Nitrogen removal in constructed wetlands receiving nitrified wastewaters can be limited by insufficient organic carbon for denitrification. Experiments were undertaken to determine the importance of decaying plant material, in a floating mat of the wetland plant Glyceria maxima, as a source of organic carbon and anoxic sites for denitrification in surface-flow wetlands. In the laboratory, a mat of G. maxima floating on a nitrified meat processing effluent (87 g m−3 NO3-N) promoted a denitrification rate of 3.8 g m−2 day−1 at 20°C. Under strictly anoxic conditions, and where G. maxima leaves were the sole carbon source for denitrification, about 2.1 g of the biomass carbon was consumed for every gram of NO3−N removed. The biomass carbon produced by G. maxima growing on meat processing effluent has the potential to sustain a denitrification rate of 2.4 - 4.8 g m−2 day−1. In a pilot-scale wetland completely covered with G. maxima and receiving a nitrified meat processing effluent, nitrogen removal rates ranged from 0.6 g m−2 day−1 in winter to 3.0 g m−2 day−1 in summer. The floating plant mat and the sediment are the most active denitrification sites in the wetland. Nitrogen removal may potentially be enhanced by improving the contact between the wastewater and the decaying plant material.

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