| Dianchi Lake is one of the most eutrophication lakes in China.Until 2015,the lake is still in moderate eutrophication state and water quality is worse than the surface water classⅤstandard.There are 35 rivers in the basin which are the main channels for pollutants inflowing into the lake.Therefore,the key strategy of controlling Dianchi Lake pollution is to regulate the rivers.The rivers in the Dianchi Lake basin can be divided into three types:urban river（15）,suburban river（8）and multi-type river（12）.Among them,the multi-type river has the pollution characteristics of both urban and suburban rivers,including complicated causes,high pollution load and different spatial distributio Therefore,Xinbaoxiang River,which is the largesr multi-type river in Dianchi Lake basin,was selected as a model river for this study.By investigating the current situation of river pollution to summarize the existing problems in the river basin,corresponding suggestions to improve the water quality were proposed,including pollution control of suburban combined overflow in midstream,technical assessment of urban non-point source pollution in downstream and treatment of the low-pollution water in the lake estuary.The application of the technologies in the demonstration project showed that the technology research could obviously decrease the pollution.Investigation of pollution state in Xinbaoxiang river.Through the monitoring data,sample survey and analysis of statistical yearbooks,the riverbed state,composition of vegetation resources,river water quantity,water source,aquatic organisms were investigated within 41 km from the upper reaches to the estuary of Xinbaoxiang river.The upstream is the natural ecological river,the river bank has part of the natural vegetation.The midstream and downstream belongs to artificial river regulation and riparian vegetation is artificial planting.The river is seriously short of water,and the natural water supply is less than 2/3 of the ecological water demand.From the upstream to the downstream,the river Banks,vegetation and water quantity of the Xinbaoxang river were affected by human activities and the water pollution was steadily aggravated.The organic pollution load in the midstream was more than twice that of the upstream.The nitrogen and phosphorus load in the midstream were 3 times that of the upstream.In the downstream,the concentration of NO3--N increased and the concentration of TN was significantly higher than that of the midstream.Construction of assessment system and problem diagnosis.Based on the analysis of survey,a comprehensive river assessment index system was constructed with hydrology characteristics,water quality,riparian zone,morphology and aquatic organisms.Such a system was subsequently executed for the assessment and problem diagnosis of the upstream,midstream and downstream of Xinbaoxiang river.The main problem of the upstream suburban river is the lack of ecological water demand,while the middle suffers from not only water shortage but water pollution problems as well,followed by the poor quality of downstream urban areas ascribed to pollution from non-point sources and the inflow of low-pollution water.Hence,this paper proposed the control stratedy for river management.Regarding river replenishment,the upstream make-up water quality needs to satisfy the surface water classⅤstandard,and the midstream supply water quality shall adopt the class 1-A effluent standard of the sewage treatment plant.As for the treatment of river pollution,the overflow pollution of suburban midstream remains an essential difficulty,while the interception of urban non-point source pollution and the treatment of low-pollution water are still imperative in the urban downstream areas.Study on pollution control technology of overflow in suburban midstream river.Aiming at the problem of combined sewer overflow during the rainy season in the suburbs of the midstream,this study proposed a technology for efficient treatment of overflow sewage by a storage tank-sewage treatment plant-constructed wetland system.The overflow sewage was converted to the storage tank during the rainfall period,followed by the sedimentation and purification,after which the bottom high-concentration overflow sewage was transferred to the sewage treatment plant,and,correspondingly,the upper low-concentration one entered the wetland for treatment.In this paper,the design parameters of 7500m3 storage tank and 3436m2multi-stage wetland system were investigated,laying a foundation for the design and construction of the demonstration project.After the demonstration project was completed,the evaluation of a 6-month project treatment effect indicated that the project operated steadily,and the final effluent quality reached the classⅤstandard of surface water.This technology not only effectively solved the pollution of overflow sewage,but maximally reduced the impact of overflow sewage on the sewage treatment plant as well.Last but not least,wetlands could also assist in the purification of polluted river water during the dry season.Study and evaluation of water quality improvement technology in urban downstream river.The downstream area is the key to control the water quality of the river into the Dianchi Lake,but the investigation shows that the pollution in the downstream river is most serious.Considering serious urban non-point source pollution,this paper studied and evaluated the technical effects of riparian interception technology in urban areas,including wall-mounted rainwater interception and grassed swales pollution interception.The interception efficiency of COD,TN,TP and SS by wall-mounted rainwater interception technology was 37.3%,27.8%,30.0%and 49.8%,respectively.The interception efficiency of COD,TN,TP and SS by swales pollution interception technology was 45.0%,26.3%,38.8%and 71.9%,respectively.The results demonsatrate that these two technologies can be applied to terminal control and midway control as a riparian interception technology respectively for urban non-point source pollution control.Aiming at the problem of low pollution water in downstream caused by recharging from sewage treatment plants,we conducted the selection of wetland types,the optimization of hydraulic load and the selection of wetland species based on the estuarine wetland space and then proposed a“ditch,pond and surface”ecological wetland technology which was futher applied to the demonstration project.The evaluation on the demonstration project shows the ecological wetland technology working well both in the dry and rainy season.In the dry season,the wetland operation scale is 20,000 m3/d,which can reduce COD 43.34 t,BOD5 5.23 t,TN 8.24 t,NH3-N 4.59 t,TP 0.54 t and SS 26.43 t every year.During the rainy season,the operation scale of the wetland is 40,000m3/d,which can reduce COD 55.2 t,BOD5 15.12 t,TN 6.38 t,NH3-N 4.01 t,TP 0.79 t and SS 141.6 t every year. |
