Nitrogen transformations and removal efficiency enhancement of a constructed wetland in subtropical Taiwan.

Urbanization condenses reactive nitrogen into cities leaving threats of nitrogen pollution onto nearby environments when sewage is not properly treated. Constructed wetland is an ecological and economical way to remove reactive nitrogen. We investigated the seasonal nitrogen transformations and removal pathways in a surface-flow constructed wetland (93,000m with five treatment cells), which treats domestic wastewater in subtropical Taiwan. By using isotopic pairing technique, we found denitrification exceeds anammox dominated the nitrogen removal pathway throughout seasons. The potential denitrification (0.09 to 2.84gNmd) in the overlying water was in the same magnitude relative to that in sediments (1.26 to 4.14gNmd). The denitrification rates in sediments were highest in summer followed by autumn and winter. The concentration removal efficiencies of ammonium and dissolved inorganic nitrogen (DIN) were both highest in summer, then decreased significantly in autumn and winter. Temperature is a significant regulator for seasonal nitrogen removal. However, a positive correlation was observed between the potential denitrification rates and the amount of NO addition, indicating that nitrate addition may still stimulate denitrification under low temperature condition in winter (15.2-16.3°C). Since nitrate concentrations in porewater were much lower than that in water column for autumn and winter, we speculated NO-N (nitrite and nitrate) supply to the sediments was a limiting factor for low DIN removal efficiency. We proposed to enhance nitrate removal efficiency via denitrification by physically promoting NO-N and oxygen exchanges through the sediment-water interface.