Response Surface Optimization of Turbidity Removal Using Plant-Based Mixed Coagulant

Optimal conditions for turbidity removal from synthetic wastewater were investigated using a mixed plant-based coagulant. Synthetic wastewater was prepared by suspending kaolin powder in distilled water and adjusting the final turbidity to 500 NTU. The mixed coagulant was prepared by extracting Clerodendrum powders and cactus powders in a 0.5mol/L NaCl solution followed by filtration. The experimental ranges of the key factors were pH 3-8, coagulant dosage 30-120mg/L, and cactus ratio 30-90%. A face-centered response surface design was employed to evaluate factor interactions and to determine the optimal conditions. In the single-factor experiments, the highest turbidity removal efficiency (96%) was obtained at pH 3, and the efficiency progressively decreased with increasing pH. The maximum removal efficiency was 84% at 90mg/L of dosage and 78% at 50% of cactus ratio. In the response surface analysis, all independent variables significantly influenced turbidity removal (p< 0.05), with pH (p<0.001) and dosage (p<0.001) exerting stronger effects than the cactus ratio (p=0.006). Significant two-way interactions were observed for pH × dosage (p=0.005) and pH × cactus ratio (p=0.006). The response surface model (R<sup>2</sup>=0.93) predicted optimal conditions of pH 3.0, dosage 75 mg/L, and cactus ratio 65%, yielding a predicted turbidity removal efficiency exceeding 99.9%. These findings demonstrate that Clerodendrum (charge neutralization) and cactus (bridging effect) act synergistically, providing enhanced turbidity removal performance and overcoming the limitations of single plant-based coagulants.