Analysis on phosphorus removal in series horizontal subsurface flow constructed wetland based on hydraulic model

Abstract: In order to explore the internal water flow patterns of series horizontal subsurface flow constructed wetland and the treatment efficiency of wastewater from the domestically raised waterfowl, treatment performance on the waterfowl wastewater by a horizontal subsurface flow constructed wetland with three stages in series (P1, P2, P3) was monitored continuously. Changes in the removal efficiency of total phosphorus (TP) per area base by the horizontal subsurface flow constructed wetland in different months were analyzed. The effects of the influent flow rate and hydraulic retention time on the effluent concentration of TP in the horizontal subsurface flow constructed wetland were analyzed. In addition, the actual flow morphologic characteristics in the horizontal subsurface flow constructed wetland were described through the use of three different hydraulic models including the plug-flow model, the dispersed-flow model and the tanks-in-series model. Results showed that an overall decrease of the removal efficiency of TP was observed as the number of treatment ponds increased. The averaged removal efficiencies of TP were (0.11±0.09) g/(m2·d), (0.08±0.03) g/(m2·d) and (0.05±0.01) g/(m2·d) respectively for the treatment ponds P1, P2 and P3 throughout the experiment. The increase of influent flow rate was conducive to the increase of the removal efficiency of TP in the treatment pond P1 when the influent flow rates were maintained at a level lower than 55 m3/d. However, the removal efficiencies of TP in the treatment ponds P2 and P3 declined as the influent load increased when the influent flow rates were lower than 55 m3/d. While the increase of hydraulic retention time was beneficial to the increase of the removal efficiencies of TP, resulting in decrease of effluent concentrations of TP in different treatment ponds. The removal efficiencies of TP in the treatment ponds P2 and P3 appeared to be more sensitive to the changes in the hydraulic retention time conditions when the hydraulic retention time was longer than 85 d, and exhibited greater fluctuations than the removal efficiencies of TP when the hydraulic retention time was less than 85 d. The maximum removal efficiencies of TP were reached under the condition of optimal combinations of influent flow rate and hydraulic retention time: 55 m3/d and 166 d; 3.4 m3/d and 151 d and 9.5 m3/d and 176 d, respectively for the treatment ponds P1, P2 and P3. Seasonal variations in the removal efficiency of TP were observed in the horizontal subsurface flow constructed wetland. The treatment pond P1 appeared to be efficient in the removal of TP in summer as indicated by higher removal efficiencies of TP in the summer than in the autumn. While the treatment pond P3 experienced a longer time of net discharges of TP in the summer than in the autumn. The removal efficiency of TP in the treatment pond P2 exhibited a great fluctuation as the experiment proceeded and net discharges of TP occurred both in the summer and in the autumn. Compared with the treatment ponds P2 and P3, the treatment pond P1 with a more favorable physical and chemical conditions in the horizontal subsurface flow constructed wetland was more efficient in the removal of phosphorus, as indicated by the higher first-order removal coefficients ranged from 0.663 cm/d to 0.751 cm/d than treatment ponds P2 (0.641 to 0.722 cm/d) and P3 (0.429 to 0.458 cm/d). Results of the simulations demonstrated that the tanks-in-series model with higher determination coefficients of 0.567~0.883 (P < 0.05) fit the observed data better than the dispersed-flow model (R2: 0.510~0.723, P < 0.05) and the plug?flow model (R2: 0.465~0.626, P < 0.05), indicating that the actual flow morphologic characteristics in the horizontal subsurface flow constructed wetland was close to mixed flow. Results of this study were benefit in deepening the understanding of the internal flow patterns and distributions, as well as the transportation and dispersion mechanisms of phosphorus under different operating conditions of horizontal subsurface flow constructed wetlands.