Influence of second dose on coagulation efficiency, floc re-growth, fractal structure and residual Al of the effluent in humic acid (HA) coagulation with Al-13 polymer ([Al13O4(OH)(24)(H2O)(12)](7+)) and PACl were comparatively investigated in this study. Effects of breakage shear on the floc properties generated in the coagulation with and without additional dose were also investigated. The results indicated that additional dose during breakage could essentially improve the HA removal efficiency and floc re-growth in both Al-13 and PACl coagulations. Second doses of Al-13 at 0.5 and 1.0 mg/L resulted in better turbidity and UV254 removal as well as floc re-growth rather than higher additional dose of 1.5 and 2.0 mg/L; while in PACl coagulation, more efficient HA removal and better floc re-growth were obtained at higher additional doses (1.0, 1.5 and 2.0 mg/L). Small additional Al-13 could apparently increase the D-f of re-formed flocs while the additional PACl displayed inconspicuous effect on floc D-f The additional coagulant dose could alleviate the further decrease of re-grown floc size with increased breakage shear for both coagulants. The residual Al analysis implied that two-stage addition contributed to lower residual Al in effluent than one-time addition mode with the same total coagulant concentration. (C) 2012 Elsevier B.V. All rights reserved.