Cement-Based Solidification for the Safe Disposal of Heavy Metal Contaminated Sewage Sludge

Abstract

Cement-based solidification processes for the safe disposal of metal contaminated sewage sludges in landfill have been evaluated as a possible alternative to conventional methods, such as disposal to agricultural land or sea dumping. The treatments evaluated were based on ordinary Portland cement (OPC) alone and OPC containing soluble sodium silicate as an additive. The abilities of the systems to stabilize the metal contaminants present in the sludge were determined by extensive leaching tests, using water or buffered acetic acid over a 9 day cycle, designed to simulate the most severe conditions likely to be met in a landfill. For both the OPC/sludge mix and the OPC/silicate/sludge mix, these tests indicated that the release of metals from the cement matrix was low and the patterns of release could be divided into two distinct groups. The first group, comprising zinc, lead and cadmium, gave pHdependent releases under equilibrium (crushed block) test conditions, suggesting that the metals are bound up in the cement matrix as the insoluble hydroxides at high pH. The second group, consisting of copper and nickel, were released throughout the test, suggesting that the metals were complexed by ligands present in the sludge and were, therefore, soluble under alkaline, as well as acid, leaching conditions.