Flexible biogas production from co-digestion of food waste and waste activated sludge via two-phase anaerobic digestion enhanced by eggshell-mediated acidogenesis

In this study, eggshells were supplemented into the acidogenic phase to improve hydrolysis efficiency and reinforce acidogenesis, thereby promoting acetate generation and acting as a trigger for subsequent flexible biogas production in the two-phase anaerobic digestion of food waste and sewage sludge. Experimental results showed that the addition of eggshells significantly increased the acetate fraction in the acidogenic phase, and under intensified HRT shocks, the biogas production rate was increased by up to 108 %. Further analysis using the Gompertz model fitting and flexibility indicators (e.g., biogas production ramp-up rate, m bpr_up , and biogas production ramp-down rate, m bpr_down ) demonstrated that eggshell addition markedly enhanced operational flexibility, with superior performance under more severe HRT shocks. Meanwhile, VFA accumulation in the methanogenic phase was alleviated, and system alkalinity increased by 16.7 %, indicating improved resilience against perturbations. Microbial community analysis revealed higher diversity and a significant enrichment of Methanosarcina with eggshell addition, while metagenomic results suggested that both hydrogenotrophic and methylotrophic methanogenesis pathways were strengthened, accompanied by tighter syntrophic interactions between Methanosarcina and hydrolytic–acidogenic bacteria, thereby jointly facilitating a rapid methanogenic response. Overall, eggshells, as a cost-effective and waste-derived additive, offer a practical solution to enhance the flexibility of biogas production. • Eggshell-enhanced acidogenesis improves flexible biogas production. • Acetic-acid-dominated VFA stream strengthens methanogenic stability. • Methane yield and peak production rate increase under HRT shocks. • Methanosarcina enrichment promotes robust syntrophic methanogenesis. • Low-cost waste-derived additive enables economic flexible biogas systems.