Effect of biochar derived at different pyrolysis temperatures on heavy metal speciation during sewage sludge anaerobic digestion process

The effects of biochar prepared at different pyrolysis temperatures (300-800 ℃) on the anaerobic digestion efficiency, chemical speciation distribution, and environmental risks of heavy metals in sewage sludge were investigated in this study. The results showed that the addition of biochar could keep the pH of the anaerobic digestion system stable at 7.43-7.77, and alleviate ammonia nitrogen accumulation. The volume of cumulative methane production increased by 14.31-151.32% after the addition of biochar, and biochar prepared at 600 ℃ showed the best efficiency in promoting methane production. Meanwhile, the heavy metal content in the anaerobic digested sludge increased by 15.47-432.71% compared with the control group. However, the chemical speciation of heavy metals transformed from unstable forms to stable forms. The Nemerow pollution indexes (NPI) of heavy metals in the digested sludge increased to different degrees, but the NPIs were all less than 1, indicating a low risk level. The potential ecological risk indexes (RI) of heavy metals in the digested sludge with biochar addition decreased significantly, and the RI value of the heavy metals of the experimental group with the addition of biochar prepared at 600 ℃ was reduced by 43.36%.