Abstract:

Objective To explore the effect of zero-valent iron on anaerobic ammonium oxidation system and reveal its cumulative effect mechanism. Method We established three groups of zero-valent iron dosage—L0 (0 g/L), L1 (2 g/L), and L2 (4 g/L), and then determined the nitrogen and phosphorus removal performance, sludge characteristics, microbial community structure and nitrogen metabolism-related functional genes of the anaerobic ammonia oxidation system at different zero-valent iron dosages. Result An appropriate amount of zero-valent iron (L1) can promote the denitrification process through corrosion in the short term (3–22 days), provide more nitrite substrates for anaerobic ammonia oxidation, and generate Fe-P precipitation to achieve synchronous phosphorus removal. This increases the average removal rates of nitrite nitrogen, total nitrogen, and total phosphorus in L1 by 17.64%, 10.52%, and 84.33%, respectively, compared to L0. However, excessive accumulation of zero-valent iron (L2) leads to significant negative effects: the corrosion products of excess iron accumulate on the surface of sludge and hinder mass transfer, and the competitive reduction of nitrite by Fe²⁺ leads to an imbalance of anaerobic ammonia oxidation substrates, resulting in a decrease of 31.16%, 15.56%, and 13.94% in the average removal rates of ammonia nitrogen, nitrite nitrogen, and total nitrogen in L2 on days 23 to 32 compared to L0, respectively. Meanwhile, the accumulation of zero-valent iron leads to a decrease in the relative abundance of Candidatus Kuenenia, Candidatus Brocadia, and SM1A02, while the relative abundance of Denitratisoma and Limnobacter increases. The upregulation of nitrogen metabolism-related functional genes norB and hao in both L1 and L2 is caused by changes in the physical and chemical environment of the anaerobic ammonia oxidation system triggered by zero-valent iron. Conclusion Moderate zero-valent iron can effectively improve the nitrogen and phosphorus removal performance of anaerobic ammonia oxidation systems, but there is a cumulative inhibitory effect. Its mechanism of action is a chain-like process containing chemical reduction, physical coverage, and changes in microbial ecology and metabolic functions.

Key words: zero-valent iron, anammox, nitrogen and phosphorus removal, microbial community structure, functional gene, mechanism of action