Nitrogen cycling, plant biomass, and carbon dioxide evolution in a subsurface flow wetland

Abstract

Subsurface flow wetlands have recently become a popular means of wastewater treatment. Although wetlands are being used in many places, sufficient data has not been collected concerning mechanisms of water improvement. This study was done to ascertain the fate of nitrogen in a constructed wetland and the rate of bioremediation as indicated by carbon dioxide evolution. Research included a study of nitrogen uptake by plants and nitrification. A tracer isotope of nitrogen,¹⁵N, was used to follow nitrogen through the wetland. To quantify the nitrogen acquired during uptake, plant samples were taken from three types of plants: Typha sp.-cattail, Thalia sp.-red thalia, and Cyperus sp.-umbrella palm. Isotope ratio mass spectrometry results indicated plants took up 5% of the added ¹⁵N and analysis of the effluent wastewater indicated only 10% of added ¹⁵N was present there, accounting for only 15% of the total ¹⁵N. since not all of ¹⁵N could be accounted for, a lab study was conducted to find out the capacity for nitrifucation only occurred on biofilms, that covered gravel, and not in the wastewater itself. The results indicated that there was not sufficient nitrification to significantly reduce the amount of nitrogen in the wastewater. Therefore, NH₄⁺ volatilization likely was responsible for most of the nitrogen loss. The amount of CO₂ flux from the wetland was measured from the influent end to the effluent end. Carbon dioxide evolution decreased as water passed through the wetland. The average CO₂ flux level 30 cm into the wetland was 6.08 g CO₂ m⁻²d⁻¹ (0.092 mol CO₂ m⁻²d⁻¹) and decreased steadily to 1.52 g CO₂ m⁻²d⁻¹ (0.023 mol CO₂ m⁻²d⁻¹) at the outlet of the wetland. It appears that microbial activity and treatment was mostly taking place in the front half of the wetland. The results indicated that plants did not remove a significant portion of the nitrogen and most biodegradation was being accomplished in the first 2/3 of the wetland.