Removal of nitrogen and phosphorus from agricultural irrigation runoff in subsurface horizontal-flow wetland.

The subsurface horizontal-flow wetland planted Phragmites communis was constructed to study the long-term performance nitrogen and phosphorus removal from agricultural irrigation runoff (TN was about 7mg/L, TP was about 0.5mg/L). TP and TN removal rates were above 87% and 68% at the hydraulic remain time (HRT) of 2, 4, 6d. The influence of HRT on the removal of TN, NH4+-N and TP by wetland was relatively great (P0.05) and less for PO43--P removal. Under all HRT conditions the removal rates of NH4+-N, NO2--N, NO3--N and PO43--P were all higher than 93%, with their concentrations generally below 0.04mg/L. The TP and TN in the wetland effluent were mainly of organic from, due to the release of organic nitrogen and organic phosphorus induced by internal circulation of nitrogen and phosphorus in wetland system. There existed TP and TN background levels in the wetland effluent. Wetland possessed very good relativity with TN removal rate and load (R2=0.9968); but relatively poor with TP (R2=0.5987). Based on 2d HRT, the Phragmites communis bed with surface area 1m2 had 0.1m3/d ability of treating the agriculture irrigation runoff and the effluent TN and TP concentration could be controlled below 0.50mg/L and 0.154mg/L.