Small-scale Constructed Wetland Experiment And Hydrodynamic Simulation Study

At present,as a water pollution treatment method that can efficiently remove a variety of pollutants,constructed wetland technology has quickly become a hot issue for water environmental workers,and it has been widely used in practical work.However,the hydraulic parameters are usually based on past engineering experience,and inevitably there are stagnation and short flow phenomena,which greatly reduces the purification effect and water treatment efficiency of the constructed wetland,which hinders the widespread application of artificial wetland technology.Studying the hydraulic conditions of constructed wetlands can provide a reference and basis for the technological design parameters of constructed wetlands,and ensure that it exerts efficient decontamination and purification functions,which is crucial for the further development and application of constructed wetland technologies.This paper designs and constructs a small artificial wetland experimental field to carry out experiments on the removal effect of pollutants and NaCl pulse tracer experiments under different hydraulic conditions of constructed wetlands.Based on experimental research,a two-dimensional simplified hydrodynamic model of an up-and-down baffled submerged artificial wetland system was constructed using computational fluid dynamics?CFD?technology,and the effects of baffle settings and substrate laying on the hydraulic performance of the submerged artificial wetland were analyzed.The main conclusions are as follows:?1?In general,the removal rate of organic matter,nitrogen and phosphorus in vertical flow constructed wetlands is higher than that of horizontal subsurface flow and surface flow constructed wetlands.Extending the HRT and reducing the HLR can improve the removal rate of organic matter,nitrogen and phosphorus to a certain extent.Under relatively moderate HRT and HLR conditions,the added baffle can play a role in improving the removal rate.?2?The optimal HRT for TN,TP and COD removal of the six types of constructed wetlands is 72h,48h and 48h,respectively.The optimal HRT for NH₃-N removal is relatively complex:The removal rate of NH3-N in vertically descending constructed wetlands has no obvious rules with the extension of HRT;The optimal HRT for removing NH3-N in horizontal subsurface flow and vertical upward constructed wetland is 24h;The optimal HRT for surface flow,surface flow with baffle and horizontal submerged flow with baffle to remove NH3-N is48h.When the HLR is 15cm/d,the removal rate of COD is the highest,and when the HLR is5cm/d,the removal rates of TN,TP and NH₃-N reach the maximum.?3?Among the six small-scale constructed wetlands,the hydraulic parameters of the baffle-containing horizontal subsurface constructed wetlands perform best,with a tracer recovery rate of 43.67%and a hydraulic efficiency of 0.81.The increase of the baffle slows down the flow velocity,the average residence time and the peak time increase,so that the short flow phenomenon is improved.Although a certain degree of dead zone will be generated,the kinetic energy generated during the baffling process can well eliminate the retention problem caused by the complicated flow state,so the recovery rate of the tracer is improved and the hydraulic efficiency is increased.?4?The normalized RTD curve was fitted with a CSTRs+PFD parallel model to obtain the values of various parameters in 6 small artificial wetland models.The slope,peak time,and average residence time of the fitted curve have a good match with the measured values,and the model achieves a good fitting effect.?5?The effect of the number of baffles on the hydraulic efficiency of the constructed wetland is not obvious.The model with the first baffle at the bottom of the substrate has greater hydraulic efficiency than the model with the first baffle at the top of the substrate,and it becomes more obvious as the number of baffles increases;In the method of laying the substrate layer,increasing the thickness of the intermediate substrate can improve the low flow rate caused by the small porosity substrate region of the upper layer and the rapid outflow phenomenon of the lower substrate,improve the utilization efficiency of the intermediate substrate layer,and increase the hydraulic force of the submerged artificial wetland effect.