Geotechnical characterization of dewatered sewage sludge for landfill disposal

Abstract

The disposal of increased quantities of dewatered sewage sludge to landfills may cause geoenvironmental problems such as leachability, total and differential settlement, and slope stability of the waste during disposal. This study investigates the geotechnical properties of the dewatered sewage sludge generated from chemically enhanced primary treatment (CEPT) in Hong Kong. Compaction test results show that CEPT sludge exhibits compaction characteristics similar to those of clayey soils; however, the lowest hydraulic conductivity of CEPT sludge does not occur near the wet side of its optimum water content. The consolidation behavior of CEPT sludge does not follow the conventional Terzaghi's theory. Under an overburden pressure higher than 24 kPa, the relationship between the logarithm of the hydraulic conductivity (logk) and the void ratio (e) is linear. Results from direct shear tests reveal that, for CEPT sludge, slopes of 20° can probably be constructed during the landfill operation without causing sliding problems. Compared with the disposal of pure CEPT sludge, the co-disposal of dewatered sewage sludge with other solid wastes commonly found in Hong Kong results in lower compressibility, smaller settlement, but higher hydraulic conductivity.Key words: co-disposal, geotechnical properties, landfill disposal, sewage sludge.