The Experimental Study On The Purification Efficiency Of The Secondary Biochemical Tailwater By Integrated Vertical-flow And Horizontal-flow Constructed Wetland

The secondary tailwater has a large number of emissions and higher nitrogen and phosphorus concentrations in the sewage treatment plant in China,which the discharge of the tailwater in the higher nitrogen and phosphorus concentrations still cause the surface Water pollution and the eutrophication phenomone for the limited self-purification capacity or the recipient water that had been polluted.In the arid and semi-arid areas of northwest China,due to the geographical and climatic reasons,the concentration of pollutants?especially N,P?in most sewage treatment plants can only reach the first B standard of?pollutant discharge standard of urban sewage treatment plants??GB18918-2002?,and sometimes the water effluent concentration of nitrogen and phosphorus is even higher.On the other hand,the water resources are showing the status that The total amount is much but less per capita,water environment is seriously polluted and the discharge of wastewater has the rapid growth year after year.Therefore,it is the first choice that advance wastewater treatment and reuse of tailwater to reduce pollution load of the receiving waters,alleviate the pressure water self-purification,promote the process of urbanization and solve the problem of urban water shortage.According to the characteristics of the low content of organics and the relatively high nitrogen and phosphorus concentration in the actual effluent quality of some urban sewage treatment plants in the northwest China,on the basis of the study of the adsorption and phosphorus removal efficiency of waste bricks and shale ceramsite and taking the common plant—reed and cattail as the main plants of constructed wetland,and a experimental device of the integrated vertical flow-horizontal subsurface flow constructed wetland was structured under the outdoor natural conditions.The removal efficiency of COD,NH₄⁺-N,TN and TP for the secondary biochemical tailwater was studied at the start-up phase,the stable operation stage and after harvesting.At the same time,in order to investigate the effect of higher concentration of nitrogen and phosphorus on the two wetland plants in the secondary biochemical tailwater,the growth and physiological characteristics and photosynthetic parameters of two kindswetland plants were detected at the early stage of plant transplanting and harvesting early in the downflow pool,the upstream flow and the horizontal flow of the constructed wetland system.Furthermore,the physiological and physiological characteristics of reed and cattail were analyzed before and after the treatment of secondary biochemical tailwater,and finally the following conclusions were reached:?1?At the beginning of the experiment,with reeds and cattails transplanting survival gradually grow,the number of aerobic,aerobic and anaerobic microorganisms in the matrix increased gradually,and the removal rate of COD,NH₄⁺-N and TP was increased from 52.94%,37.80%,and 53.51% to 68.98%,76.61% and 81.57%,respectively.The average height of Phragmites communis and Typha angustifolia increased from 38 cm and 32 cm to 68 cm and 63 cm,and the IVCW plants grow better than the HF segment.?2?In the stage of stable operation,the removal efficiency of NH₄⁺-N,TN and TP in wetland system gradually decreased with the increase of hydraulic load,while the effect of COD and BOD5 was less affected by hydraulic load.Under the low hydraulic load condition of 0.15m3/?m2·d?,when the concentration of NH₄⁺-N,TN and TP was between 8.40¹4.81mg/L,11.02¹8.97mg/L and 0.81¹.53mg/L,the concentrations of COD,BOD5 and TP in the effluent of the integrated vertical flow-horizontal flow constructed wetland system were lower than the?,?,? standard of ?Surface Water Environment Quality Standard??GB 3838-2002?,and the effluent NH₄⁺-N,TN has far less than the standard A ?urban sewage treatment plant pollutant discharge standards??GB18918-2002?.?3?In the stage of stable operation,the removal load of COD,NH₄⁺-N,TN and TP was increased linearly with the increase of influent load,and did not show a trend of slowing down,which indicated that the integrated vertical flow-horizontal flow constructed wetland can also carry a higher water pollution load to give full play to its capacity to decontaminate.?4?Under the different hydraulic load conditions of 0.15m3/?m2·d?,0.25m3/?m2·d?and 0.35m3/?m2·d?,the concentration of COD,NH₄⁺-N,TN and TP along the integrated vertical flow-horizontal flow constructed wetland system showed a trend of decreasing first and then decreasing slowly,and the removal efficiency of N and P in the wetland system is higher than that in the middle and lower layers,especially there is a rapid degradation process in the range of 0³0cm in the surface of the inlet of the integrated vertical wetting bed.?5?There were some differences in the uptake and enrichment ability of nitrogen and phosphorus from different plants.The nitrogen uptake of different wetland plants was higher than that of phosphorus uptake,and the ability of plant harvesting had obvious effect on nitrogen removal ability.Therefore,wetland plants that is suitable for different processes should be reasonable selected and allocated on the basis of the strength of the stain and the amount of nitrogen and phosphorus absorption in the practical application.Minewhile,wetland plants also should choose the appropriate time to harvest to strengthen the constructed wetland pollutant purification effect.?6?During the whole experiment,MDA and Pro content of plant leaves increased with the increase of pollutant concentration in wetland system,which indicated that the two kinds of plants has a certain self-defense ability to be good to adapt to the secondary biochemical tailwater pollutant stress.In addition,the net photosynthetic rate?Pn?,stomatal conductance?Gs?and intercellular CO2 concentration?Ci?of the two plants were increased with the integrated vertical flow-horizontal flow constructed wetland system,and the apparent mesophyll?AMS?,the percentage of stomatal limitation?Ls?is the opposite,which indicated that the amount of organic matter,nitrogen and phosphorus in the secondary biochemical tailwater can provide some nutrients for the cattails and reeds to enhance the photosynthetic capacity of the plant.Therefore,It is reasonable to prove that the experiment chose reed and cattail as the constructed wetland plant to deal with the tail water is reasonable and feasible.?7?The integrated vertical flow-horizontal flow Constructed wetland system can effectively reduce the load of influent pollutants in three stages after starting and stabilizing operation and is suitable for the treatment of urban sewage,which the secondary horizontal flow wetland bed can improve the treatment efficiency of 15²5% on the basis of the treatment efficiency of the integrated vertical-flow wetland.Therefore,it is important to select wetland plants with well established root system and ventilation organization,strong pollution tolerance and wide,loose porous and stable wetland matrix to strengthen the decontamination effect of constructed wetland.