Influence of thermal hydrolysis treatment on chemical speciation and bioleaching behavior of heavy metals in the sewage sludge

Abstract

Abstract In this study, the transformation of chemical speciation of Cr, Mn, As and Cd in the sewage sludge before and after thermal hydrolysis treatment was investigated using modified BCR method. The effect of thermal hydrolysis treatment and chemical speciation change on the subsequent bioleaching behavior was also researched. The results showed that the concentrations of Cr, Mn, As and Cd in oxidizable fraction decreased in the sludge treated by thermal hydrolysis. Meanwhile, the proportions of Cr, Mn and As in the mobile fractions (acid-soluble/exchangeable and reducible fraction) all decreased, while Cd was concentrated in the sludge treated by thermal hydrolysis. The final pH value of bioleached sludge treated by thermal hydrolysis was lower than that in the bioleached raw sewage sludge. And faster increase of oxidation-reduction potential (ORP) was also found in the bioleaching process of the sludge treated by thermal hydrolysis. The removal percentage of Mn and Cd increased in the bioleached sludge treated by thermal hydrolysis. Thermal hydrolysis treatment can promote the bioleaching to some extent. Furthermore, the environmental risk of Cr, Mn, As and Cd in the bioleached sludge treated by thermal hydrolysis was all alleviated according to risk assessment analysis compared with the bioleached raw sewage sludge.