This work studied the simultaneous removal of turbidity and humic acid from high turbidity synthetic raw water using polyaluminum chloride (PACl) as the coagulant. The response surface method with the Box–Behnken design of experiments was adopted to investigate the effects of pH, turbidity and alkalinity of suspension, the doses of PACl and the dosed amount of humic acid. The non-dimensional correlations for the residual turbidity and humic acid in the supernatant were obtained by regression analysis. Graphical presentation facilitates data interpretation from designed experiments. The most significant variables to the coagulation and sedimentation process were identified. Simultaneous removal of both turbidity and humic acid from the high turbidity water is reachable at neutral pH, and the PACl dose is selected if the raw water turbidity has not become too high. Extremely high turbidity stormwater may be treated using single-stage PACl coagulation to satisfactorily remove both the turbidity and humic acid from the water. The proposed strategy was applied to successfully treat the extremely high turbidity stormwater, brought by storm Nari on September 16–19, 2001.