Long-term performance of vertical-flow and horizontal-flow constructed wetlands as affected by season, N load, and operating stage for treating nitrogen from domestic sewage

To investigate the long-term nitrogen treatment efficiency in vertical-flow (VF)–horizontal-flow (HF) hybrid constructed wetlands (CWs), the nitrogen removal efficiency under different seasons, N loads, and three operating stages (representing age of the wetland) were evaluated over a 12-year period. The average total nitrogen (TN) removal efficiencies in the effluent during the operation period were in the following order: summer (75.2 %) > spring (73.4 %) ≒ autumn (72.6 %) > winter (66.4 %). The removal efficiencies of TN in summer, autumn, and spring were generally higher than those in winter. At different stages of operation (years), the average TN removal rates in the effluent were in the following order: middle stage (73.4 %; years 2006–2009) > last stage (72.0 %; years 2010–2013) > beginning stage (70.1 %; years 2002–2005). In VF–HF CWs, the amount of average TN removal (mg N m⁻² day⁻¹) over the 12-year period was in the order of summer (5.5) ≒ autumn (5.1) > spring (4.3) ≒ winter (4.2) for the VF bed and in the order of summer (3.5) ≒ spring (3.5) ≒ autumn (3.3) > winter (2.7) for the HF bed, showing that the amount of TN removal per unit area (m²) in summer was slightly greater than that in other seasons. The amount of TN removal in the VF bed was slightly greater than that in the HF bed. Using three-dimensional simulation graphs, the maximum TN removal rate was at inflow N loads below 2.7 g m⁻² day⁻¹ in the summer season, whereas the minimum TN removal rate was at inflow N loads below 1.4 g m⁻² day⁻¹ in the winter season. Consequently, the TN removal efficiency was very stable over the 12 years of operation in VF–HF hybrid CWs. Results demonstrate that the VF–HF hybrid CWs possess good buffer capacity for treating TN from domestic sewage for extended periods of time.