Abstract: The municipal sludge (sludge) and coal samples were collected and prepared in this study. The constituent characteristics and microstructures of sludge were tested by Fourier transform infrared spectroscopy and scanning elec- tron microscope. The isothermal adsorption of the dispersant (sulfonated acetone-formaldehyde dispersant (SAF)) on- to sludge and coal surface were studied,respectively. Furthermore,the adsorption kinetics were also researched. Addi- tionally,the coal sludge slurries (CSSs) were prepared to study the influences of sludge additive amount and disper- sant quantity. The results showed that there were strongly polar hydroxyl functional groups in sludge,associating by in- termolecular hydrogen bonds. A plenty of flocs composed by filamentous microorganism were discovered in sludge. The extent of saturated adsorption of SAF by sludge particles was 7. 98 mg / g,greater than the value of 4. 91 mg / g for the saturated adsorption of SAF by coal particles. The adsorption kinetics of sludge adsorbing SAF closely fitted the first- order kinetics model. However,the pattern of coal adsorbing SAF trended toward the second-order kinetics model. The research also indicated that the determinant factor of sludge adsorbing SAF was the diffusion of SAF in a porous struc- ture built by sludge particles. With the increases of sludge additive amount,the fixed-viscosity concentrations and the slurryabilities of CSSs decreased. The constituents and microstructures of sludge were the main reasons for the competi- tion between sludge and coal to adsorb SAF,and it also deeply influenced the slurryability of CSSs which could be im- proved by adding dispersant amount.