Unraveling mechanism of denitrification process for treating low carbon-to-nitrogen ratio wastewater with different zero-valent iron exogenous enhancement strategies

This study systematically evaluated the effects of intermittent versus continuous addition of zero-valent iron (ZVI) as exogenous enhancers on nitrogen removal performance in denitrification systems. Results shown that under carbon-to‑nitrogen ratios (C/N) was 3, 2, and 1, continuous addition of ZVI significantly improved total nitrogen removal efficiency (TNRE) by 8.57 %, 19.34 %, and 11.45 %, respectively, compared to the control system. Concurrently, extracellular polymeric substance (EPS) production increased by 32.43 %, 62.91 %, and 35.93 %, to that of intermittent ZVI addition system at the same phases. The continuous ZVI-added system enriched more mixotrophic denitrification bacteria and increased soluble iron ion concentrations through EPS-mediated electron shuttling and endogenous protection mechanisms. While, under C/N was 0, the TNRE in continuously ZVI-added system reduced to 8.3 ± 3.24 % due to iron mineral crust formation. Specifically, the TNRE of system with intermittent addition of ZVI stabilized at 17.03 ± 3.24 %, enriched more autotrophic denitrification bacteria and avoided excessive accumulation of iron oxides. Therefore, this experiment provided new insights into the stable operation of the ZVI-enhanced denitrification process for treating low C/N wastewater.