Simulation Of Water Transportation And Multicomponent Reactive Transportation In Two-stage Vertical Flow Constructed Wetlands

There will be a severer urban disease since the city grows bigger and population explosion,especially the disaster of urban rainstorm runoff,in today`s China.Vertical Flow Constructed Wetlands（VF CWs）,which have the simplicity of low-cost operations and the reliability as well as efficiency of their treatment,is an ideally advanced technology for control,storage and treatment of urban rainstorm runoff based on the idea of “sponge city”.However,current researches of VF CWs are widely use the approach of experiment and their conclusions can`t explain the microorganism based reaction mechanism inside the VF CWs correctly and accurately.This paper is aimed at modeling variably saturated water flow and multicomponent reactive transport in VF CWs.The two-dimension dynamical model of two-stage VF CWs describes the coupling function of water movement,solute diffusion and microbial action.With the help of this dynamical model,this paper was modeled the variable conditions of effluent discharging modes,substrate combining modes and temperatures in treating rainstorm runoff of Chongqing using a two-stage VF CWs.By analyzing the simulation results,the main conclusions as following:For different effluent discharging modes,uniformly-spaced effluent over the seepage face can increase both effective volume ratio and hydraulic efficiency meanwhile shows a better performance on the transportation of dissolved oxygen and the treatment efficiency of COD、nitrogen and phosphorus than the rest.There is a higher DO concentration and a longer high concentration time after inflow.The better transportation of O₂ provides a more favorable environment for the microorganism which leads to a better removal of organic matters.For different substrate combining modes,relatively smaller substrate,sand（0²mm or 0.06⁴mm）for instance,can provide a better distribution of DO.The better transportation of O₂ leads to a better performance of nitration directly and therefore leads to a better performance of denitrification indirectly.Three filtration lays should be setting from top to bottom as the following rules: middle→small→lagre for size,top> middle > bottom for θ_s（saturated water content）and bottom≥top> middle for K_s（saturated hydraulic conductivity）,which can guarantee a higher effective volume ratio and hydraulic efficiency meanwhile shows a good removal of nitrogen.For different temperatures,comparing 20℃,nitration gets faster on its best fit temperature 25℃.The simulation result shows that nitrobacteria is more sensitive than nitrosomonas when temperature rise.Denitrification gets obviously faster on its best fit temperature 30℃,which is 40% faster and maximum concentration of N₂-N is 20% higher than 20℃,and the nitration gets twice as faster as per 10℃ higher.At lower temperatures,15℃ and 10℃ for instance,unlike nitrobacteria,temperature-insensitive nitrosomonas shows a steady performance of NH₄⁺-N transformation.And denitrification is obviously inhibited on 10℃.In the summer of Chongqing,runoff temperature is between 22³0℃ approximately.There is no need for heating the water of VF CWs.However in the winter of Chongqing,runoff temperature is between 9¹6℃ approximately.Putting the denitrification zone,which is the lower part of VF CWs,into underground is a heat preservation approach for keeping a steady removal of nitrogen.