The effect of plant and influent organic matter on nitrogen removal was investigated in four experimental surface-flow wetlands treating a nitrified meat processing effluent. Three wetlands contained a floating mat of the plant Glyceria maxima (and associated decaying plant litter). The fourth contained a simulated plant mat constructed of nylon fabric. The influent or 50% effluent recycle was irrigated over two of the plant-containing wetlands, while the other wetlands (and the one with 50% recycle) received influent at one end as per normal practice. Nitrogen removal in the planted wetlands during the final year of the study was about twice that in the wetland without plants, and averaged 46-49% (5.2-5.5 g N m−2 d−1) at a high average loading rate of 11.2 g N m−2 d−1. Summertime nitrogen removal reached 75% (approx. 9.5 g m−2 d−1). About 87% of the nitrogen removed by the planted wetlands was due to denitrification, with 13% due to accumulation in sediment and plant biomass. The plants (including plant litter) and influent organic matter were each responsible for about 50% of nitrogen removal, mainly through supplying organic carbon and creating anaerobic conditions for denitrification. Irrigation of wastewater over the plant mat did not enhance nitrogen removal.

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