Addition of Fe2+ increase nitrate removal in vertical subsurface flow constructed wetlands

Fe2+ can assist in the process of denitrification as electron donor to enhance nitrogen removal in constructed wetlands (CWs). In this study, Fe2+ was added to the influent of CWs aiming to investigate the effects of external Fe2+ addition on denitrification efficiency at different carbon/nitrogen (C/N). Community-level physiological profile (CLPP) was assessed according to substrate utilization patterns with Biolog Eco Plates™. Results showed that external Fe2+ remarkably improved the capability of denitrification in constructed wetlands. Nitrate removal rate was largely increased after the addition of 30 mg/L Fe2+ to the CWs reactor for a 2-day hydraulic retention time (HRT) at the presence of organic carbon source with low C/N ratio of 2. In reactor without adding organic carbon source, Fe(II)-Fe(III) cycle reaction could not be completed, thus forming hydroxide precipitation leading to low effluent Fe2+ and total iron. However, the enhancement of Fe2+ on denitrification was not obvious at high C/N of 6 and 4 in that organic carbon acted as dominant electron donor in participating denitrification processes. Substrate microbial community structure and diversity in CWs were influenced by organic carbon source and Fe2+. The Shannon, Pielou and Simpson indices in the CWs with external Fe2+ were higher than the control. Principal component analysis (PCA) showed that the CLPPs in constructed wetlands of different influents were significantly different. The constructed wetlands with added organic carbon sources and Fe2+ were grouped together, while the wetlands with the low C/N and the wetlands with high C/N were detached.