The dye adsorption characteristics of sludge particulates, both primary and secondary, were studied. Major factors such as pH, SS (suspended solid), and ionic strength on dye adsorption were evaluated. An anionic dye, new coccine acid red #18 was used as the adsorbate. Results show that dye adsorption is a fast process and can reach equilibrium in 30 min. Results also show that pH is the most important factor determining the dye adsorption. Decreasing pH can significantly increase the adsorption of the dye. Greater adsorption density can be achieved at SS less than 1 g/L. However, at SS concentration greater than 1 g/L, the dye adsorption density remains relatively unchanged. It is found that the increase in the ionic strength decreases the dye adsorption density. Dye adsorption can be described by a multilayer adsorption isotherm. The multilayer adsorption model allows the calculation of both the monolayer adsorption density (i.e., the maximum first layer adsorption density) and the equilibrium constants for the first layer and subsequent adsorption. Based on the monolayer adsorption capacity, it is possible to determine the specific surface area of various sludge particles.