Study On Constructed Subsurface Flow Wetland In Advanced Treatment For The Effluent Of Wastewater Treatment Plant

With the rapid development of China's economy,the pollution of water environment is becoming increasingly serious.As the standard of contaminants for municipal wastewater treatment plant is upgraded constantly,the traditional technology of sewage treatment cannot meet the present situation and the choice of advanced treatment process more and more people's attention.As an ecological treatment technology,the subsurface constructed wetland has several advantages such as high treatment efficiency,low investment cost,etc.,and this process can further remove the contaminants,which act as a good barrier protecting the natural water body from the discharged contaminants.In addition,the treated urban sewage treatment plant secondary effluent by subsurface constructed wetland treatment can be reclaimed,which exhibit good ecological and environmental benefits.In this study,six pilot-scale subsurface flow wetlands were constructed based on different packing and grading?the first group and the sixth group of packing and grading conditions are the same,the other four groups are different?.First of all,the characteristics of overall contaminants removal efficacy in six subsurface flow wetlands treating the secondary effluent were investigated.When the average influent concentration of COD_Cr was 59.46 mg/L,the average effluent reached an amount of18.13 mg/L,with an average removal efficiency of 69.50%;When the average influent concentration of NH₄⁺-N and TN was 4.01 mg/L and 17.23mg/L respectively,the average effluent reached an amount of 0.61 mg/L and 5.61 mg/L,The average removal efficiencies got to 84.76%and 67.51%respectively;Similarly,when the average concentration of TP in influent was 1.29 mg/L,the average concentration of TP in effluent from each wetland was 0.14 mg/L and the average removal efficiency could reach 89.03%.Secondly,according to the law of removal of characteristic contaminants,the optimal packing type of subsurface flow wetlands was determined.The results showed that the combination of steel slag and zeolite had the best COD_Cr removal efficiency and the average removal efficiency reached 76.50%.NH₄⁺-N and TN removal efficiency with zeolite filler were the best,the average removal efficiency could reach 92.57%and71.54%.For TP removal efficiency,the best choice of filler was steel slag,the average removal efficiency was up to 98%.In addition,with the increase of hydraulic load,the removal efficiencies of COD_Cr and NH₄⁺-N in 6 wetland systems show different reduction degree,but when the hydraulic load increases to a certain extent,the removal efficiency gradually stabilized.The removal efficiencies of TP and TN in 6 wetland systems were less affected by the increase of hydraulic loading.Three plants?lythrum,sedge and iris?were selected to investigate their effects on the removal efficiency of constructed subsurface flow wetland systems.The results showed that,the performance of lythrum was the best for the removal of characteristic contaminants,followed by sedge,the iris were the worst.In addition,the seasonal variation had a greater impact on the removal of contaminants in subsurface wetlands,the best removal efficiency was in summer while the worst in winter.Finally,based on the above research,the fuzzy evaluation method is used to evaluate whether constructed subsurface flow wetland effluent can be used for replenishment of surface landscape water.The results showed that secondary effluent of urban sewage treatment plant treated by constructed subsurface flow wetland could reach the surface water replenishment water quality requirements.