This paper deals with results from laboratory scale experiments with model dye effluents comprising of the commercially used textile diazo dye, CI "Reactive Black 5", coagulated with ZETAG type primary coagulants. Size and structure analysis of flocs in coagulated dye sludge was undertaken in order to evaluate their separation abilities. The particle size distribution was estimated by use of a Galai CIS-100 particle counting system working on a time-of-transition principle, while their fractal dimension was obtained from laser scattering instrument in LALS mode. An image analysis of the flocculated dye-sludge has also been carried out. In parallel to the flocs characterization, the measurement of surface charge density of coagulated dye sludge was performed with the aim of linking surface charge data with the floc characteristics, and on this basis, to outline the predominant mechanism of color removal. It was found that flocs produced at optimal dosage are characterized by large sizes and a high value of fractal dimension, which is manifested in a very good level of color removal by sedimentation. The evident correlation between the surface charge density progression of coagulated dye flocs and color removal, suggests adsorption and charge neutralization as the predominant mechanism of dye destabilization.