Assessment Of Influence Factors And Evaluation Of Engineering Efficiency On The Retention Of Nitrogen And Phosphorus In The Multiple Pond-constructed Wetlands In A Basin

Non–point source(NPS)pollution,as an important source of pollution load into lake in China,seriously affected the health of aquatic ecological environment and the safety of drinking water.Large-scale integrated multiple pond-constructed wetlands(MPCWs)were firstly applied to control NPS pollution in Erhai Basin.It aimed to intercept NPS pollution in the basin and reduce the pollution load into lake.This study focused on comprehensively analyzing the interception of NPS pollution in MPCWs,identifing the influence factors of pollutant purification performance,and revealling the driving mechanism of process parameters on microbial community distribution.Finally,both an interval analysis model on the intensification of purification performance by changing process parameters and an assessment model on the interception of NPS pollution were constructed,which optimized the configuration of process parameters and spatial layout of MPCWs in a basin.The major conclusions were outlined below:(1)The interception of NPS pollution by integrated MPCWs in Erhai basin was comprehensively analyzed.The influent pollutants were characterized by high nitrogen concentration,low phosphorus and low carbon concentration,which were linked with the agriculture-based local industrial structure,farmland-based land use and the status of fertilization based on farmyard manure.According to the characteristics of influent,evaporation and rainfall,the water seasons in MPCWs were divided into wet season(July to October),normal season(November to December and May to June),dry season(January to April).Due to this,the characteristic process parameters of MPCWs,such as hydraulic loading rate(HLR),mass loading rate(MLR),hydraulic retention time(HRT)and water depth,showed seasonal variability.During wet season,MPCWs ran in a high-water-level because of concentrated rainfall,and achieved the largest interception of NPS pollution.In contrast,higher evaporation than the rainfall during both dry and normal season caused significant decline of water level.The accumulated and purified reclaimed water was available for the nearby rural agricultural water use.It can bring about water saving,pollution reduction,water resource allocation and sewage reuse by MPCWs.(2)The purification performance was identified,and the contribution of different process parameters to pollutant interception was traced to achieve scientific classification and management of integrated MPCWs.The purification performance was good and the mass removal rate of TN and TP in wet season(660.8 mg/m~2/day and47.9 mg/m~2/day)were higher than those in normal(305.7 mg/m~2/day and 22.2 mg/m~2/day)and dry season(221.0 mg/m~2/day and 12.5 mg/m~2/day).Influent condition(MLR and HLR),temperature,HRT and water depth significantly affected the purification performance,among which the influent condition was the main control factor.According to the water depth and process characteristics,the integrated MPCWs were divided into four categories:a.shallow free water surface flow constructed wetlands(water depth<0.8 m)had small storage capacity but excellent purification performance;b.medium-depth free water surface flow constructed wetlands(water depth=0.8-1.5m)had medium storage capacity and general purification performance;c.deep pond-constructed wetlands(water depth>1.5 m)had the best storage capacity,but poor purification performance;d.eco-floating treatment wetlands(water depth>1.5 m)had great storage capacity,and purification performance.(3)Further study was conducted with microbial high-throughput sequencing on three MPCWs to reveal the driving mechanism of combination process and parameters on microbial community distribution.The abundance of functional bacteria in BT,HL and XY MPCW were 1.75%?6.95%,1.81%?7.77%and 3.68%?24.60%,respectively.A few dominant species appeared in BT MPCW,while rich dominant genus species presented in XY MPCW,indicating that the combination process(pond-free water surface flow CW-pond-floating treatment wetlands-pond)promoted the flourish of functional genera.The correlation analysis results displayed that the concentration of influent pollutants and hydraulic retention time positively correlated with the flourish of dominant and functional bacteria.(4)Both an interval analysis model based on process parameters to enhance purification performance and an assessment model on the interception of non-point source pollution were constructed to optimize the configuration of process parameters and spatial layout of MPCWs.Operational influent condition parameters were quantified based on the interval analysis model:MLR(TN)was in the range of5000?7500 mg/m~2/day,MLR(TP)ranging from 200 to 400 mg/m~2/day and HLR from0.3 to 0.6 m~3/m~2/day,with which good purification performance could be obtained.Based on the assessment model,the contribution rates of TN and TP interception in MPCWs were estimated at 2.14%and 2.67%,respectively.Shallow free water surface flow constructed wetlands could be used in populous areas with small amount of highly polluted water and eco-floating treatment wetlands could be used in small population area with large amount of highly polluted water.Reasonable site selection,increasing the number and areas of MPCWs as well as optimized spatial layout may help to increase the interception of NPS pollution by MPCWs.The design of enhancing efficiency and reasonable spatial layout of MPCWs in a basin required various consideration to other factors,such as local special land resources endowment,pollution source structure,and the allocation of rural agricultural water resources.Overall,this study comprehensively evaluated the engineering efficiency of MPCWs for controlling NPS pollution in a basin.It provided basis of decision for the effective application of multiple functions as‘regulation,storage,saving,utilization and purification'and the scientific management of MPCWs for controlling NPS pollution in Lake Basin.