Mechanism of nano-scale zero-valent iron modified biochar for enhancing low-nitrogen anammox process resistance to low temperatures

Two anaerobic ammonia oxidation (anammox) systems, one with adding nano-scale zero-valent iron modified biochar (nZVI@BC) and the other with adding biochar, were constructed to explore the feasibility of nZVI@BC for enhancing the resistance of low-nitrogen anammox processes to low temperatures. The results showed that the average nitrogen removal efficiency with nZVI@BC addition at low temperatures was maintained at about 80%, while that with biochar addition gradually decreased to 69.49%. The heme-c content of biomass with nZVI@BC was significantly higher by 36.60%-91.45%. Additional, nZVI@BC addition resulted in more extracellular polymeric substances, better biomass granulation, and a higher abundance of anammox bacteria. In particularly, anammox genes hzsA/B/C, hzo and hdh played a pivotal role in maintaining nitrogen removal performance at 15°C. These findings suggest that nZVI@BC has the potential to enhance the resistance of low-nitrogen anammox processes to low temperatures, making it a valuable approach for practical applications in low-nitrogen and low-temperature wastewater treatment.