Effects Of Biochar On Gas-generation Characteristics And Passivation Of Heavy Metals In Wet Anaerobic Digestion Of Pig Manure

In recent years,the environmental pollution has become increasingly serious,caused by a large amount of pig manure produced by pig breeding.Anaerobic digestion is an effective way to recycle pig manure.One crucial problem which restricting the utilization of pig manure resources lies on its heavy metal pollution.The pollution of heavy metals is not only determined by its concentration in pig manure,but also by its speciation.For the problems of low gas production in large and medium-sized biogas digesters and heavy metal pollution of digestates,the effect of adding biochar on the gas production and methane content during wet anaerobic fermentation of pig manure were studied.And the improved BCR sequential extraction technology was used to analyze the changes of speciation of heavy metals(Cd,Ni,As,Pb,Cr,Mn,Cu,Zn).The heavy metal risk assessment of digestate was also carried out.The main results of this study are as following:(1)The addition of biochar had no significant effect on total gas production and gas production efficiency(P>0.05),while methane content significantly increased by 6.2% ~10.4%(P<0.05).But,the add ratio of biochar in this paper had no significant effct on methane content(P>0.05).The addition of biochar also significantly increased the peak value of daily gas production by 30.0%~73.7%(P<0.05).(2)98.3%~99.6% of heavy metals remain in the digestates after anaerobic digestion,and compared with pig manure,the concentration of heavy metals in the digestates increased.There are very few heavy metals transfer to biogas effluent,specifically Cd,Ni,Pb and Cr are not detected,and the addition of biochar had no significant effect on the concentration of heavy metals in the biogas effluent.(3)The heavy metals in the pig manure before digestion are mainly in acid soluble fraction and reducible fraction.After anaerobic digestion,the speciation of heavy metals changed significantly.The proportion of acid soluble fraction of As increased by 18.3%~50.0%,while the proportion of acid soluble fraction of Cd,Ni,Pb,Cr,Mn,Cu and Zn decreased.The proportion of residual fraction of all metals increased by 8.9%~91.5%.The addition of biochar significantly increased the proportion of residual fraction of As,Ni,Pb,Mn and Zn(P<0.05).(4)The addition of biochar can effectively improve the passivation rate of heavy metals and the maximum passivation of As,Pb,Cd and Mn was found in the group of 5% biochar.(5)Ni,As,Pb,Cr,and Mn in the digestate are less harmful to the environment,and the heavy metals risk in the digestate mainly comes from Cd,Cu and Zn.The potential ecological risk index indicated that the digestates were all classified as moderate ecological risk.