The effect of coagulant aid sodium alginate (SA) on coagulation behavior and floc characteristics of aluminum sulfate (AS) was investigated for synthetic dye wastewater treatment. Coagulation performance and floc characteristics of AS were investigated for comparison. The results showed that AS plus SA exhibited synergic effect on color removal depending on the dose of AS and SA. Color removal was more enhanced by SA at low AS doses than at higher ones. Floc properties including floc growth rate, size, strength and reversibility were investigated by Photometric Dispersion Analyzer (PDA). AS plus polymer SA exhibited an obvious improvement on floc size and floc growth rate. Besides, the AS-SA dual-coagulants significantly improved the floc recoverability as reflected by higher recovery factors compared to AS. (C) 2012 Elsevier B.V. All rights reserved.