A new coagulant aid, Enteromorpha extract (Ee), was used together with traditional coagulants in water treatment process to assess its effect on coagulation behavior and floc characteristics. Four kinds of aluminum and ferric coagulants were studied in this article, with the results indicating that aluminum sulfate (AS) was the optimal coagulant due to its high efficiency and perfect cooperation with Ee for given test water. Then coagulation behavior and floc characteristics of AS coagulation system were studied. Turbidity, UV254 (ultraviolet absorbance at 254 nm wavelength) and DOC (dissolved organic carbon) removal efficiencies were used to evaluate coagulation effects; and floc characteristics were investigated in terms of floc size, strength and recovery ability. The results obtained in this study indicated that the coagulation effect of AS could be significantly improved by around 30% when appropriate proportion of Ee was dosed. When AS was used in conjunction with Ee, the generated flocs had bigger sizes and faster growth rate than those generated by AS alone. Meanwhile, the former were also stronger and had better recovery abilities than the latter. The charge neutralization was the dominant mechanism in AS coagulation, and Ee displayed good adsorption bridging effect. Consequently, coagulation efficiency could be enhanced significantly due to the combination of two advantages. (C) 2012 Elsevier B.V. All rights reserved.