Quantitative evaluation of heavy metals' pollution hazards in liquefaction residues of sewage sludge

► The pollution hazards of heavy metals in liquefaction residues of sewage sludge were quantitatively evaluated. ► The leachability of heavy metals was well suppressed by liquefaction process. ► According to geo-accumulation index (Igeo) results, the pollution degrees of heavy metals were significantly relieved. ► Based on risk assessment code (RAC), each tested heavy metal had no or low risk to the environments after liquefaction. ► The overall risk level of heavy metals was declined from “very high risk” to “low risk” after liquefaction.