Nitrate-N removal rate variabilities in floating treatment wetland mesocosms with diverse planting and carbon amendment designs

Floating treatment wetlands are an innovative wetland design to passively improve water quality and remove pollutants from reservoirs with limited infrastructure modification or construction as compared to traditional treatment wetlands. However, design recommendations remain limited. Therefore, the objectives of this study were: 1. Determine nitrate-N removal rates in two floating treatment wetland vegetation designs entering a second growing season and 2. Evaluate nitrate-N removal performance in floating treatment wetland designs following the addition of two carbon amendments during the growing season. Three floating treatment wetland mesocosm experiments were conducted during the summer of 2019, where nitrate-N removal was evaluated. Two floating treatment wetland plant designs (Rush species and Diverse species) and three carbon conditions (no amendment, spent coffee grounds, and barley straw) were assessed in replicates of three. Floating treatment wetland vegetation design (Rush versus Diverse) was found to have significant (P < 0.05) effects on nitrate-N removal performance with Diverse floating treatment wetlands removing nitrate-N at more than twice the rate of Rush floating treatment wetlands (491 to 695 mg N m⁻² d⁻¹ and 1547 to 1937 mg N m⁻² d⁻¹, respectively). However, impacts of nitrate-N removal following carbon amendments were insignificant compared to no amendment Controls. Diverse wetland species should be considered in floating treatment wetland design while alternative methods to add carbon amendments to floating treatment wetland mats should be further explored.