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Highlights

A floating treatment wetland design was evaluated for water quality improvements.

Nitrate-N removal rates were quantified using spent coffee grounds as a carbon source.

Nitrate-N removal rates increased throughout the growing season

Abstract. The Midwestern U.S. is vulnerable to eutrophic conditions from high nutrient concentrations. Floating treatment wetlands (FTWs) are an innovative wetland design for nutrient removal from nonpoint sources and provide a unique treatment. The objectives of this project were to quantify nitrate removal in traditional and carbon-amended FTWs planted with Midwestern plant species during the establishment year. Three greenhouse experiments were conducted throughout the growing season using 18 mesocosms. Two vegetation designs were evaluated: rush species ( and ) and diverse species (, , , , , and ). Spent coffee grounds were applied to 9 of the 18 mesocosms as a carbon amendment. Nitrate-N removal increased during the establishment growing season in the FTW systems (Spring: 15.0% to 17.3%, Summer 1: 82.8% to 92.6%, Summer 2: 86.4% to 94.7%). Nitrate-N removal was also impacted by carbon amendments (FTW without amendment: 82.8% to 94.7%, FTW with amendment: 88.4% to 96.1%). Carbon additions were found to enhance denitrifying conditions even in the absence of FTWs (decreased dissolved oxygen, increased available organic carbon). Significant differences in nitrate-N removal were not observed between FTW vegetation designs. This study provides new insight on the impacts of the growing season, plant species, and carbon amendments on FTW nitrate-N removal performance during the establishment year.