Hydraulic Characteristic And Removal Of Organic Pollutants In The Subsurface Flow Contructed Wetland

The constructed wetland is a kind of low-cost and high-efficiency ecosystem engineering technology for wastewater treatment and water reuse.Compared to the surface flow constructed wetland,more and more attentions are paid to the subsurface flow constructed wetland(SSFW) because of the virtues of good sanitary condition,high removal of contamination,little influenced by weather,and so on.In the SSFW system design,the hydraulic factor is very important. The traditional methods to investigate the hydraulics of SSFWs are usually based the model experiment using the physical tracer experiment to get the rule of the flow.The method has great limitation in the display of the flow pattern.It not only because the physical tracer experiment itself is associated with the problems such as the selection and injection of the tracer and so on,but also because there has the vegetable root in the SSFW,and the media in the SSFW with large surface and biologic film,which make the selection of the tracer more difficult.In addition, the method is labor intensive,expensive and time-consuming,and can not provide the information of flow filed.The development of SSFW has created the need to better understand the flow patterns in wetlands and how to use mathematical models as design tools.The appearance and development of the computational fluid dynamics(CFD)technology make there has another choice to study the hydraulics except the traditional method which use model experiment associated with physical tracer.The CFD technology presented a sophisticated design and analysis tool to simulate the flow of mass and momentum throughout a fluid continuum.It is a good method to study the hydraulics,because of its low cost and because it can provide the full flow field,and can be scaled up.The technique allows a computational model to be used under many different design constraints and is effective in water treatment device design and optimization,such as wastewater oxidation ponds.However,to the authors' knowledge,no one has provided a CFD model for SSFWs to now.In this study,a two-dimensional model was developed for the SSFW based on the reasonable simplification of the SSFW system running at Jinzhou considered using a commercial code Fluent 6.22(Fluent Inc.). The flow in the SSFW was simulated by porous media model and the removal of the organic pollutants in the SSFW was simulated by wall surface reaction model.The effect of the layered purge area,and distribution and catchment area on the hydraulic performance of the SSFW was discussed.Moreover,the treatment of the organic pollutants in the wastewater by the SSFW was investigated.These would benefit the design of the SSFW.The main works and results are listed as follows:1.The hydraulic characteristic of the SSFW(1)The hydraulic characteristic of the S SFW with layered purge areaThe research of the effect of wetland configuration(the inlet location, media resistance,and protection area)on the hydraulic performance indicated that the SSFW with an inlet at the middle edge of the upper media had better hydraulic efficiency than the SSFW with an inlet at the top and bottom edge of the upper media.The constructed media affected the hydraulic performance by the ratio of the upper and lower media resistance(K).The higher resistance in the protection area benefited the hydraulic efficiency.The operating condition(the inlet velocity and outlet pressure)on the hydraulic performance was investigated.The results indicated that the effect of operating condition was slighter than that of wetland configuration.The inlet velocity markedly affected the mean resident time.It was necessary to determine appropriate inlet velocity to get not only good hydraulic efficiency but also enough resident time.The effect of the outlet pressure caused by the water level can be neglected.In the engineering design,the water lever in the weir at the outlet can be adjusted according to the need.(2)The hydraulic characteristic of the SSFW with distribution and /or catchment areaThe effect of two types(the layer and protection pattern)of catchment/distribution area on the hydraulic performance was investigated.The simulation results confirmed that the protection pattern catchment/distribution area was better than the layer pattern.Compare to the SSFW with the layer pattern catchment area,the one with protection pattern catchment area had high hydraulic efficiency and by decreasing the catchment width,the hydraulic efficiency can be improved and the effect of the resistance ratio on it can be weakened.The research of the hydraulic characteristic of the SSFW with distribution and/or catchment area indicated that the design of distribution and catchment area in a vertical direction was better than in a horizontal direction.The hydraulic efficiency of the SSFW with vertical distribution and catchment area slightly affected by the variation of the distribution and catchment area in the vertical direction,and can be kept in a high level.2.The removal of the organic pollutants in the SSFW(1)The removal of the organic pollutants in the,SSFW with layered purge areaThe effect of the inlet location on the removal of the organic pollutants demonstrated the removal efficiency was slightly affected by the inlet location and when the inlet location at the top,middle and bottom edge of the upper media,they were 0.753,0.707 and 0.719 respectively.The removal efficiency was markedly affected by the inlet velocity,and increased with the decrease of the inlet velocity because of the increase of resident time.(2)The removal of the organic pollutants in the SSFW with distribution and catchment areasThe effect of the distribution and catchment areas design indicated that the removal efficiency in the SSFW with protection pattern distribution and catchment areas was slightly higher than that in the SSFWs with layer pattern distribution and catchment areas because of the larger purge area in this design.The microbe had significant impact on the removal efficiency because deferent kinds of microbe had deferent activation energy which had great effect on the removal efficiency.The removal efficiency decreased with the increase of the activation energy.