Organic matter removal from landfill leachate using a biochar-enhanced microbial electrolytic cell-anaerobic digestion system at different HRT

• The biochar addition for landfill leachate treatment by MEC-AD system was investigated. • HRT change on landfill leachate treatment performance in MEC-AD system • The degradation pathway of complex organic matter was identified by combining UV–vis, GC–MS, and microbial community analysis. The integration of microbial electrolytic cell with anaerobic digestion (MEC-AD) system is a promising method for improving landfill leachate treatment. Biochar addition further enhances overall process efficiency. However, the impact of varying hydraulic retention times (HRT) on operational efficiency and microbial dynamics is not well understood. This study examines the effects of different HRT on the degradation efficiency and microbial composition of biochar-amended MEC-AD system. Here, optimal performance was observed at 48 h HRT, achieving 70.67% of chemical oxygen demand (COD) removal efficiency, with a notable enrichment of functional microbial strains, which represented a 67.58% increase compared with the removal efficiency at 24 h of HRT. Furthermore, the COD removal efficiency was enhanced by approximately 4% with biochar addition under 24 h of HRT. Essentially, maintaining appreciable HRT coupled with biochar addition is a plausible strategy for enhanced landfill leachate treatment. Mechanistically, this enhanced system performance, specifically efficient organic matter degradation involved the disruption of key structural components within contaminants, including heterocyclic aromatic hydrocarbons and conjugated unsaturated bonds as indicated by GC–MS analysis. Microbial community dynamics revealed that the addition of biochar and the extension of HRT both facilitate Acidobacteria proliferation, while the addition of biochar particularly promoted the enrichment of functional microbes like Pseudomonas . Essentially, both HRT regulation and biochar addition were critical in microbial community structuring. Enrichment of acidogens including Azoarcus and Longilinea , facilitates the subsequent production of acetic acid production, optimizing carbon metabolism. Finally, investigating the scalability of biochar-amended MEC-AD systems under optimized operational parameter (HRT) conditions is urgent to ensure sustainable enhanced landfill leachate treatment and resource recovery.