Advanced Treatment Of Tail Water From Sewage Plant By Constructed Wetland + Ecology Ponds

With the rapid development of China's social economy,water shortage and pollution phenomenon has gradually become the focus of attention.And the tail water of sewage treatment plant has the characteristics of large amount and high concentration of pollutants.Discharging this type of tail water directly into the receiving water body will cause different levels of pollution.The advanced treatment of the tail water from sewage treatment plant can alleviate the current situation of water shortage and pollution in our country,and achieve the sustainable development of water resources.Compared with the traditional water treatment technology,the technology of constructed wetland and ecological pond has more advantages in advanced treatment.The combination of the them can integrate the advantages of them,make up for the lack of depth purification,and stabilize water quality.Shenzhen Binhe sewage treatment plant tail water as the research object,discusses the application of high-efficiency vertical flow constructed wetland +multistage ecological ponds in advanced treatment of tail water,which intends to provide a theoretical basis for the practical engineering application of combined purification system.This study begins with laboratory test on purification of ecological pond,to select optimal ecological pond,which is high efficient and has landscape effect: construct ecological pond with 5 kinds of submerged plants with purification capacity in Guangdong Province,and study ecological pond system in water purification and the stability from nutrient salt tolerance?oxygen recovery rate and the removal efficiency of pollutants,to screen dominant ecological pond for constructing combined system in pilot test.Then construct pilot scale combined system with vertical flow constructed wetland and ecological pond to study the optimal hydraulic conditions based on the optimal ecological pond,and study the trial operation effect and the differences between different points of ecological pond in water purification efficiency under optimal hydraulic conditions.Finally,on the basis of pilot test monitoring data,thewater quality evaluation and benefit evaluation of the combined process are studied,and the actual operation effect of the combined process is discussed to provide the parameter basis for the practical engineering application.The main conclusions of this study are as follows:(1)purification experiment of ecological pond :Different ecological pond showed different degree of tolerance to ammonia nitrogen,the most obvious is that the removal rate of Hydrilla verticillata to CODCr and TP is higher when ammonia nitrogen is 0.2 mg/L,however the removal rate of other ecological pond to CODCr and TP is higher when ammonia nitrogen is 4.0 mg/L.Different ecological pond showed different oxygen recovery ability: oxygen recovery ability of Ceratophyllum demersum and Vallisneria natansand oxygen capacity is higher,oxygen recovery rate can reach 99.85%,99.85% respectively.Different ecological pond showed different pollutants removal efficiency: the CODCr removal rate of ecological pond is not high,Ceratophyllum demersum with the highest removal rate can only remove 35.13%;The NH3-N removal rate of ecological pond range from50% to 60%,the NH3-N removal rate of Myriophyllum spicatum and Vallisneria natans is higher,which can be as high as 66.45%,63.74% respectively;The TN removal efficiency of ecological pond range from 60% to 85%,the TN removal rate of Ceratophyllum demersum and Potamogeton malaianus is higher,which can be as high as 81.16%,77.29% respectively;The TP removal efficiency of ecological pond range from 30% to 50%,the TP removal rate of Hydrilla verticillata and Vallisneria natans is higher,which can be as high as 46.29%,43.74% respectively;(2)pilot scale test of vertical flow constructed wetland and ecological pond:The purification of vertical flow constructed wetland and ecological pond is effected by HLR,and the rule of influence is different.With the increasing of HLR,the removal rate of CODCr in vertical flow constructed wetland system increase firstly and then decrease,but the change is not obvious,the removal rate of NH3-N and TN decrease gradually,and the removal rate of TP increase firstly and then decrease.With the increasing of HLR in ecological pond system,the removal rule of the pollutants are the same,the removal efficiency of CODCr,NH3-N,TN and TP all decrease with theincreasing of HLR;The best operating condition is when the hydraulic loading of VFCWs is 545.5mm/d and the hydraulic loading of ecological ponds is112.5mm/d;The combined process has obvious purifying effect on the tail water from sewage treatment plant,the removal effect of CODCr,NH3-N,TN and TP can be up to59.22%,81.06%,93.11%,55.81%,respectively,the effluent can reach grade IV of?surface water environmental quality standard?(GB3838-2002);wetland,ecological pond and the combined process of CODCr,NH3-N,TN of the treatment effect is consistent with Shenzhen temperature trend,the treatment effect was positively correlated with temperature,while the removal efficiency of TP is independent of temperature;with the prolongation of flow the concentration of pollutants fall,and the removal rate increase gradually.(3)the operation effect evaluation of pilot project:Single factor pollution evaluation method ? comprehensive pollution index evaluation method?eutrophication pollution index method and fuzzy comprehensive evaluation method were adopted to evaluate water quality of pilot system.Water quality assessment is based on ? surface water environment quality standard ?(GB3838-2002),the evaluation results show that: effluent of the combined system can almost be up to the standards,only a small number of monitoring data is beyond the standard in the running of a year.Items which exceeding the standard are NH3-N?TN?TP,and this phenomenon happens in the winter,and exceeding standard rate is no more than 6.67%?4.67%?6.67%.The compliance rates of CODCr?NH3-N?TN and TP is as high as 100%?94.44%?93.75%?97.92%,respectively;effluent quality grade is basically qualified in winter(December,January,February,March),effluent quality grade is qualified in other time of the year;the eutrophication index of the influent is51.46-57.22,which belongs to the mild eutrophication.The effluent eutrophication index is in the range of 36.31-49.21,which belongs to the middle nutrition;in November,December,January,February and March membership degree of effluent to Class IV water quality standard is 0.3621,0.5202,0.6142,0.6331,0.2992,respectively,the effluent for class IV water quality standard,for class III water quality standard in other periods,water quality is poorer in winter than in other seasons,but still can reach the standards of class ?;The operation of the combined process can significantly reduce the pollution load of the receiving water body,and effectively improve the surface water environment.Stable operation of one year,at least can reduce CODCr emissions by9.85 t /a,NH3-N emissions by 1.73t/a,TN emissions by 4.96t/a,and TP emissions by0.16t/a;effluent can be used as supplementary water for river and green water,which has good environmental and economic benefits.