Direct reuse of two deep-dewatered sludge cakes without a solidifying agent as landfill cover: geotechnical properties and heavy metal leaching characteristics

Two types of deep-dewatered sewage sludge cakes were produced from pilot-scale experiments by using two composite conditioners: FeCl3 + quick lime (Fe–Lime) and Fenton's reagent + red mud (Fenton–RM). The feasibility of direct reuse without any solidifying agents of these two deep-dewatered cakes, with water content of about 60 wt%, as landfill cover materials was investigated. Geotechnical properties of these two sludge cakes were found appropriate for reuse as landfill covers. Their plasticity index values increased significantly from 11.6 (raw sludge) to 23.8 (Fe–Lime) and 35.4 (Fenton–RM). The unconfined compressive strength and direct shear strength of the two deep-dewatered sludge cakes could meet or exceed the requirement of landfill cover materials after a certain curing time. Microstructural analyses of scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that their microstructures were more porous than that of raw sludge since the skeleton builders played a role in building the rigorous framework. There was negligible leaching of Cu, Zn, Pb, Cd and Cr from the deep-dewatered sludge cake from the toxicity characteristic leaching procedure and column leaching test. Both deep-dewatered sludge cakes could be reused as effective landfill cover materials with a suitable curing time.