Hydraulic Characters And Ecological Effect Of Constructed Wetland Devices For Landscape Water Purification

As a water treatment technique developed in 1970s,Constructed wetlands has a great potential and advantages in slightly contaminated waste water purification,and has been widely used around the world recently.However,the action mechanism of constructed wetland for waste water treatment is now still unclear.It has been discovered that hydraulic characteristics plays a key role on pollutant removal efficiency in constructed wetland.Baffled flow constructed wetland is an improvement based on the above consideration,and has been widely used in urban sewage,storm water runoff and landscape water treatment.However,few studies have focused on its hydraulic mechanisms.In this study,a series of simulation devices of constructed wetland were designed,and their hydraulic characteristic parameters were obtained via a salt trace experiment.On the basis,the hydraulic characters of constructed wetland with different flow patterns were evaluated.As microbial degradation,plant assimilation and matrix absorption all could contribute to the purification effect of constructed wetland,hydraulic characteristics is just one factor for treatment efficiency of constructed wetland.So it is necessary to study the treatment efficiency of the constructed wetland with hydraulic optimization on actual operation.Based on the above considerations,a group of devices with different flow patterns and different matrix was built,with their purification effect was examined in half of one year.With experiment in this section,the improvement effect of flow pattern on treatment efficiency was confirmed.Constructed wetland will emit greenhouse gases,mainly CO_2,CH₄,N₂O during operation.Greenhouse gas emissions by natural wetland have been studied extensively,whereas the condition for constructed wetland has been rarely explored.To study the effect of flow pattern and matrix on ecosystem functions of constructed wetland,we established four constructed wetlands to treat landscape water.The flow patterns as well as matrix of the four constructed wetlands were horizontal push flow with gravel matrix,horizontal push flow with ceramic matrix,horizontal baffle flow with gravel matrix,and vertical baffle flow with gravel matrix,respectively.The results of hydraulic characteristic evaluation indicated that:（1）recycling rate of horizontal push device with a 2:1 length to width ratio is far less than that of the horizontal push device with a 4:1 length and width ratio,indicating that a retention phenomenon would be caused by a lower length and width ratio.（2）Without matrix filling hydraulic characteristics of horizontal baffled flow devices and the horizontal push device were basically the same,whereas this value decreases sharply to less than 20%after filling of matrix,revealed that hydraulic characteristics of this baffled type are poor.（3）In the vertical baffled flow device with four galleries,dead zone was significantly reduced compared with other devices,and a high recycling rate（1.5h 64.56%）was guaranteed,which possess the most satisfactory hydraulic characteristic.The results of treatment efficiency of landscape water by constructed wetland with different flow patterns and matrix have shown that,vertical baffle flow gravel wetland exhibit the best among the 4 devices.Its treatment efficiency for TP,TN,NO₃^--N,NH₄⁺-N are 42.47%,28.92%,32.72%and 52.92%respectively,higher than the horizontal baffle flow gravel wetland（41.67%,23.76%,28.20%and 38.93%for TP,TN,NO₃^--N,NH₄⁺-N）.Treatment efficiency of horizontal push flow wetland with ceramic aggragate matrix for COD,TP,NH₄⁺-N are 24.39%,39.14%and22.39%,higher than that of the horizontal push flow wetland with gravel matirx（14.71%,37.19%and 12.23%）.This is mainly due to the ceramic has better adsorption capacity.The result of green hourse gas emission of contratructed wetland with different flow patterns and matrix have shown that,CO₂ emission fluxes of vertical buffle flow wetland with gravel matrix was 409.3mg/m²·h,significantly higher than the others;CO₂ emission flux and the air temperature showed a positive correlation.Emission flux of CH₄ of horizontal baffle flow wetland with gravel matrix was 0.83mg/m²·h,significantly higher than the other three devices;CH₄ emission flux also increases with the increase of temperature.N₂O emission fluxes of vertical buffle flow wetland with gravel matrix was 20.3mg/m²·h,higher than the others;horizontal push flow wetland with ceramic sand matrix emited more N₂O than horizonal push flow wetland with gravel matirx,indicating that the matrix type has an impact on the N₂O emission flux of wetland.