Effects of the substrate depth on purification performance of a hybrid constructed wetland treating domestic sewage

The depth of substrate in constructed wetlands (CWs) has a significant effect on the construction investment and the purification performance of CWs. In this study, a pilot scale CW system was operated in a domestic sewage treatment plant in Xi'an, China. The experimental systems included three-series CWs systems with substrate depths of 0.1m, 0.3 m and 0.6 m, respectively. Each series was composed of a hydroponic ditch, a horizontal subsurface flow CW and a vertical flow CW. The effluent from the primary clarifier in the sewage treatment plant was intermittently conducted to the wetlands at a flow rate of 0.3 m3/d. The hydraulic loading rate of each CWs system was regulated at 0.1 m3/m2.d and the hydraulic retention time was 3 days. Canna indica L. was planted both in the hydroponic ditches and the CWs systems. Results showed that the highest removal efficiency of NH+4-N and TP was obtained in the hybrid CW with 0.1 m substrate depth. The average removal efficiency for NH+4-N and TP were 90.6 % and 80.0 %, respectively. The highest average removal efficiency of COD was obtained in hybrid CWs system with 0.6 m substrate depth. Therefore, a simultaneous removal of COD and nutrients can be achieved through the combination of different wetlands using different substrate depths. In addition, the substrate depth presents significant effects on the concentration of DO and root growth characteristics of canna in the system. As a result, the highest concentration of DO (2 mg/L) and the highest amount of roots production were achieved in the 0.1 m substrate depth horizontal and vertical flow CWs.