Feasibility study on enhancing the biodegradability of fresh and old landfill leachate using combined chemical precipitation and Fenton processes

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: Fe 2+ ratio, H₂O₂ dosage, and reaction time—and the BOD 5 /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 BOD 5 /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.