Removal of wastewater nitrogen in a wetland filter

• An artificial wetland system, designed to achieve year-round wastewater treatment concomitantly with nutrient recycling, was studied for three years. As part of a joint research project, this thesis focuses on nitrogen transformations and nitrogen removal capacity.Wastewater was infiltrated into soil planted with the emergent aquatic macrophyte reed sweetgrass (Glyceria maxima) in experimental field lysimeters. The wetland filters removed 57- 76% of nitrogen applied with wastewater (313-374 g m-2) during three growing seasons and one winter. Soil permeability was influenced by wastewatcr application frequency. Low permeability resulted in more efficient removal, probably due to greater gaseous losses. This was attributed to longer duration of ponding and longer periods of soil anaerobiosis. Nitrogen losses from surface water during submerged conditions were greater after a harvest occasion than before. The major cause for this was plants shading the surface from wind and sun, rather than nutrient uptake.G!yceria maxima, like other emergent aquatic macrophytes, possesses an aerenchymatous structure for oxygen transport to roots and rhizomes under submerged conditions. Elevated soil redox potentials and possibilities for nitrification in the rhizome layer due to oxygen diffusion were thus hypothesized, but could not be detected. Instead, the decrease in redox potential after submergence with wastewater was faster in planted microplots as compared to control plots. This could be attributed to a lack of nitrate for denitrification, due to competition from plant uptake for available nitrogen. When nitrate is present, the decrease in redox potential is retarded by denitrification.Although plant uptake accounted for more than 25% of nitrogen removal during growing seasons, it could not be showed that plants were needed for efficient nitrogen removal when conditions for nitrification-denitrification were optimal. Thus, wetland filter design and management must reflect whether the primary aim is wastewater purification or nutrient recycling.Removal of phosphorus and BOD7 met the required limits for municipal wastewater effluents in Sweden. The costs for reaching this level of treatment in a wetland filter were estimated to be lower than for the existing treatment systems in urban areas with up to 1000 inhabitants.

Ämnesord

SAMHÄLLSVETENSKAP  -- Annan samhällsvetenskap -- Tvärvetenskapliga studier inom samhällsvetenskap (hsv//swe)

SOCIAL SCIENCES  -- Other Social Sciences -- Social Sciences Interdisciplinary (hsv//eng)

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INTERDISCIPLINARY RESEARCH AREAS

TVÄRVETENSKAPLIGA FORSKNINGSOMRÅDEN

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