Intensified nitrogen and phosphorus removal and mechanism revelation in constructed wetlands amended with iron-carbon based substrate towards low C/N ratio wastewater

Constructed wetlands (CWs) show a promising capability of nutrient removal from the low C/N ratio characterized wastewater with the help of functional substrates, yet the mechanism remains unclear. In this study, sponge iron mixed with biochar was adopted as substrates to construct bench-scale CWs (CW-FeC) treating simulated urban tail water (C/N = 2). Over the experiment, it achieved a prominent performance with total nitrogen (TN) and total phosphorus (TP) removal efficiency of 89.8 and 92.1 %, respectively. Meanwhile, greenhouse gas emissions such as CO2 and N2O were extensively depressed. Mechanism analysis revealed that the deposition of ferrous ions on biochar would enlarge the micro-electrolysis interfaces and amplify the binding sites between P and Fe compared to CW systems with other substrates (gravel, biochar, and gravel mixed with sponge iron remarked as CW-G, CW-C, and CW-FeG). A significant difference regarding the mechanism of N removal was proposed based on the bacterial community analysis. The released ferrous (due to electrolysis and high abundance of iron-reduced bacteria) as electron donors could be easily captured by iron-based autotrophic denitrification bacteria as evidenced by 1.98 times higher the relative abundance of denitrifying bacteria and upregulated related genes expression. In contrast, the iron-oxidized bacteria have to be acclimatized in the CW-FeG to supply electrons for denitrification via interspecies extracellular electron transfer. The positive correlation between iron-redox bacteria and denitrification bacteria further demonstrated the integrity of the iron cycle in CWs for better nitrogen removal. These findings provide an elaborated insight into the nutrient transformation and removal processes interacting with substrates in CWs.