Predictive Optimization of Coagulation-Flocculation for Sustainable Landfill Leachate Treatment Using Response Surface Methodology

Abstract

Landfill leachate is a complex and highly polluted wastewater containing elevated levels of suspended solids, colour-causing compounds, and refractory organic matter, posing significant challenges to conventional treatment systems. Ineffective leachate management can lead to severe environmental contamination of surface and groundwater resources. Coagulation–flocculation remains one of the most widely applied physico-chemical processes for leachate treatment due to its operational simplicity and effectiveness in removing colloidal and colour-forming substances. However, treatment efficiency is strongly dependent on operating parameters such as pH, coagulant dosage, and settling time, which are often optimized using trial-and-error approaches that neglect interaction effects among variables. This study applies Response Surface Methodology (RSM) to optimize the coagulation–flocculation treatment of landfill leachate, using aluminium sulphate (alum) as the coagulant. A Central Composite Design (CCD) was employed to evaluate the combined effects of pH (6–8), alum dosage (8–12 g/L), and settling time (20–40 min) on colour and turbidity removal efficiencies. Quadratic polynomial models were developed and statistically validated using analysis of variance (ANOVA). The developed models exhibited strong predictive performance, with coefficients of determination (R2) exceeding 0.98 for both response variables and non-significant lack-of-fit values. Numerical optimization identified optimal operating conditions at pH 6, alum dosage of 12–25 g/L, and settling time of 30 min, resulting in maximum turbidity and colour removals of 90–96.6% and 85–91.9%, respectively. Experimental validation confirmed close agreement between predicted and observed results. The findings demonstrate that RSM is an effective and reliable tool for optimizing coagulation–flocculation processes and provide practical insights for improving landfill leachate treatment efficiency, particularly for tropical landfill conditions.