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

Liquefaction residues (LR) are the main by-products of sewage sludge (SS) liquefaction. This study quantitatively evaluates the potential ecological risk and pollution degrees of heavy metals (Pb, Zn, Cu, Cd, Cr and Ni) in LR versus SS. The leaching rates (R1) of heavy metals in LR were much lower than those in SS, revealing that the mobility/leachability of heavy metals was well suppressed after liquefaction. Geo-accumulation index (Igeo) indicated that the liquefaction process significantly weakened the contamination degrees of heavy metals. Potential ecological risk index (RI) demonstrated that overall risks caused by heavy metals were obviously lowered from 1093.56 (very high risk) in SS to 4.72 and 1.51 (low risk) in LR1 and LR2, respectively. According to the risk assessment code (RAC), each tested heavy metal had no or low risk to the environments after liquefaction. In a word, the pollution hazards of heavy metals in LR were markedly mitigated.