,BC500 and BC700 promoted the total methane production of sludge anaerobic digestion to varying degrees.In comparison to the Control group,addition of BC300,BC500 and BC700increased the cumulative methane production by 42.4%,28.9%and 11.2%,respectively,with the addition of 1g/g TS.Correlation analysis between the electrochemical properties of biochars and the corresponding methanogenesis performance found that the EDC of biochar was positively correlated with the methanogenesis of sludge anaerobic digestion.The addition of biochars,regardless of its electrochemical properties,did not improve the hydrolysis or acidogenesis process significantly.The promotion of biochars on the methane production of sludge anaerobic digestion was mainly related to the acetogenesis-methanogenesis stage.And the analysis of the microbial community structure revealed that,EC,which reflects the conductivity of biochar,and the redox activity parameters（EDC,EAC,EEC）were opposite to the microbial community structure during the anaerobic digestion of sludge.（3）Exploring the mechanism of effect of biochars based on anaerobic digestion of organic carbon model substrate of sludges:Based on the complexity of sludge composition,carbohydrate,protein and fatty acid model substrates（dextran,bovine serum protein BSA,propionic acid）were selected to study the effect of biochars with different electrochemical properties on the methanogenesis of sludge anaerobic digestion.BC300 had the strongest promoting effect on methane production of anaerobic digestion of three types of model substrates,and BC500 had the second largest promotion effect on methane production of anaerobic digestion of these three types of model substrates,while BC700 could not promote anaerobic digestion of dextran and BSA significantly and had a relatively lower promotion effect on propionic anaerobic digestion.Correlation between the redox activity parameter EEC of biochar and microbial abundance was completely opposite to correlation between conductivity parameter EC and microbial abundance in these three types of model substrates anaerobic digestion system,which is consistent with results from sludge anaerobic digestion.Study on two methanogenesis pathways（acetoclastic methanogenesis and hydrogenotrophic methanogenesis）during anaerobic digestion of sludge have found that BC300 could promote both hydrogenotrophic methanogenesis and acetoclastic methanogenesis,while BC500 and BC700 could only promote hydrogenotrophic methanogenesis,and have no significant effect on acetoclastic methanogenesis.