This study focused on the reduction of the treatment cost of mature landfill leachate (LL) by enhancing the coagulation pre-treatment before a UVA-LED photo-Fenton process. A more efficient advanced coagulation pretreatment was designed by combining conventional coagulation (CC) and electro-coagulation (EC). Regardless of the order in which the two coagulations were applied, the combination achieved more than 73% color removal, 80% COD removal, and 27% SUVA removal. However, the coagulation order had a great influence on both final pH and total dissolved iron, which were key parameters for the UVA-LED photo-Fenton post-treatment. CC (pH = 5; 2 g L−1 of FeCl36H2O) followed by EC (pH = 5; 10 mA cm−2) resulted in a pH of 6.4 and 100 mg L−1 of dissolved iron, whereas EC (pH = 4; 10 mA cm−2) followed by CC (pH = 6; 1 g L−1 FeCl36H2O) led to a final pH of 3.4 and 210 mg L−1 dissolved iron. This last combination was therefore considered better for the posterior photo-Fenton treatment. Results at the best cost-efficient [H2O2]:COD ratio of 1.063 showed a high treatment efficiency, namely the removal of 99% of the color, 89% of the COD, and 60% of the SUVA. Conductivity was reduced by 17%, and biodegradability increased to BOD5:COD = 0.40. With this proposed treatment, a final COD of only 453 mg O2 L−1 was obtained at a treatment cost of EUR 3.42 kg COD−1.
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