Revealing the nitrogen and phosphorus removal potential: Insights into surface flow and subsurface flow constructed wetlands employing integrated iron and sulfur electron donors

Abstract

This study investigated the differences in nitrogen (N) and phosphorus (P) removal performance, nitrous oxide (N₂O) emissions, and microbial community structure between surface flow (SF) and subsurface flow (SSF) constructed wetlands (CWs) using iron scraps (ISs) and elemental sulfur (S⁰) as integrated electron donors. Four configurations were examined: control SFCW (SF-C), ISs and S⁰ SFCW (SF-Fe+S), control SSFCW (SSF-C), and ISs and S⁰ SSFCW (SSF-Fe+S). The results indicated that both CW types utilizing combined ISs and S⁰ were effective in removing N, with SF-Fe+S (35.73–72.71 %) exhibiting higher nitrate removal efficiency compared to SSF-Fe+S (10.05–61.27 %). However, SSF-Fe+S (77.88–85.72 %) demonstrated a greater efficiency in removing total phosphorus (TP) than SF-Fe+S (50.78–67.91 %). In addition, both SSFCWs, SSF-C (7.22 mg/m²/d) and SSF-Fe+S (3.39 mg/m²/d) exhibited higher N₂O emissions compared to SFCWs, SF-C (3.05 mg/m²/d) and SF-Fe+S (1.94 mg/m²/d). Microbial community analysis revealed distinct differences between CW types; Dechloromonas (22.45 %) and Ferritrophicum (18.23 %) were the dominant genera in SF-Fe+S, whereas Ferritrophicum (37.13 %) and Acinetobacter (21.80 %) predominated in SSF-Fe+S. In addition, TP removal was potentially enhanced by substrate adsorption and coprecipitation through iron (Fe²⁺ and Fe³⁺) ions released from ISs corrosion reacting with phosphate (PO₄³⁻) ions. The study reveals that the type of CW and the combination of electron donors significantly influence their effectiveness in removing N and P, reducing N₂O emissions, and enhancing microbial community composition, thus providing valuable insights.