Strength and leaching behavior of heavy metal contaminated sludge solidified/stabilized by compound binders

Heavy metals contained in sewage sludge may cause potential environmental pollution. In this study, compound binders were used to stabilize and solidify the sludge, which aims to reduce hazard of heavy metals. The strength and leaching behavior of heavy metals from treated sludge was investigated by performing a series of laboratory experiments including the unconfined compressive strength (UCS), the toxicity characteristic leaching procedure (TCLP), the sequential chemical extraction (SCE), and the semi-dynamic leaching test (Semi-DLT). The experimental results showed that the UCS of sludge was significantly improved after stabilization and solidification (S/ S) treatment, therefore it can be used as low graded material for landfill. According to the TCLP tests, the selected heavy metals (i.e. Cr, Cu, Zn, Pb, Ni) became more stable under acid conditions in short term. From the SCE tests, some heavy metals were effectively converted into stable form in S/S process. The long term leaching behavior of the heavy metals was also evaluated by the diffusion coefficients (De) and leaching index (L) calculated by the data obtained from the Semi-DLT tests. Low De values showed the S/S treatment is effective for sewage sludge, while the calculated L values also meet the environmental requirement of heavy metal stability.