Study On Deep Treatment Of Tail Water Technology By Cascade Constructed Wetland-Ecological Pond System

With the development of society,the problem of water shortage and water pollution has gradually attracted people's attention.The tail water of sewage treatment plant has the characteristics of large discharge and high nitrogen and phosphorus,and direct discharge is easy to cause eutrophication of xwater body.It can be recycled through deep processing to achieve the goal of sustainable development of water resources.Among many sewage treatment technologies,constructed wetlands are widely used due to the advantages of high efficiency and energy saving.This study constructed a cascade-type constructed wetland-ecological pond system,which can be combined with the construction of slope land according to local conditions and has multiple natural reoxygenation capabilities.This paper mainly explores the application of cascaded constructed wetland-ecological pond system in the treatment of tail water,which provides a theoretical basis for the promotion and application of the system in practical engineering.This study starts with a small test screening of ecological pond submerged plants:Four kinds of submerged plants were selected to construct different ecological ponds,and the growth of different submerged plants was investigated comprehensively,the reoxygenation efficiency of different ecological ponds and the purification efficiency of tailwaters were studied to provide a theoretical basis for the establishment of superior ecological ponds in the pilot system.On the basis of selecting a superior ecological pond,a cascade-type constructed wetland-ecological pond system will be constructed.Firstly,examine the purification efficiency of the tailwater by the pilot system,and the changes of pollutants along the process of the pilot system and the effects of different hydraulic loads and different seasons on the purification of tail water from the pilot system were studied.Secondly,compared with the traditional serial subsurface flow constructed wetland system,the advantages of the pilot system are verified.Finally,the intensive measures of recirculation were taken to study the effects and laws of different reflux ratios on the purification of tail water by the pilot system,in order to further improve the advanced treatment capacity of the pilot system to the tail water.The main conclusions of this study are as follow:(1)Screening test of submerged plants in ecological ponds showed that:The growth situation of submerged plants in different ecological ponds is different,which is characterized by the best growth of Myriophyllum spicatum and Hydrilla verticillata.Different ecological ponds showed different reoxygenation abilities,among which Hydrilla verticillata and Ceratoplhyllum demersum had better reoxygenation effect,and the reoxygenation rate was as high as 117.22%and 88.78%,respectively.Different ecological ponds have different effects on the removal of various pollutants:The removal rate of COD in each ecological pond is not high,and the black algae with the highest removal rate can only reach 46.67%;The removal effect of TP on Ceratophyllum demersum and Hydrilla verticillata was higher,and the removal rates reached 65.02%and 55.14%,respectively.The removal of NH3-N and TN by Myriophyllum spicatum and Hydrilla verticillata was better,the removal rates of NH3-N were 70%and 66%respectively,the removal rates of TN were 80.88%and 73.53%,respectively.Among the four submerged plants,Myriophyllum spicatum and Hydrilla verticillata are more prominent,preferably submerged plants in the ecological pond of the pilot system.(2)Study on the effect of step-type constructed wetland-ecological pond pilot system to purify tail water:With the extension of the pilot system process,the concentration of each pollutant gradually decreases,and the removal rate gradually increases;The removal rate of each pollutant in the pilot system decreases with the increase of hydraulic load;Under different hydraulic loads and pollution load conditions,the pilot system showed strong anti-loading ability,good removal effect and stable effluent;The removal effect of the pilot system on various pollutants in summer(July-august)is better than that in winter(december-january),the removal rate of TN and NH3-N was the most affected by the seasonal influence,while the removal rate of TP was not significantly affected by the seasonal influence(3)A comparative validation study on the effect of the pilot system on the purification of tail water shows that:The removal rates of COD in the pilot system and the traditional serial subsurface flow constructed wetland system were 66.67%and 49.02%,respectively;the removal rates for NH3-N were 86.27%and 66.67%,respectively;the removal rates for TN were 62.88%and 35.42%,respectively;the removal rate of TP was 59.09%and 39.06%,respectively.Compared with the traditional serial subsurface flow constructed wetland system,the pilot system has certain advantages for the deep removal of pollutants.(4)The effect of reflux measures on the effect of the pilot system on the purification of tail water:The reflux measures can improve the removal efficiency of COD,TN and TP in the pilot system.When the reflux ratio is 0.5,the removal effect of COD and TP is the best;when the reflux ratio is 1,the removal effect of TN is the best.Compared with non-reflux,the highest removal rates of COD,TN and TP by reflux were 16.09%,13.81%and 7.91%respectively.