Feasibility study on enhancing the biodegradability of fresh and old landfill leachate using combined chemical...

Landfill leachate (LL), due to its high concentrations of heavy metals, ammonia nitrogen (NH₃-N), and persistent organic compounds, requires effective pretreatment before biological treatment. This study evaluated a three-stage approach-chemical precipitation with lime, ammonia stripping, and Fenton oxidation-for fresh and old leachate from the Aradkooh Waste Processing Complex in Tehran. Lime dosages of 9 g/L for fresh and 18 g/L for old leachate were determined from preliminary experiments.; these values were selected due to their favorable performance in reducing heavy metals and NH₃-N. Arsenic showed the highest removal efficiency (93% in fresh leachate, 66% in old), while NH₃-N removal reached 93% in both. The Fenton process was optimized using a Box-Behnken design (BBD) and response surface methodology (RSM), with three variables-H₂O₂: Fe²⁺ ratio, H₂O₂ dosage, and reaction time-and the BOD₅/COD ratio as the response. Optimal conditions (A = 2, B = 10, C = 60 min for fresh; A = 2, B = 8.2, C = 75 min for old) increased the BOD₅/COD ratio from 0.29 to 0.67 in fresh leachate and from 0.23 to 0.73 in old leachate. Chemical oxygen demand (COD) removal efficiencies in fresh and old leachate were 88.3% and 81.3%, respectively. These findings demonstrate that combining chemical precipitation and Fenton oxidation effectively enhances leachate quality and biodegradability, supporting improved biological treatment performance.