Chemical conditioning pretreatment does not always improve sludge dewaterability sufficiently for flocculated sludges with high compressibilities. In this study, tannery sludge incineration slag (TSIS) was used as a skeleton builder, in combination with cationic polyacrylamide (CPAM), to condition tannery sludge. The results showed that pretreating the sludge with a combination of TSIS and CPAM considerably improved sludge dewaterability over CPAM conditioning alone. The optimum TSIS and CPAM dosages were 150% dry solid (DS) and 10 kg/t DS, respectively, which generated a maximal net sludge solids yield (10.3 kg/m(2) h) and a minimal time to filter (7 s). The negative charge of the sludge particles was neutralized by the positive charge of the dissolved metal ions of the TSIS, resulting in larger floc sizes. Compressibility and porosity measurements and scanning electron microscope (SEM) images indicated that the TSIS formed a porous and incompressible structure during mechanical dewatering. (C) 2013 Elsevier B.V. All rights reserved.