Simultaneous recovery of biogas/biochar and degradation of 3,5-Dimethylphenol in anaerobic landfill leachate treatment

• Hardwood biochar (10 wt.%) mitigated 3,5-DMP toxicity in anaerobic baffled reactors. • 3,5-DMP removal reached 74.8% via synergistic chemisorption and biodegradation. • Biochar increased enzyme activity by 47.7% and stimulated key phenolic degraders. • Pyrolysis of excess sludge into biochar closed the loop for circular waste management. • LCA results advocate for biochar soil application to support the food-energy-water nexus. Biochar was employed as an additive in an anaerobic bioreactor to remove 3,5-Dimethylphenol (3,5-DMP) from landfill leachate (LL) and promote the growth of phenol-degrading bacteria, followed by sludge valorization into valuable-added products. An optimum biochar dosage of 100 mg/g volatile solids was applied to operate a pilot-scale anaerobic baffled reactor at 12 h-hydraulic retention time and a 3,5-DMP loading rate of 198.3±10.8 g/(m 3 ⋅d). The influent chemical oxygen demand mass was converted into CH 4 (24 wt.%), biomass (16 wt.%), and soluble metabolites (42 wt.%), demonstrating a 3,5-DMP removal efficiency of 74.8±3.2%. The secretion of extracellular polymeric substances and enzymatic activities (i.e., dehydrogenase, alcohol dehydrogenase, and aldehyde dehydrogenase) were improved by 27.3% and 47.7%, respectively, after biochar supplementation. This finding was accompanied by increasing the relative abundances of Actinobacteriota, Bacteroidota, Caldatribacteriota, Campylobacterota, Firmicutes , and Hydrogenedentes by 133.4%, 35.0%, 151.8%, 369.1%, 70.8%, and 123.0%, respectively, facilitating the phenolic ring cleavage process and utilizing 3,5-DMP as a carbon source via different metabolic pathways. Through controlled pyrolysis (at 550°C for a 2.0 h residence time), the resulting sludge was transformed into a value-added biochar. The abundance of surface oxygen-containing functional groups on this biochar played a critical role in polishing the effluent by adsorbing the remaining 3,5-DMP. Following the validation of the study's outputs via life cycle assessment and techno-economic studies, it is imperative to conduct long-term monitoring of the metabolic byproducts of 3,5-DMP to fully characterize their environmental impact and persistence.