Transfer of Nitrogen and Phosphorus Nutrients in Vegetated Buffer Zones Receiving Treatment Plant Effluent

Abstract

Vegetated buffer zones (VBZs) are a type of constructed wetlands well known for their purifying capacity. They are built between wastewater treatment plants and receive compartments for a complementary treatment of eutrophicants. This study presents the results of the monitoring of nitrogen and phosphorus transfers in two VBZs with the objective to specify the processes and control factors. Two VBZs, located in Liebsdorf and Raon in the east of France, having contrasting characteristics of surface area, hydraulic load, soil texture and plant species, were monitored for 28 months in order to analyze mineral forms of nitrogen and phosphorus in water, sediment and plants. The decrease in nitrate at the Raon VBZ is linked to low dissolved oxygen (DO) in effluent, which probably triggered denitrification. By contrast, ammonium content remains high. At the Liebsdorf VBZ, high nitrate concentration in water and low ammonium content in soil show that nitrification could be the main process involved, favored by high DO in effluent and helophytes in the ditches. However, the low hydraulic retention time and large hydraulic load limit microbiological processes in the improvement of effluent water quality. Both zones show few changes in phosphorus concentration in water. The content of nutrients in plants is high by comparison with natural wetlands, and twice as high in Liebsdorf as in Raon. However, despite the high retention capacity of plants, the vegetation plays a minor role ( 20%). This study highlights the main factors (soil texture, ratio hydraulic load/surface) which managers can control in order to improve the efficiency of VBZs in nutrient removal.