Cold Season Nitrogen Removal in a High Loaded Free Water Surface Wetland with Emergent Vegetation

The aim of this study was to quantify nitrogen removal in high loaded free water surface wetlands dominated by emergent vegetation. It was undertaken in two subsystems of the full-scale wetland Alhagen in Nynäshamn, Sweden. Time proportional samples were taken at the inlets and outlets for 2 weeks in September and November 2005, respectively, and the water flow was monitored. The samples were analysed for ammonium-N (NH

4

×

-N), nitrate-N (NO

3

–

-N), nitrite-N (NO

2

–

-N) and total-N, and the mass nitrogen removal was calculated. Sediment cores were randomly collected to measure potential denitrification, and the result was related to the actual mass nitrogen removal. Zero total-N removal could be detected in the subsystem with 6 hours hydraulic retention time (HRT). In the one with 3–4 days HRT, the total-N removal rates were 0.6 g N m

–2

day

–1

in September and 0.2 g N m

–2

day

–1

in November. The potential denitrification rate was 8 times higher than the observed removal in September and 48 times higher in November. This deviation was likely related both to relatively high oxygen levels and to the amount of available organic carbon.