Biochar supplementation into anaerobic baffled reactor to mitigate the inhibitory effect of heavy metals on real hazardous landfill leachate detoxification

While anaerobic digestion has been widely used for treating hazardous landfill leachate (HLL), this biological system should be upgraded to maintain stable methanogenic performance under highly fluctuating influent compositions. This study established a biochar-amended anaerobic digester to enhance the sludge's ability to detoxify HLL laden with heavy metals and polychlorinated biphenyls (PCBs), followed by bio-CH₄ recovery. A multistage anaerobic baffled reactor (MABR) was inoculated with biochar and operated at a hydraulic retention time of 18 h and an organic loading rate of 5.56 ± 1.64 g/(L⋅d). Phase#1 of HLL treatment was composed of a total heavy metal concentration of 29.7 mg/L, which was substantially increased to 212.6 mg/L in phase#2. This operational condition showed a chemical oxygen demand (COD) removal efficiency of 6.3 ± 5.3 % and a CH₄ yield of 111.8 mL/g COD_removed. These values were improved by more than 100 % after adding a biochar dosage of 100 mg/g volatile solids (VS) to the MABR module, which was also validated by improving PCBs degradation by ≈ 94 % compared to phase#2. This phase was accompanied by enhancing the dominance of Euryarchaeota (66.7 %), Halobacterota (16.7 %), Methanobacteria (8.0 %), and Methanosarcinia (2.0 %), further facilitating extracellular polymeric substance (EPS) secretion, volatile fatty acid (VFA) bio-conversion into CH₄, and aromatic ring cleavage. While the study outputs are beneficial in avoiding environmental pollution arising from landfills, further studies are required to determine the initial investment and operating costs of the biochar-amendment bio-system to provide valuable insights into the financial stability of HLL detoxification.