Enhanced Nitrogen Removal from Tailwater in Constructed Wetlands with Plant Carbon Source addition and Z-nZVI Substrate

Abstract

The discharge of treated effluent from sewage treatment plants, commonly referred to as tailwater, can lead to particular pollution in the receiving water bodies. Constructed wetlands (CWs) represent a cost-effective and straightforward approach to tailwater treatment. However, they often have a limited capacity for removing residual chemical oxygen demand (COD) and nitrogen due to poor biodegradability of the remaining organic substances, resulting in insufficient carbon sources for denitrification. To address these constraints, a synergistic combination of plant-derived carbon sources and zeolite-loaded nano-zero-valent iron (Z-nZVI) materials were employed to enhance the nitrogen removal efficiency within CWs. Z-nZVI facilitated the transformation of nitrate (NO₃⁻–N) and nitrite (NO₂⁻–N) into ammonium (NH₄⁺–N), which was subsequently eliminated via zeolite adsorption through physical–chemical processes. Then, introducing plant carbon source (PCS) augmented biological denitrification. Under optimized conditions with a C/N ratio of 3.5, the addition of both PCS and Z-nZVI led to removal rates of 85.34 ± 2.44% for NH₄⁺–N, 85.24 ± 1.58% for NO₃⁻–N, and 84.32 ± 0.77% for total nitrogen (TN) in the CW system. These represented average improvements of 17.37%, 60.48%, and 51.50% for NH₄⁺–N, NO₃⁻–N, and TN respectively, compared to the control wetland without intervention. This study introduces a novel strategy to boost nitrogen removal efficacy in treated wastewater within CWs, utilizing PCS and Z-nZVI, thus offering a valuable reference for treating low C/N ratio wastewater in CWs.