Understanding Zerovalent Iron Exposure on Biological Nitrogen Removal: From Impacts to Potential Mechanisms

Microorganism-dominated nitrogen conversion in wastewater treatment is of great significance to the nitrogen cycle. Until now, Fe⁰ has been widely used in sludge dewaterability, sulfide control, and bioenergy recovery. However, there is limited information about the comprehensive assessment of Fe⁰ on multiple biological nitrogen removal processes. Here, the impact of Fe⁰ dosage on the activity of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, anammox bacteria, and denitrifiers was evaluated. The results revealed that anammox has a more sensitive response to iron dosages (5.34 mM), and improved intracellular iron (216%) is the key to stimulating microbial metabolism by accelerating electron transfer, enzymatic activity, and ATP biosynthesis. Moreover, the long-term operation confirmed that additional Fe⁰ increased the relative abundance of ammonia-oxidizing bacteria, anammox bacteria, and denitrifiers, and the enriched nitrogen removal pathways further improved the nitrogen removal to 93.3% from 79.2% in an oxygen-limited system. This study helps us deeply understand the underlying mechanism of microbial activity stimulated by iron.