The Simulation Study On The Horizontal Subsurface Flow Constructed Wetlands By Using The CFD

Horizontal subsurface flow constructed wetlands have good removal efficiency to sewage and waste water,its hydraulic performance and operating parameters have an effect on removal efficiency.However,its design depends on empirical equation for a long time and its primary design parameters remain to be surface loading and hydraulic loading up to now.The objective of this study was to establish a hydraulic model of constructed wetlands,understand hydraulic performance of wetlands and optimize its structural parameters by means of computational fluid dynamics(CFD),which can help to build mathematical model of water treatment of horizontal subsurface flow constructed wetlands.The model was built in three pairs of horizontal subsurface flow constructed wetlands which with the same superficial area and with the following aspect ratio:1:1,1.5:1 and 2:1;The size of the granular medium of each bed within each pair was divided into coarse granitic gravel(D60=10mm)and small granitic gravel(D60=3.5mm);The initial water depth of the entire simulation object was set to 0.5 m and hydraulic load rate was set to 36mm/day.A mathematical solution of hydraulic model and water treatment model was realized by simulation technology of computational fluid dynamics.Its simulated date of tracer showed good agreement with experimental data,so correctness of porous media model and water treatment model of constructed wetlands was verified.The effects of aspect ratio,hydraulic load rate and the saturated hydraulic conductivity of wetlands on the hydraulic retention time distribution density(RTD)function have be discussed and hydraulic efficiency of constructed wetlands was calculated.The results showed that aspect ratio and the saturated hydraulic conductivity have profound influence on the hydraulic efficiency of constructed wetlands,and hydraulic efficiency increased with the increase of aspect ratio,hydraulic efficiency decreased with the increase of the saturated hydraulic conductivity,a higher aspect ratio and finer medium reinforce the hydraulic behavior of the wetland system by increasing internal dispersion,which can improve hydraulic efficiency.Although hydraulic load rate does not have profound influence on the hydraulic efficiency,it can influence the shape of RTD function and make hydraulic retention time change greatly.Mathematical model of water treatment of wetlands has been verified,which can apply to similar horizontal subsurface flow constructed wetlands.The results showed that heterotrophic microorganism have greater concentration in the wetlands inlet where 'sewage go into the wetlands and in this place,organic pollutants removal rate accounted for the largest;The water deep of wetlands have great impact on water treatment efficiency,water quality decreased with the increase of water depth,distribution of dissolved oxygen maybe the reason why the water treatment efficiency in the wetland surface better the that in the wetlands bottom;with the increasing of water hydraulic,hydraulic retention time reduce,the water treatment efficiency decreased.So,with regard to the simulated constructed wetlands,its water treatment efficiency can reach the primary sewage discharge standard when the water hydraulic less than 40mm/d.