Modeling and optimizing of coagulation–flocculation process by response surface methodology for rehabilitation of tannery wastewater treatment plant

This work aims to optimize coagulation–flocculation operational parameters using chitosan as a biocoagulant to reach a satisfying reduction rate of chemical oxygen demand (COD), chloride concentration and suspended materials in an alkaline pH. This process is suggested as pretreatment before the biological step and a comparison study with aluminium sulphate is conducted. In the first part, nine factors were screened by Plackett–Burman experimental design to identify the most significant, that is, for coagulant: weight percentage of chitosan mixed to aluminium sulphate, dosage, mixing rate and stirring time, for flocculent: dosage, stirring time, mixing rate also initial pH and settling time. In the second part, modelling and optimization of the chosen responses were established by Doehlert design. The target responses, that is, pH: 8.6, COD: 592 mg O2/L, suspended matter (SM): 12.8 mg/L and [Cl]: 2,889 mg/L were obtained with the following optimal conditions: dosage of chitosan = 612.8 mg/L, a stirring speed of 6 rpm at an initial pH of 8.8, and 5.6 mg/L of flocculant with a stirring speed of 43 rpm followed by a settling time of 22 min. The most significant abatements obtained for COD, SM and [Cl] are 68.90%, 95.24% and 39.72%, respectively. Chitosan was found to be an efficient eco-friendly coagulant, forming less sludge than aluminium sulphate and allowing the industry to reduce treatment costs.