Bachand, Philip A. M., and Alexander J. Horne. Denitrification in constructed free-water surface wetlands: II. Effects of vegetation and temperature. Ecological Engineering 14: 17-32.
Abstract: Constructed wetlands are increasingly being used for treating nitrogen-rich wastewaters. Of the 115 treatment wetlands listed in the North American Treatment Wetland Database which record nitrogen data, a large portion are used for treating secondary treated or lower quality (e.g. primary, agricultural runoff, stormwater) wastewater. Twenty-five percent treat agricultural and stormwater runoff, and only seven are used for either advanced secondary or tertiary treatment. Yet constructed wetlands may provide an attractive and economical alternative to conventional treatment plants for denitrifying high quality, nitrified wastewater. In populated areas where this is most needed, high land costs will increase the capital costs of this technology. Moreover, in semi-arid regions like the western and southwestern USA, high evaporation and evapotranspiration rates may hinder this technology by concentrating total dissolved solids (TDS) and dissolved organic carbon (DOC) concentrations. Implementation of management and design practices for denitrification may be one method to increase efficiencies, reduce costs and increase reliability. One relatively unknown variable in denitrification is the role of different plant species. If one plant provides substantially better conditions for denitrification, wetlands designed for denitrification could be smaller and less expensive. Three commonly used free-surface marsh vegetation treatments (bulrush Scirpus spp., cattail Typha spp., and a mixed stand of macrophytes and grasses) were used in replicated macrocosms to determine nitrate removal rates. Nitrate removal rates between vegetation types were large and differed significantly (PB0.001; cattails 565 mg N m 2 day 1, bulrush 261 mg N m 2 day 1, and mixed 835 mg N m 2 day 1). Mass balance calculations demonstrated that bacterial denitrification rather than plant uptake was the main mechanism for nitrate removal. Both water temperature (temperature–activity coefficient u 1.15–1.22) and organic carbon availability affected denitrification rates whereas surface water dissolved oxygen (DO) and nitrogen concentrations did not. This experiment could not distinguish why the different vegetation types resulted in different denitrification rates. Plant productivity differed between treatments. Plant physical structure, waterfowl grazing pressures and wind disturbance affected the rate litter entered the water column. The literature reports that plant decomposition rates depend upon the plants C:Nlitter ratio and the plant fiber content. All these factors likely affected the rate bioavailable organic carbon was made available to microbial denitrifiers. Based on our study and a literature review, in organic carbon–limited free-surface wetlands, a mixture of labile (submergent, floating) and more recalcitrant (emergent, grasses) are recommended for improving denitrification rates.
Key words: Denitrification; Constructed free-water surface wetlands; Vegetation; Temperature
Reproduced by permission
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