Migration and risk assessment of heavy metals in sewage sludge during hydrothermal treatment combined with pyrolysis

- HTP was used for SS disposal and HM immobilization.
- Enhanced dehydration and a reduction in the SS volume can be achieved after HTP.
- HMs migrated from the bioavailable fractions to the more stable fractions after HTP.
- The directly toxic HMs and their leaching potential was lower after HTP.
- Based on the RAC, the ecological risk of HMs in the biochar decreased to the lowest pollution level.

In this study, the immobilization and risk assessment of heavy metals (HMs) in sewage sludge (SS) during hydrothermal treatment combined with pyrolysis (HTP) was investigated. The experimental results showed that the immobilization of HMs in SS was significantly affected and that the eco-toxicity/risk of SS to the environment was reduced through HTP, in addition to the enhancement of dehydration and a reduction in the volume of the sewage sludge. After the HTP process, although the majority of HMs were accumulated in the biochar, the HMs could be transformed from bioavailable fractions to more stable fractions, such as the acid soluble/exchangeable fractions (F1) of Zn, Ni and Cd in the biochar, which decreased to less than 6.31%, 2.36% and 3.64%, respectively. Moreover, the leaching potential of HMs decreased after the HTP process.