Nitrogen removal in a combined vertical and horizontal subsurface-flow constructed wetland system

This study was conducted to (1) determine appropriate conditions for nitrogen removal by a combined constructed wetland (CW) system and (2) evaluate the effects of hydraulic loading rate (HLR), nitrogen loading rate (NLR), effluent recirculation, plant uptake, and seasonal change on the removal efficiency of nitrogen. A three-stage experimental CW system consisting of a vertical flow-gravel filtration (v-GF) bed without plants, a horizontal-subsurface flow (h-SSF) bed planted with Iris australis, and a vertical-subsurface flow (v-SSF) bed planted with Phragmites australis in series were fed with primary-treated domestic wastewaters. The beds with plants produced effluents of better quality than the v-GF bed without plants. It was observed that the average removal efficiencies increased as the HLR (from 100 to 30 L/m2 d) decreased and the recycle ratios (to 100%) increased. The average removal efficiencies in the summer period were found higher of approximately 6% for the v-GF and h-SSF beds and 11% for v-SSF bed. Nitrogen removal by plant uptake was very low and only 3% (Iris) to 7% (Phragmites) of the N-input compared to other mechanisms such as nitrification and denitrification. The volume-base first-order nitrification and denitrification removal constants (1/d) were 0.388 and 0.785 for the h-SSF bed, and 0.412 and 0.293 for the v-SSF bed, respectively.

The results demonstrated the effects of seasonal changes, hydraulic and nitrogen loading rates, recycling, rapid drainage, and plant uptake on the removal of nitrogen in a combined constructed wetland system. The main conclusions that can be drawn from this study are as follows: 1. Average nitrogen removal efficiencies were higher in summer when average temperature was 238C. À 2. The h-SSF wetland system had higher NO3 + removal but lower NH4 removal. Apparently, this system provided adequate environment for denitrification but limited conditions for nitrification. The v-SSF wetland system had + a higher NH4 removal efficiency because of aerobic conditions (by the oxygenation of the rhizosphere and the aeration of the top layer) developed within system. Average + À NH4 and NO3 removal efficiencies of the complete system were 91% and 89%, respectively. 3. At the same HLRs, both nitrification in the v-SSF bed and denitrification efficiency in 8.