Horizontal subsurface flow constructed wetlands (CWs) planted with Phragmites australis were set up to analyze the effect of external ferrous iron (Fe2+) addition on chemical oxygen demand (COD) and nitrogen removal. The results showed that external Fe2+ addition has no significant effect on COD removal, while the COD removal efficiencies in CWs with Fe2+ addition were slightly lower than those in CWs without Fe2+ addition, since Fe2+ as an electron donor for denitrification may decrease the consumption of organic carbon. However, external Fe2+ addition significantly enhanced the nitrogen removal capability of the CWs. With an increase in external Fe2+ concentration, the removal efficiencies for total nitrogen (TN), nitrate nitrogen (NO3N), and ammonium nitrogen (NH4N) all increased. The removal efficiencies for TN and NH4N were greatest for an influent Fe2+ concentration of 50 mg L-1, while the greatest removal efficiencies for NO3N were observed at an influent Fe2+ concentration of 150 mg L-1. With increasing hydraulic retention time (HRT), the COD and NO3N removal efficiencies in the CWs with external Fe2+ addition increase sharply and then became stable, while the removal efficiency for TN exhibited a continuous increase. The removal efficiency for NH4N was greatest at an HRT of 5 d-7 d with Fe2+ addition. The change in pH with increasing HRT indicated that external Fe2+ addition did not significantly affect the pH value of the effluent water, but that the wetland systems caused an increase in effluent pH. Fe2+ addition remarkably reduced the oxygen-reduction potential of both the influent and effluent water, which was beneficial to denitrification of microorganisms.
Keywords: COD removal; Constructed wetlands; Ferrous iron; Hydraulic retention time; Nitrogen removal.
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