Anaerobic digestion is widely used for sewage sludge stabilization. Different anaerobic digestion methods have different impacts on the speciation and stability of heavy metals in sludge. This study investigated the response of the speciation and stability of heavy metals in sludge to the treatment of mesophilic anaerobic digestion-mechanical dewatering and thermal hydrolysis pre-treatment-thermophilic anaerobic digestion. The results showed that the process of mesophilic anaerobic digestion increased the contents of Cd, Cr, Cu, Ni, and Zn in the sludge, increasing its contamination level and potential ecological risks. In contrast, the thermophilic anaerobic digestion process increased the contents of Cd, Cr, and Pb in the sludge, reducing the level of heavy metal contamination and potential ecological risks. It was found that the main contaminated metals were Cd and Zn in Plant N and Cd in Plant S. Cd has the highest risk factor and is the highest risk contributor to the potential ecological risk of all the types of sludge. The sum of reducible and the exchangeable fractions of Cd, Ni, Pb, and Zn reduced, and the residual fraction of Cd, Cr, Cu, and Ni decreased after mesophilic anaerobic digestion. This indicated that the process of mesophilic anaerobic digestion changed the proportion of potential toxicity and steady state of the heavy metals into direct toxicity. After thermophilic anaerobic digestion, the exchangeable proportion of Cd, Cr, Cu, Pb, and Zn decreased, and the steady state of Cd, Cr, Cu, Ni, Pb, and Zn decreased. After the thermophilic anaerobic digestion process, the proportions of bio-availability and mobility of heavy metals therefore decreased, and the proportions of steady state also reduced. The potential toxicity increased.
Keywords: heavy metal; mesophilic anaerobic digestion; potential ecological risk; speciation and stability; thermophilic anaerobic digestion.