High Nitrogen Removal in a Constructed Wetland Receiving Treated Wastewater in a Cold Climate

Constructed wetlands provide cost-efficient nutrient removal, with minimal input of human labor and energy, and their number is globally increasing. However, in northern latitudes, wetlands are rarely utilized, because their nutrient removal efficiency has been questioned due to the cold climate. Here, we studied nutrient retention and nitrogen removal in a boreal constructed wetland (4-ha) receiving treated nitrogen-rich wastewater. On a yearly basis, most of the inorganic nutrients were retained by the wetland. The highest retention efficiency was found during the ice-free period, being 79% for ammonium-nitrogen (NH₄⁺-N), 71% for nitrate-nitrogen (NO₃^--N), and 88% for phosphate-phosphorus (PO₄^3--P). Wetland also acted as a buffer zone during the disturbed nitrification process of the wastewater treatment plant. Denitrification varied between 106 and 252 mg N m^-2 d^-1 during the ice-free period. During the ice-cover period, total gaseous nitrogen removal was 147 mg N m^-2 d^-1, from which 66% was removed as N₂, 28.5% as N₂O through denitrification, and 5.5% as N₂ through anammox. Nearly 2600 kg N y^-1 was estimated to be removed through microbial gaseous N-production which equaled 72% of NO₃^--N and 60% of TN yearly retention in the wetland. The wetland retained nutrients even in winter, when good oxygen conditions prevailed under ice. The results suggest that constructed wetlands are an efficient option for wastewater nitrogen removal and nutrient retention also in cold climates.