Effects of nano valent iron size on two-phase anaerobic digestion of food and kitchen waste.

The addition of zero-valent iron (ZVI) to the anaerobic digestion of food and kitchen waste (FKW) can significantly improve methane production efficiency. However, the impact of nano ZVI (nZVI) addition during both acidification and methanogenic phases of the two-phase anaerobic digestion of FKW remains unclear. This study investigated the effect of different nZVI particle sizes (50, 100, and 300 nm) introduced during the acidification phase on the overall performance of two-phase anaerobic digestion. The results revealed that nZVI improved the performance of the acidification phase. Particularly, 50 nm nZVI increased protein concentrations, likely owing to its toxicity, which caused microbial cell damage. The addition of 300 nm nZVI led to higher concentrations of soluble chemical oxygen demand (SCOD) and total volatile fatty acids (TVFAs), reaching 40,302.45 and 10,375.00 mg/L, respectively. In the methanogenic phase, 300 nm nZVI achieved the highest methane production, reaching 799.78 mL/g VS, which was enhanced by the optimal concentrations of TVFAs and Fe²⁺. Moreover, the addition of 300 nm nZVI enriched Bifidobacterium (32.74%) and Clostridium sensu stricto 1 (37.57%), both of which promoted TVFA generation, increased Methanobacterium abundance, and facilitated rapid methane production. Furthermore, 300 nm nZVI enhanced key metabolic pathways, such as transport, catabolism, and amino acid metabolism, thereby increasing methane production in the anaerobic digestion system.