Effects of outlet positions and plant species on rural domestic sewage purification in engineered vertical flow wetland

In this study, four different types of engineered vertical flow constructed wetland (VF-CW) — Canna indica Linn wetland (M), Pennisetum alopecuroides (Linn.) Spreng wetland (L), Sorghum sudanense (Piper) Stapf wetland (S) and control wetland without plant (CK) — were used to determine the effects of outlet positions (high, middle and low position) and plant species on pollutant removal efficiency in rural domestic sewages. Domestic sewages from anaerobic tanks in the Changshu Agro-ecological Experimental Station were intermittently used to irrigate VF-CW at hydraulic loading rate of 0.15 m³·m^-2·d^-1. Results showed that the outlet position significantly determined the removal efficiency of NH₄⁺-N, NO₃^- -N and COD (chemical oxygen demand) in rural domestic sewages. The removal rate of NH₄⁺-N significantly increased with lowering of the outlet position. The maximum removal rate of NH₄⁺-N was 98.3%. However, the removal rate of NO₃^--N and COD significantly decreased with lowering of the outlet position. The removal rates of NO₃^--N and COD of the highest outlet were -47.4% and 64.5%, respectively. The removal rate of total nitrogen (TN) of the high outlet increased by 22.5%~27.6% compared with that of the middle and low outlets. On the contrary, the removal efficiency of total phosphorus (TP) of the high outlet was 20.6%~28.9% lower than that of middle and low outlets. Compared with the control treatment (CK), the removal rates of NO₃^--N, TN, TP and COD in VF-CW with C. indica, P. alopecuroides and S. sudanense significantly increased by 74.4%~98.6%, 11.3%~17.8%, 8.60%~16.3% and 14.1%~19.0%, respectively. However, no significant differences were noted among VF-CWs with three plant species regarding the removal of NO₃^--N, TN, TP and COD. In term of the removal rate of NH₄⁺-N, C. indica wetland performed significantly less than the other plant species VF-CWs. These results suggested that pollutant removal efficiency improved by controlling outlet position and growing plants in VF-CW systems.