The purpose of this study was to determine the key components and adsorption capacity of the DWTP sludge for adsorption of cadmium ion in water. The humic acid was extracted from the sludge according to the method of International Humic Substances Society (IHSS) (Stenven, 1994) and several artificial sludges were synthesized by following the process in DWTP in order to determine the key components due to adsorption of cadmium ion in water.