Study On The Occurrence And Transformation Mechanism Of Methylmercury During Advanced Anaerobic Digestion Of Sludge

The environmental health risk of heavy metals is the key to determine the land use of sludge to realize its resource utilization.As one of the heavy metals,mercury can be converted into methylmercury with strong bioaccumulation and fat solubility.However,there are still few studies about the migration and transformation of methylmercury during sludge treatment and disposal,especially in the process of advanced anaerobic digestion.In this study,mercury and methylmercury in sludge during the process of advanced anaerobic digestion with high-temperature thermal hydrolysis were analyzed.Further based on laboratory experiments and molecular biological analysis,the migration and transformation of mercury and methylmercury in the process of advanced anaerobic digestion were investigated.The methylation and demethylation of mercury and the effect of sulfate during anaerobic digestion of sludge were also studied.Furthermore,the evolution of microbial community and functional genes during advanced anaerobic digestion were investigated,to reveal the methanism of migration and transformation of methylmercury.This study provided a theoretical basis for the control of heavy metal pollution for its environmental health risk in sludge treatment and resource ultilization.The results showed that:（1）According the survey of advanced anaerobic digestion engineering in Beijing for one year,the total mercury content in sludge increased gradually with the decomposition and mineralization of organic matter during advanced anaerobic digestion,which was increased from（4.35±0.59）mg·kg^-1 in the raw sludge to（5.58±1.42）mg·kg^-1 after thermal hydrolysis,and further increased to（6.37±1.05）mg·kg^-1after anaerobic digestion.The digested sludge could hardly meet the standard for its land use in acid soil（p H<6.5）.However,advanced anaerobic digestion significantly reduced the content and mass flow of methylmercury in the sludge.The content of methylmercury in the sludge was reduced from（5.99±3.55）μg·kg^-1 of the raw sludge to（1.59±0.96）μg·kg^-1 after thermal hydrolysis,and further decreased to（0.82±0.55）μg·kg^-1 after anaerobic digestion.In particular,the thermal hydrolysis process was prominent in reducing methylmercury content,which played an important role in reducing the environmental health risk of mercury.（2）In the practical advanced anaerobic digestion process,the physic-chemical properties of sludge changed a lot due to the breakdown of sludge flocs and microbial cells in thermal hydrolysis as well as the decomposition and mineralization of organic matters in anaerobic digestion.The results of RDA analysis showed that the content of methylmercury in sludge was positively correlated with the content of phosphate,Fe and VFAs,but negatively correlated with p H,ammonia and proteins.Firmicutes,Bacteroidetes,chlorofloxi,Proteobacteria and Thermoogae were the dominant microorganisms in anaerobic digestion.Mercury methylation related microorganisms（Desulfovibrio,methanamassificus,etc.）and functional genes（hgc A,dsr A,mer A）were detected,but the abundance of which was low.It seemed that the methylation of mercury existed in the anaerobic digestion process.（3）Laboratory research showed that mercury methylation and demethylation occured in different stages of anaerobic digestion,and mercury methylation was the dominant in the early stage of anaerobic digestion,which may be related to the existence of sufficient microbial substrates in sludge during the early stage of anaerobic digestion.While the demethylation of mercury occured at 5 d of anaerobic digestion,which may be related to the change of p H in sludge and the metabolic activity of related microorganisms.The addition of sulfate in the early stage of anaerobic digestion had an inhibitory effect on mercury methylation.However,there was no significant effect on mercury methylation in the whole process of sludge anaerobic digestion.（4）Further microbiological analysis showed that microbial methylation and demethylation of mercury occurred in the anaerobic digestion process,while sulfate could hardly affect the microbial methylation.Specifically,the content of methylmercury in sludge was correlated with some microbial groups（Proteobacteria,Bacteroides and Euryarchaeota）,and the functional genes such as hgc A,mer A and dsr A.RDA analysis showed that there was a significantly negative correlation between mer A and methylmercury content during anaerobic digestion.Based on pearson network structure analysis,hgc A seemed to exist in Pseudomonas,paenalcaligens,unclassified＿f＿＿XI＿o＿＿Clostridiales,Tissierella,unclassified＿f＿＿Ruminococcaceae and Gelria,while unclassified＿f＿XI＿o＿Clostridiales,Paenalcaligenes and Acetomicrobium seemed to be the host of mer A.The addition of sulfate had no significant impact on the evolution of microbial community and functional genes.