ZVI combined with alkaline pretreatment optimizes FW and RS anaerobic co-digestion: methanogenic potential, acidification mitigation, and microbial community regulation

Food waste and residual sludge co-digestion suffers from poor biodegradability and frequent acid inhibition, due to abundant refractory organics and dense extracellular polymers. Batch anaerobic digestion experiments were performed to explore the combined promotion effect of alkaline pretreatment combined with zero-valent iron (ZVI) on the co-digestion of food waste and residual sludge. Preliminary trials confirmed that 3% NaOH was optimal for substrate disintegration. Under the tested conditions, the numerically highest cumulative methane yield was achieved at the F4 group (5 g/L ZVI), which was 62.72% and 19.61% higher than the untreated KB and alkali-only F0 groups, respectively. Moderate ZVI improved system pH buffering, lowered oxidation–reduction potential (ORP), accelerated the conversion of acetic and propionic acids, and reduced acidification risk. Fe2+ presented a three-stage variation governed by ZVI corrosion, chemical precipitation and microbial assimilation. High-throughput sequencing revealed ZVI enriched hydrolytic Bacteroidetes and mixotrophic Methanosarcina while decreasing the proportion of hydrogenotrophic Methanobacterium. Redundancy analysis (RDA) supported indirectly Fe2+, ORP and pH correlated with microbial succession and methanogenic performance. Excessive ZVI triggered performance decline, which may arise from potential salting-out effects, further experimental validation is required. This study clarifies the coupling mechanism of alkali-ZVI and provides technical guidance for organic solid waste resource recovery.