Advanced treatment of the output-stream of biological leachate treatment by appropriate combination of coagulation/flocculation and oxidation

Abstract

An integrated technique consisting of oxidation and coagulation–flocculation processes was investigated in order to provide an effective method for the treatment of biologically pre-treated leachate and to reduce pollutants to values under the limit required for discharging to the receiving waters. Leachate containing 985 mg L⁻¹ chemical oxygen demand (COD) was treated by using oxidation with NaClO, coagulation/flocculation with aluminium sulfate (alum) or poly-ferric sulfate (PFS) as well as combined pre-oxidation followed by coagulation/flocculation. Oxidation resulted in the removal of COD and colour up to 78.9 and 88.4%, respectively, however residual COD values were over 200 mg L⁻¹ even under intensive experimental conditions (high dosage of NaClO and prolonged oxidation time). Coagulation with PFS was found to be more efficient in comparison with alum, and it yielded a residual COD of 184 mg L⁻¹ the optimum coagulation conditions (polyacrylamide (PAM) 5 mg L⁻¹, PFS 30 mg L⁻¹, pH 6, reaction time 20 min). The combined process involving oxidation followed by coagulation/flocculation with PFS resulted in residual COD values lower than 100 mg L⁻¹ the following conditions: 40–60 min pre-oxidation with 30 mg L⁻¹ NaClO followed by coagulation with 200–250 mg L⁻¹. Hence, pre-oxidation with NaClO followed by coagulation with PFS was recommended for removal of biologically-treated substances from the biologically-treated or stabilized leachate.