Heavy-Metal Leaching from Cement Stabilized Waste Sludge

The purpose of this paper is (1) to review the current Japanese status on the contamination of sediments, (2) to evaluate the leaching characteristics from cement stabilized sludge containing heavy metals, and (3) to discuss some factors affecting the leaching characteristics. Waste sludge is discharged from dredging works in harbors, lakes and rivers. Waste sludge has a high water content and is often contaminated with toxic substances. The reuse of treated sludge in geotechnical applications should be encouraged if the sludge can be sufficiently treated to minimize adverse environmental impact. Column and batch leaching tests are conducted on the cement stabilized sludge containing heavy metals (lead and chromium). The batch test is in accordance with the Japanese regulatory requirement. The column leaching test is performed to account for hydraulic conductivity. The accumulated mass of heavy metal leached from a stabilized sludge with a unit thickness (1 m) per unit area (1 m2) under a one-dimensional vertical seepage flow with a unit hydraulic gradient (i = 1) is calculated from the column test results in order to discuss the environmental impact when the stabilized sludge is reused in geotechnical applications. When the contaminated sludge with high water content (300%) is directly stabilized, significant leaching occurs. For instance, in 0.8 year, a total of 300 mg/m2 of Cr leached from 1 m thick stabilized soil, and 2 g/m2 of Pb leached. When sludge is stabilized after the dewatering to a water content of 80%, the mass leaching is substantially reduced due to the lower hydraulic conductivity of the stabilized soil. For example, cumulative mass leaching is expected to be approximately 1 g/m2 in 200 years for Pb, and less than 50 mg for Cr mass in 300 years. However, if the stabilized sludge is crushed, subjected to drying, or exposed to acid, higher quantities of Pb and Cr are expected to leach. In conclusion, the contaminated sludge can be used as earthen material if it is stabilized properly, that is, with low water content, without exposure to acid and drying, and without being crushed.