Study And Modeling Of Organic Pollutants Removal In Constructed Wetlands Under Unsteady Inflow Condition

Study on pollutant removal mechanism of constructed wetland(CW) with unsteady state becomes emergent for the increasingly widely use of constructed wetland in non-point source pollution control such as management of agriculture and urban runoff. However, traditional wastewater treatment theories are generally based on steady flow condition. The theories and study methods are now inadequate on wastewater treatment under unsteady condition. Therefore, the analysis of the influence of unsteady conditions on contaminant removal mechanism for constructed wetland and contaminant removal regularity for constructed wetland have an important realistic meaning for the application in the non-point source pollution control.This test takes horizontal flow constructed wetland and vertical flow constructed wetland as examples, based on the timeline of multipoint tracer test, analyses organic matter removal efficiency for constructed wetland under unsteady state process, established pollutant removal model based on residence time distribution for constructed wetland under unsteady state, verifies and analyses model.The test results show that the horizontal flow constructed wetland(HFCW) and vertical flow constructed wetland(VFCW) under different unsteady state conditions, constructed wetland showed a good pollutants removal performance for CODCr, TOC and DOC. The event-mean concentrations(EMC) of CODCr, TOC and DOC are less than 40 mg.L-1, 20 mg.L-1 and 17 mg.L-1 respectively, average removal rates of CODCr, TOC and DOC for HFCW and VFCW are more than 50%, 48% respectively. EMC of CODCr can reach the surface water environment V class water quality standard(GB3838-2002) and urban greening of mixed water standard(GB/T18920-2002), river ornamental landscape water standard(GB/T18921-2002). The average removal rates of HFCW are higher and the average removal loads of VFCW are higher under the same unsteady state conditions. From the shock load, HFCW has a stronger ability to resist impact load. The effluent pollutants concentration is affected by the water quantity and water quality, but the fluctuation of water quantity is more significant influence on effluent pollutant concentration.The pollutant removal model is used to simulate the effluent pollutant concentration of CW under unsteady state conditions. It is a mathematical concept of the model that column vector C(t) of the effluent pollutant concentration is equal to the product of the residence time distribution matrix X and first-order kinetics column vector F(t). The main parameters of the model are the residence time distribution C￠(f)of CW and the first-order reaction rate constant nk(d-1), the residence time distribution C￠(f) can be calculated by CSTRs+PFD parallel model. When z=0.324, f=0.890, n=2.2, D=0.279, nk =5.63d-1, the DOC removal model can predict the effluent DOC concentration changes of HFCW under different unsteady state conditions, and in 5 times unimoda,8 times unimodal and 5 times bimodal shock loads experiments, the correlation coefficient of the measured and simulated effluent DOC concentration are 0.9483, 0.945 and 0.8782 respectively; When z=0.631, f=0.849, n=1.6, D=0.110, nk =1.40d-1, the DOC removal model can predict the effluent DOC concentration changes of VFCW under different unsteady state conditions, and in 5 times unimodal, 8 times unimodal and 5 times bimodal shock loads experiments, the correlation coefficient of the measured effluent DOC concentration and simulated effluent DOC concentration are 0.841, 0.9264 and 0.8735 respectively.