Structure properties of flocs(size,fractal dimension(Df),etc.)have a high impact on coag-ulation efficiency.In this work,the influences of three different additives(ferric salt(Fe),phosphate(P),and citric acid(CA))on coagulation process/efficiency were investigated.Re-sults showed that a small amount of extra Fe can facilitate the growth of Al flocs by providing more'active sites'.Although zeta potential and Df showed a limited change,the average floc size increased apparently and the increment was more obvious when Fe was added after the formation of the flocs.In contrast,P addition during the rapid mixing period will decrease the final average floc size,while the influence is less significant when P was added after the growth of the flocs.In terms of CA,a more striking negative effect on the growth ability of the flocs was observed compared to P.The strong complexing/coordination interactions between CA and aluminum hydroxide is the main reason behind the influence.CA also sig-nificantly decreased the Df value of the flocs compared to P,and Df showed a comparatively higher decrease when P or CA was added during the rapid mixing stage compared to the addition after the flocs formation.These results indicated that the addition of CA or P dur-ing the rapid mixing stage'inactivated'or occupied more'active sites'on the preliminarily formed Al NPs during the hydrolysis process,and therefore presented stronger impact on the morphology/size of the formed flocs.