Effect of second coagulant addition on coagulation efficiency, floc properties and residual Al for humic acid treatment by Al13 polymer and polyaluminum chloride (PACl)

► Both charge neutralization and adsorption occurred at second-time coagulation. ► Small second Al13 could increase coagulation efficiency, floc re-growth and Df. ► Second dose alleviated decrease of re-grown floc size with increasing shear. ► Two-stage addition led to lower residual Al than 1-shot addition at the same dose.