Landfill leachate pretreatment by coagulation-flocculation process using iron-based coagulants: Optimization by response surface methodology

Abstract Humic acids (HAs) are characterized as recalcitrant compounds in landfill leachate. In the present study, considering the HA removal as well as some traditional indexes such as chemical oxygen demand (COD Cr ), color and turbidity removals from landfill leachate, the performance of coagulation–flocculation (CF) process using iron-based coagulants was optimized by response surface methodology (RSM). The results achieved using polyferric sulfate (PFS) were compared with those using conventional coagulants such as ferric chloride 6-hydrate (FeCl 3 ·6H 2 O), ferric sulfate 7-hydrate (Fe 2 (SO 4 ) 3 ·7H 2 O). The quadratic models developed for responses indicated the optimum conditions were PFS dose of 8 g/L at pH 6.0, FeCl 3 ·6H 2 O dose of 10 g/L at pH 8.0 and Fe 2 (SO 4 ) 3 ·7H 2 O dose of 12 g/L at pH 7.5. Compromising to simultaneously optimize, the optimum conditions resulted in the COD Cr , color, turbidity and HA removal of 56.38%, 63.38%, 89.79%, 70.41% for PFS whereas 68.65%, 93.31%, 98.85%, 80.18% for FeCl 3 ·6H 2 O and 55.87%, 74.65%, 94.13%, 53.64% for Fe 2 (SO 4 ) 3 ·7H 2 O, respectively. The results of confirmatory experiments agreed well with the model predictions, which demonstrated that RSM can achieve good predictions with the least number of required experiments. Meanwhile, the relatively appreciable removals of HA and COD Cr indicated that CF process as a pretreatment could remove recalcitrant compounds in leachate effectively.