Anaerobic co-digestion of leachate from biowaste composting and waste glycerine: methane yield, microbial community...

Effective treatment of leachate from biowaste composting (L_CB) is essential for closing the loops of biowaste composting in circular bioresource management. Owing to its high biodegradability but nutrient imbalance, anaerobic digestion of L_CB may suffer from instability. The effect of co-digesting L_CB with 3% waste glycerine (G) was tested in up-flow anaerobic sludge blanket reactors operated at organic loading rates of 2-6 kg COD/(m³·d). Glycerine addition improved the COD/TKN ratio, enhanced process performance, and significantly modified microbial community structure and methanogenic pathways. Compared to L_CB mono-digestion, co-digestion increased biogas yield by 8.85-16.78% and methane content by 3.8-8.4%, while improving organic removal efficiency, as reflected by lower effluent COD (1221.25-2370.16 mg/L) and VFA (600.8-1059.7 mg/L) concentrations. Microbial analysis revealed a marked enrichment of hydrolytic and acidogenic taxa (Firmicutes, Bacteroidota, Actinobacteria, Synergistetes) and syntrophic oxidizers (Syntrophobacter, Syntrophotalea, Syntrophobacterium), which collectively represented ∼80% of acetoclastic-related genera. A key finding of the study was the co-existence of syntrophic oxidizers and hydrogenotrophic methanogens, which was linked to high content of glycerol in G. Glycerol was primarily converted through propionate into acetate, likely increasing proton (H⁺) release and hydrogen partial pressure, thereby stimulating hydrogenotrophic methanogenesis alongside acetoclastic pathways. This dual-pathway methane formation improved the efficiency and stability of co-digestion. Conversely, L_CB mono-digestion was dominated by caproic and heptanoic acid accumulation, whose multi-step degradation constrained methane production. Overall, co-digestion with G enhanced methane production, stabilized reactor performance, and steered microbial communities toward syntrophic and hydrogenotrophic methanogens cooperation, demonstrating a practical strategy for the valorization of L_CB.