Chemical speciation of heavy metals in excess sludge treatment by thermal hydrolysis and anaerobic digestion process

Abstract

Adequate treatment of excess sludge from municipal wastewater treatment plants (WWTP) is a critical issue nowadays in WWTP management. Thermal hydrolysis and anaerobic digestion are the methods often used to reduce the organic content of the excess sludge. However, these two methods have very limited effects on heavy metals’ removal. As the toxicity of heavy metals is closely related to not only their quantity but also in their chemical speciations, more in-depth studies on heavy metals speciation distribution are needed. In this paper, the speciation of eight kinds of heavy metals before and after thermal hydrolysis and anaerobic digestion were investigated using a modified BCR (Bureau Communautaire de Référence: a four-step sequential extraction process proposed by the European Community Bureau of Reference in 1992) sequential extraction process. The results showed that the stable content of eight kinds of heavy metals increased by various extents after thermal hydrolysis. The chemical speciation of heavy metals during thermal hydrolysis and anaerobic digestion exhibited significant changes from non-stable to relatively stable state. The existence of H2S can reduce the activity of anammox bacteria, and thus can lead to the deterioration of anaerobic biological treatment system. However, the toxicity of H2S and heavy metal ions can be significantly inhibited simultaneously by the interaction between heavy metals and H2S without secondary pollution, which is beneficial for excess sludge disposal.