Stability assessment of heavy metals in sewage sludge pyrolysis biochar based on the chemical-enhanced primary treatment (CEPT) process

Chemical enhanced primary treatment (CEPT) process has attracted attention for reducing energy consumption in sewage water treatment plants. However, most research in this area has focused on the performance evaluation of the CEPT process and not considered the environmental risk of the sewage sludge produced by the CEPT process. Therefore, this study compared the stability of heavy metals in the biochar derived from CEPT sewage sludge (CS) with that of the biochar from conventional biological treatment sewage sludge (BS). Heavy metal retention ratio was lower in CS (48.2–83.0 %) compared to BS (50.0–115.1 %) in all pyrolysis temperatures applied. The results on leaching potential and sequential extraction of heavy metals indicated that CS had lower heavy metal stability compared to BS. In particular, the mobility of heavy metals in CS increased rapidly above 800 °C, increasing the risk index value from 8.0 to 23.7. However, both biochars pyrolyzed at 550 °C showed high stability even after 40 wet-dry aging cycles. Therefore, this research shows that the chemical treatment process produces sewage sludge with a high risk of heavy metals compared to the conventional biological treatment process, but the risk can be lowered or controlled by introducing a suitable treatment process, such as pyrolysis.