Study And Modeling Of Diclofenac Removal In Constructed Wetlands Under Unsteady State

With the development of society and economy,wastewater pollution under steady state such as urban sewage treatment has been gradually controlled,and treatment of wastewater under unsteady state,such as runoff pollution,has caused more and more attention.The water quality and water volume of wastewater under unsteady state have obvious fluctuations.Industrial wastewater,urban runoff and agricultural runoff are typical examples of wastewater under unsteady state.Constructed wetland wastewater treatment technology can adapt to the drastic fluctuation of the influent water quantity and water quality,and has advantages of low operating cost and simple management,which makes it more and more widely used in the treatment of unsteady wastewater.However,the existing studies of constructed wetlands are mostly based on steady-state conditions where the influent water quantity and water quality are relatively constant,lack of research on the hydrodynamic characteristics of constructed wetlands under unsteady state conditions and their effects on the removal of pollutants,which hinders the development and application of constructed wetlands.Organic drugs are attracting attention as a new pollutant.Many studies have demonstrated chronic aquatic toxicity to environmental organisms.Moreover,these drugs may act on the food chain through bioaccumulation and biomagnification effects,causing adverse effects on the ecosystem.With the passage of time and the development of human society,the use of organic drugs is bound to increase,and the impact on humans and other organisms is unpredictable.In this study,a typical organic drug,diclofenac,was chosen as the target contaminant,which is also an important component in many unsteady wastewaters.This study was conducted in a horizontal subsurface flow constructed wetlands and a vertical subsurface flow constructed wetlands.A multi-point tracer test based on the time axis was used to study the hydrodynamic characteristics and variation of constructed wetlands under unsteady state conditions.Combining the characteristics of influent water quality and volume under unsteady state,the diclofenac removal test under unsteady state was carried out,and the mechanism of the influence of hydrodynamic characteristics on the kinetics of decontamination reaction under unsteady state conditions was explored.Finally,the hydrodynamic characteristics of the constructed wetland under non-steady state conditions are coupled with the kinetics of the decontamination reaction to establish a pollutant removal model under non-steady state conditions.The results show that the hydraulic impact under unsteady state shorten the average residence time of the influent liquid in the constructed wetland at different times,and this effect is particularly noticeable for the water which enters the constructed wetland at the beginning of the water impact and during the impact.Using the dimensionless time variable ? to normalize the original residence time distribution(RTD)curve,the concentration curve of normalized RTD presents a similar peak time and leaning.Then use the CSTRs+PFD parallel model to simulate it and describe the characteristics of the flow in the unsteady state of the constructed wetland.When the model parameters n,f,z,D are 1.790,0.920,0.324,and 0.451,respectively,the model can simulate the horizontal subsurface flow constructed wetland well.When the model parameters n,f,z,D are 1.518,0.760,0.531,and 200,respectively,the model can simulatete the vertical subsurface flow constructed wetland well.Combining the CSTRs+PFD parallel model with the primary decontamination kinetics,the diclofenac removal model is established.The model contains two parameters,namely the residence time distribution C'(?)and the first order reaction kinetic rate constant k_v.C'(?)is calculated from the CSTRs+PFD parallel model.k_v is affected by the change of unsteady flow,and it is a piecewise univariate cubic function of Q.The model can predict the effluent concentration of diclofenac under unsteady state of horizontal subsurface flow constructed wetland and vertical subsurface flow constructed wetland well.