Enhanced ozone catalytic treatment of mature landfill leachate with low C/N ratio using transition metal-modified biochar

The treatment of mature landfill leachate poses notable challenges due to the inhibitory effect of high NH₄⁺ -N concentrations on microbial nitrification. In this study, an innovative internal circulation system was developed by coupling enhanced catalytic ozonation using transition metals (Fe, Co, and Ni) loaded biochar derived from residual sludge with a membrane bioreactor (MBR) for effective leachate treatment. The incorporation of Ni remarkably altered the biochar structure, yielding an ID/IG value of 1.24 and creating more defect sites and disordered edges, which facilitated the generation of hydroxyl radicals (·OH), superoxide radicals (·O₂^-), and singlet oxygen (¹O₂). Parallel factor analysis (PARAFAC) indicated that the humification index (HIX) decreased from 0.79 to 0.39 during the treatment process, proving that the humus substances in the leachate were effectively degraded. The Shannon index of microorganisms decreased from 3.97 to 3.42, and the Shannoneven decreased from 0.78 to 0.67, which indicated that the microorganism diversity in the MBR unit increased and the uniformity decreased after the internal circulation reaction system. The Bacteroidetes and Proteobacteria played a key role in the removal of NH₄⁺-N and organic matter. After 25 days of operation, MBR-ozone-catalyzed internal circulation system achieved 99.6 % NH₄⁺-N removal with concentration decreasing from 1952 mg/L to 8.8 mg/L, and 96.9 % COD removal by decreasing from 698 mg/L to 21.5 mg/L, which demonstrated the synergistic effect of the biochemical system and the catalytic system. This study provided a substantial step forward for solving the difficult problem for the treatment of mature landfill leachate containing high NH₄⁺-N.