Synergistic and Stable Anaerobic Codigestion of Food Waste and Landfill Leachate at Ultrahigh Organic Load in an Internal Circulation Reactor

Abstract Separate anaerobic digestion of food waste often leads to system acidification and instability. Landfill leachate treatment is limited by ammonia inhibition and carbon deficiency. Conventional anaerobic processes struggle to maintain high treatment efficiency and operational stability under ultrahigh organic loading rates. The aim of this study was to construct a synergistic codigestion system for food waste and landfill leachate in an internal-circulation (IC) reactor in order to overcome this stability bottleneck and determine the underlying microbial mechanisms. A laboratory-scale IC reactor was operated continuously with substrates mixed at a volumetric ratio of 1 ∶ 1 . The organic loading rate (OLR) was increased progressively to 20.3 kg chemical oxygen demand ( COD ) / m 3 · day in order to evaluate pollutant removal, methane production, and key parameters, and high-throughput sequencing was employed to analyze the microbial community. The results showed that the system could operate stably at an ultrahigh OLR, achieving a maximum chemical oxygen demand removal efficiency of 88.3% and a volumetric methane production rate of 0.44 L CH 4 / L · day . The leachate provided crucial alkalinity and trace elements, which effectively neutralized acid accumulation and maintained a pH level of 6.9–7.6. Microbial analysis revealed a synergistic metabolic network in which the acetoclastic ( Methanothrix , 38.62%) and hydrogenotrophic ( Methanobacterium , 19.50%) methanogenesis pathways cooperated with the hydrolytic and syntrophic bacteria. In conclusion, integrating an IC reactor with a codigestion strategy successfully achieves a dual breakthrough in treatment performance and operational stability under ultrahigh organic loading. This offers a reliable technical solution for the efficient recovery of energy from complex organic waste.