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 (CODCr), 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 (FeCl3·6H2O), ferric sulfate 7-hydrate (Fe2(SO4)3·7H2O). The quadratic models developed for responses indicated the optimum conditions were PFS dose of 8g/L at pH 6.0, FeCl3·6H2O dose of 10g/L at pH 8.0 and Fe2(SO4)3·7H2O dose of 12g/L at pH 7.5. Compromising to simultaneously optimize, the optimum conditions resulted in the CODCr, 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 FeCl3·6H2O and 55.87%, 74.65%, 94.13%, 53.64% for Fe2(SO4)3·7H2O, 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 CODCr indicated that CF process as a pretreatment could remove recalcitrant compounds in leachate effectively.