The Long-term Nitrogen Removal Efficiency from Agricultural Runoff in Phragmites Australis Packed Surface Flow Constructed Wetland

The low C/N ratio and high nitrate proportion characteristics of agricultural runoff makes its nitrate removal difficult. To resolve such problem, the economically and easily obtained Phragmites australis stems and leaves were applied and packed in the surface layer of the surface flow constructed wetland (PSFW) as external carbon sources to study the long-term nitrogen removal from agricultural runoff. Compared with control PSFW₀, obviously higher nitrate removal efficiency was observed in Phragmites australis packed PSFW during 29-149 d operation. When the influent NO₃^--N and TN were (16.4±1.0) mg/L and (17.7±2.0) mg/L and the HRTs were 2 d, 3 d and 4 d, the highest nitrogen removal efficiency was achieved under 4 d HRT condition, and the NO₃^--N and TN removal efficiency in PSFW were (87.4±6.0)% and (74.1%±6.0)%. Those removal in PSFW₀ were (14.4±4.0)% and (14.4±3.0)% respectively. Slight higher denitrification efficiency was obtained in PSFW than that in PSFW₀ during 150-269 d operation. When the influent NO₃^--N and TN were (10.4±1.0) mg/L and (10.8±1.0) mg/L and the HRTs were 3 d, 2d and 1d, the highest nitrogen removal efficiency was achieved under 3d HRT condition, and the NO₃^--N and TN removal efficiency in PSFW were (91.9±7.0)% and (90.2±7.0)%. Those removal were (91.3±5.0)% and (86.4±6.0)% in PSFW₀ accordingly. Slight lower denitrification efficiency was obtained in PSFW than that in PSFW₀ during 270-334 d operation. When the influent NO₃^--N and TN were (5.7±0.4) mg/L and (7.2±0.8) mg/L and the HRT was 3 d, the NO₃^--N and TN removal efficiency in PSFW were (88.6±10.0)% and (82.5±7.0)%. Those removal were (94.0±6.0)%和(87.8±3.0)% in PSFW₀ accordingly.