Pollutant removal and greenhouse gas emissions in horizontal subsurface flow constructed wetlands with iron ore treating ammonium-rich wastewater

Horizontal subsurface flow constructed wetlands (HFCWs) are capable of eliminating organic matter and nitrogen while emitting less methane (CH₄) and nitrous oxide (N₂O) than free water surface flow wetlands. However, the simultaneous removal of pollutants and reduction of greenhouse gases (GHG) emissions from high-strength wastewater containing high levels of organic matter and ammonium nitrogen (NH₄⁺-N) has not get been investigated. The influent COD concentration affected the efficiency of nitrogen removal, GHG emissions and the presence of iron from iron ore, but the COD and TP removal efficiencies remained unaffected. CO₂ and CH₄ fluxes were significantly influenced by influent COD concentrations, whereas less N₂O emissions were obtained during 7d. The highest CO₂ and CH₄ fluxes, along with the GHG emissions, were observed in HFCWs with COD concentrations of 375.6 mg/L and NH₄⁺-N concentrations of 159.0 mg/L at a COD/N ratio of 2.4. Conversely, the lowest CH₄ (-1.72 mg/m²/h) and N₂O fluxes (0.13 mg/m²/h) were recorded in HFCWs with COD concentrations of 375.6 mg/L and NH₄⁺-N concentrations of 162.4 mg/L at a COD/N of 4.5, although nitrogen removal was weak in these HFCWs. HFCWs at a COD/N ratio of 3.6 exhibited greater removal of nitrogen and other pollutants, along with a lower global warming potential (GWP). In conclusion, the concentrations of organic matter and NH₄⁺-N in wastewater affected both pollutant removal and GHG emissions. The simultaneous enhancement of pollutant removal and the reduction of GHG emissions can be achieved in HFCWs with a COD/N ratio of 3.6.